DRYING EASTERNHARDWOOD LUMBER

ByJOHN M. McMILLEN

andEUGENE M. WENGERT

Forest Products LaboratoryForest Service

U. S. Department of Agriculture

(Maintained at Madison, Wisconsin, incooperation with the University of Wisconsin)

AGRICULTURE HANDBOOK NO. 528September 1978

Library of Congress Catalog No. 77-600073

MCMILLEN, JOHN M.

1977. Drying eastern

and WENGERT, EUGENE M.

hardwood lumber. U.S. Dep. Agric., Ag-ric. Handb. 528, 104 p.

Presents recommendations based on recent research andindustry practice for drying eastern hardwood lumber, in-cluding dimension items. Accent is on comparing methodsfor energy-saving management decisions, but practical guid-ance is also given to wood drying personnel. Air drying,accelerated air drying, and kiln drying are covered.

KEYWORDS: Hardwoods, drying, air drying, kiln drying,forced-air drying, high-temperature drying, steaming, de-humidifier drying, cost accounting, energy saving, degradereduction.

ABOUT THE AUTHORS . . .

JOHN M. MCMILLEN is a forest products technologist who hasspecialized in the drying of wood for more than three decadesat the Forest Products Laboratory.

EUGENE M. WENGERT, formerly a member of the wood dry-ing group at the Laboratory, is now extension specialist inwood technology at Virginia Polytechnic Institute and StateUniversity, Blacksburg, Va.

For sale by the Superintendent of DocumentsU.S. Government Printing Office

Washington, D.C. 20402Stock Number 001-000-03761-7

ii

PREFACEInterest in broader utilization of hardwoods has been spurred by shortages of softwoods

and an apparent surplus of hardwoods. However, to greatly increase use of hardwoods willrequire simplified drying procedures. At the same time, the better grades of hardwoods must beseasoned by methods less wasteful of both material and the energy required to dry it. Thesefacts make this publication desirable.

The handbook is confined to eastern hardwoods because most of the hardwood timber growsin the Eastern United States. Much of the general methodology will apply to westernhardwoods, but some species have special problems and some localities have severe air dryingweather; western hardwood users should consult local authorities on specific questions.

Without supplanting the two existing handbooks on kiln drying and air drying lumber, thishandbook combines improvements in practice with new research findings to provide savings incosts and energy. It assembles scattered technology on accelerated air drying and presents newfacts and interpretations on air drying. Kiln drying information is concentrated here, and othermethods of drying are discussed. An overall guide is also provided to combine various dryingprocedures most economically for various product requirements.

This publication should be of the greatest value to operating and managerial personnelresponsible for hardwood lumber processing in hardwood mills, custom drying operations, andfurniture plants. It should also help individuals dry small quantities of lumber inexpensivelywithout sacrificing quality. Teachers and students should find this handbook helpful in careerdevelopment programs.

The authors acknowledge the assistance of Val Mitchell, State Climatologist, University ofWisconsin, Madison in supplying and interpreting some of the climatological informationneeded for this publication. Other vital contributions were made by many Forest Servicecolleagues and coworkers in industry. Particular recognition goes to Walton Smith and PaulBois for photographs, to Kenneth Compton for specific information on steaming of walnut, andto members of the U.S. Forest Products Laboratory research unit on improvements in dryingtechnology. For all these forms of assistance, we are most grateful.

JOHN M. MCM ILLEN

E UGENE M. WE N G E R T

Use of trade, firm, or corporation names in this publication is forthe information and convenience of the reader. Such use doesnot constitute an official endorsement or approval of any prod-uct or service by the U.S. Department of Agriculture to theexclusion of others that may be suitable.

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CONTENTS

Drying Hardwoods . . . . . . . . . . . . . . . . .

Chapter I.—Comparison of DryingMethods for Hardwoods . . . . . . . . . . . .

Basic Drying Concepts . . . . . . . . . . .Methods . . . . . . . . . . . . . . . . . . . . . . . . . .How Dry is Dry Enough . . . . . . . . . .Literature Cited . . . . . . . . . . . . . . . . . .

Chapter 2.—Stock Preparation andStacking . . . . . . . . . . . . . . . . . . . . . . . . . . .

From Logs to Lumber . . . . . . . . . . . .Sorting . . . . . . . . . . . . . . . . . . . . . . . . . . .Stacking . . . . . . . . . . . . . . . . . . . . . . . . .Further Warp Control . . . . . . . . . . . .Literature Cited . . . . . . . . . . . . . . . . . .

Chapter 3.—Air Drying . . . . . . . . . . . . .Advantages and Limitations of

Air Drying . . . . . . . . . . . . . . . . . . . . .Utilizing Air Movement . . . . . . . . . .Other Factors Relating to Drying

Rate and Defects . . . . . . . . . . . . . . .Drying Time and Final Moisture

Content . . . . . . . . . . . . . . . . . . . . . . . .Deterioration of Lumber While

Air Drying . . . . . . . . . . . . . . . . . . . . .Air Drying Small Quantities

of Lumber . . . . . . . . . . . . . . . . . . . . . .A Quick Guide for Improving Air

Drying Efficiency of HardwoodsLiterature Cited . . . . . . . . . . . . . . . . . .

Chapter 4.—Accelerated Air DryingResearch Basis . . . . . . . . . . . . . . . . . . .Types of Dryers and Procedures . .Advantages and Disadvantages of

Accelerated Air Drying . . . . . . . . .Drying Times . . . . . . . . . . . . . . . . . . . .Literature Cited . . . . . . . . . . . . . . . . . .

Chapter 5.—Conventional KilnDrying . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Drying Procedures . . . . . . . . . . . . . . .Operational Considerations . . . . . . .Drying Times . . . . . . . . . . . . . . . . . . . .Literature Cited . . . . . . . . . . . . . . . . . .

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Chapter 6.—High-TemperatureDrying . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Basic Concepts . . . . . . . . . . . . . . . . . . .Research and Practice to Date . . .Drying Times and Species

Potentialities . . . . . . . . . . . . . . . . . . .Apparent Process Requirements .Literature Cited . . . . . . . . . . . . . . . . . .

Chapter 7.—Special PredryingTreatments . . . . . . . . . . . . . . . . . . . . . . . .

Steaming . . . . . . . . . . . . . . . . . . . . . . . . .Chemical Seasoning . . . . . . . . . . . . . .Polyethylene Glycol Process . . . . . .Surface Treatments to Prevent

Checking . . . . . . . . . . . . . . . . . . . . . . .Other Pretreatments . . . . . . . . . . . . .Literature Cited . . . . . . . . . . . . . . . . . .

Chapter 8.—Other Methods ofDrying . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Heated Room Drying . . . . . . . . . . . . .Dehumidifier Drying . . . . . . . . . . . . .Solar Drying . . . . . . . . . . . . . . . . . . . . .Press Drying . . . . . . . . . . . . . . . . . . . . .High-Frequency and Microwave

Heating . . . . . . . . . . . . . . . . . . . . . . . .Solvent Seasoning . . . . . . . . . . . . . . . .Minor Special Methods . . . . . . . . . . .Literature Cited . . . . . . . . . . . . . . . . . .

Chapter 9.—Storage of DriedLumber . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Air-Dried Lumber . . . . . . . . . . . . . . . .Kiln-Dried Lumber . . . . . . . . . . . . . . .

Chapter 10.—Economics and EnergyTypical Costs for Various Drying

Procedures . . . . . . . . . . . . . . . . . . . . .Degrade Costs and Causes . . . . . . .A Cost Accounting Approach . . . . .Energy Considerations . . . . . . . . . . .Literature Cited . . . . . . . . . . . . . . . . . .

Appendix A.—Species, Thicknesses,Drying Schedules, andComments . . . . . . . . . . . . . . . . . . . . . . . .

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CONTENTS (Con.)Page Page

Appendix B.—Mixed Drying of Some Appendix D.—Lumber Names, TreeSpecies by Simplified Species, Seasoning Types, andSchedules . . . . . . . . . . . . . . . . . . . . . . . . 87 Botanical Names Mentioned in

This Handbook . . . . . . . . . . . . . . . . . . . 92Appendix C.—Special Kiln Schedules

for Special Purposes . . . . . . . . . . . . . . 89 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

Requests for copies of illustrations contained in this publicationshould be directed to the Forest Products Laboratory, ForestService, U.S. Department of Agriculture, P. O. Box 5130, Madi-son, Wis. 53705.

LIST OF TEXT TABLESTable

1

2

3

4

5

6

Title

Expected average interiorrelative humidities andrecommended moisturecontent values for mostwood items for interioruses . . . . . . . . . . . . . . . . . . . . . . .

Estimated time to air drygreen 1- and 2-inch hard-wood lumber to approxi-mately 20 percent averagemoisture content . . . . . . . . . .

Forced-air dryer sched-ules for selected easternhardwoods . . . . . . . . . . . . . . . . .

Index of forced-air dryingschedules . . . . . . . . . . . . . . . . . .

Low-temperature kilnschedules for selectedeastern hardwoods . . . . . . . .

Index of low-temperaturekiln schedules . . . . . . . . . . . . .

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18

27

28

29

30

Table

7

8

9

10

11

12

Title

Approximate Canadianlow-temperature kilnschedules for 4/4 mapleand birch . . . . . . . . . . . . . . . . . .

Suggested low-temperaturekiln schedule for rough-sawed 4/4 red andwhite oak . . . . . . . . . . . . . . . . . .

Accelerated air drying timesfor common hardwooditems in slightly heatedforced-air dryers (low-temperature kilns) . . . . . . . . .

Grouping species for basickiln schedules . . . . . . . . . . . . .

Alphabetical index ofspecies and basic kilnschedule table numbers . . .

Basic kiln schedules forred and white oak, south-ern lowland . . . . . . . . . . . . . . . .

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36

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38

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LIST OF TEXT TABLES (Con.)Table

13

14

15

16

17

18

19

20

21

Title

Basic kiln schedules for redoak, northern or upland . . .

Basic kiln schedules forwhite oak, northernor upland . . . . . . . . . . . . . . . . . .

Basic kiln schedules for rockelm, dogwood, persimmon,and similar woods . . . . . . . . .

Basic kiln schedules forAmerican and slippery elm,holly, and black walnut . . . .

Basic kiln schedules forbeech, sugar maple,and pecan . . . . . . . . . . . . . . . . .

Basic kiln schedules forwhite ash, yellow birchcherry, sweetgumand similar woods . . . . . . . . .

Basic kiln schedules formagnolia, paper birch,butternut, normal cotton-wood, and similar woods . . .

Basic kiln schedules for yellow-poplar and cucumber-tree .

Basic kiln schedules forblack tupelo (black gum)and sweetgum sapwood(sap gum) . . . . . . . . . . . . . . . . . .

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38

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41

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Table

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23

24

25

26

27

Title

Basic kiln schedules forbasswood, aspen (sap-wood or box lumber). . . . . . . .

Accelerated kiln schedulefor northern or uplandred oak 4/4, 5/4 witha high initial moisturecontent (some risk ofslight surface checking) . . .

Approximate kiln-dryingtimes for 4/4 hardwoodlumber in conventionalinternal fan kilns . . . . . . . . . .

Eastern hardwood speciesfor which high-temperaturekiln-drying potential has beenindicated, and estimateddrying time for 1-inchlumber . . . . . . . . . . . . . . . . . . . .

Amount temperature mustbe raised above averageoutdoor temperature forvarious equilibrium mois-ture content values inheated-room drying . . . . . . . .

Energy consumption and

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54

cost estimates in kilndrying hardwood lumber . .

Mention of a chemical in this handbook does not constitute arecommendation; only those chemicals registered by the U.S.Environmental Protection Agency may be recommended, andthen only for uses as prescribed in the registration—and in themanner and at the concentration prescribed. The list of regis-tered chemicals varies from time to time; prospective users,therefore, should get current information on registration statusfrom Pesticides Regulation Division, Environmental ProtectionAgency, Washington, D.C. 20460.

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68

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DRYING HARDWOODS

Drying is a vital but sometimes trouble-some procedure in the manufacturing of mosthardwood products. Only after much of themoisture has been removed from wood is thematerial ready to be made into useful prod-ucts. And drying raises problems-speciallywith hardwoods. During drying, degrade canoccur, resulting in both loss of value and lossof valuable wood resource; considerableamounts of energy can be wasted; and mis-takes can be made that will cause problems insubsequent manufacturing. Fortunately, avariety of procedures are available that satis-factorily dry hardwood lumber for shipmentand ultimate use.

This report combines the informationpublished by many public laboratories, uni-versities, and associations, with other infor-mation developed at the Forest Products Lab-oratory and other units of the Forest Service,U.S. Department of Agriculture. It concen-trates on the common ways of drying 1-inch

through 2-inch thick lumber, free from dryingdefects, for the finest uses. It also suggestsways to economize on drying time, cost, andenergy demands whenever hardwoods are tobe used for construction or less demandinguses. It also provides a standardized method ofcomputing drying costs.

Much general information needed for airdrying and kiln drying hardwoods properly iscontained in two Agriculture Handbooks: The“Dry Kiln Operator’s Manual” 1 and “Air Dry-ing of Lumber.” 2 This report does notattempt to repeat all details from those vol-umes. Rather, it presents new information onthese methods and on accelerated air drying,as well as other methods available for dryinghardwoods.

1 Rasmussen, Edmund F. 1961. Dry Kiln Operator’sManual. U.S. Dep. Agric., Agric. Handb. 188,197 p., illus.

2 Rietz, Raymond C., and Rufus H. Page. 1971. Air Dry-ing of Lumber: A Guide to Industry Practices. U.S. Dep.Agric., Agric. Handb. 402, 110 p., illus.

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CHAPTER 1COMPARISON OF DRYING METHODS FOR HARDWOODS

Basic Drying Concepts

A great deal of scientific knowledge is notnecessary to dry wood. Some appreciation ofthe basic concepts, however, will help in theselection of drying methods and in the appli-cation of the general information on drying tospecific situations.

To dry wood, three basic requirementsmust be met:(1) Energy must be supplied to evaporatemoisture throughout the drying process.As the wood becomes drier, additionalenergy is needed to release moisture fromthe hydroscopic forces that bind some ofthe water to the wood. Very green woodwith large amounts of free water requires580 calories per gram of water evapo-rated. Below 30 percent moisture content,more energy is required (fig. 1). Energyalso is required to raise the temperatureof the wood to increase the rate of mois-ture movement from the interior to thesurface and to increase the rate of evap-oration.(2) The environment adjacent to thewood must be capable of receiving mois-ture from the wood’s surface. In air and

kiln drying, the relative humidity must bebelow 100 percent. The lower the relativehumidity, the faster the drying. (But rel-ative humidity must not be so low as tocause degrade. )

(3) Air movement through a stack oflumber must be adequate to bring heatenergy in, to remove evaporated mois-ture, and to maintain desired relativehumidities within the stack. The relativehumidity within a stack of drying lumbertends to rise above the general relativehumidity in the drying yard or kiln. Thusthe inside of the stack tends to dry slowerthan the outside.

These three factors can be manipulated tocontrol the drying process, minimizing de-fects while drying the wood as rapidly as pos-sible. See Dry Kiln Operator’s Manual and AirDrying of Lumber for more information onwood properties, the drying process, causes ofdrying defects, and the methods of avoidingthem. More highly technical information isgiven in other references (Brown et al 1952,MCMillen 1969, Siau 1971, and Stamm 1964).

Methods

A number of methods are available for dry-ing hardwood lumber, ranging from air andkiln drying to special seasoning processes.The customary method in the United States,except in the northernmost States, has beento air dry the lumber to 20 percent moisturecontent, then kiln dry it as much as needed. Iflumber is in short supply, more lumber is kilndried from the partly air dry or green condi-tion. Kiln drying from the green condition hasbeen common where air drying conditions arepoor most of the year.

During years of adequate energy and woodsupplies, the shortest air drying time and thebest target moisture contents for air driedlumber were not well defined. Today these aremuch more important; the combination of airdrying and kiln drying probably is the best

energy-conserving method for drying betterquality hardwood lumber in most of theUnited States. A later chapter on air-dryingcontains a map of the average number ofmonths of good air-drying weather availablein various regions, and a table of air-dryingtimes by species and thicknesses in these re-gions.

In regions where good air-drying weatherprevails only 4 to 6 months of the year, con-sideration should be given to accelerated airdrying procedures. These have good promisefor reducing lumber inventories and bringinglumber production closer to the market. Theyalso permit quality drying of hardwoods withcharacter marks, or minor natural defects.The chapter on this subject includes a discus-sion of the several types of equipment avail-

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M 144691

Figure 1.—Energy required in drying of wood as woodmoisture content changes (after Skaar and Simpson1968).

able, the schedules recently perfected for avariety of Appalachian hardwoods at theForest Products Marketing Laboratory,3 anda table of drying times for common hardwooditems in slightly heated forced-air dryers.

Very small quantities of hardwood lumbercan be air dried and then stored in a slightlyheated or dehumidified room to bring mois-ture content low enough for furniture andother indoor uses. If fully air dried, such smallquantities can be kiln dried with almost anyother species of hardwood in a commercialkiln, if a person can find a custom kiln dryingfirm that will accept them.

Drying hardwood lumber in a conventionalkiln has several advantages. Modern conven-tional kilns have been designed to acceleratedrying greatly by using temperatures from100° to 180° F with air velocities from 200 to600 feet per minute through the load. Underthese conditions, a schedule of relativehumidities high enough to prevent drying de-fects can be maintained in reasonably tightsite-constructed or prefabricated buildings.

One advantage of kiln drying is the abilityto achieve a low, uniform final moisture con-

3 Northeastern Forest Experiment Station, ForestService, U.S. Department of Agriculture, located atPrinceton, W. Va.

tent. Another is to relieve drying stresses(case-hardening). Relief of drying stresses isnecessary to prevent warp if more wood isplaned off of one side of a board than theother, or if the wood is to be resawed ormachined deeply from one side. Uniformity ofmoisture content and relief of drying stressesare especially important to large-scale opera-tions that produce hardwood dimension cut-tings from small trees. If the stock is notproperly equalized and conditioned, warpageand poor joints occur when the cuttings areglued into panels.

While high-temperature drying (above 212°F) is commonly used for softwoods, it has gen-erally been considered inapplicable to mosthardwoods. However, recent research and in-dustrial applications indicate more optimisticprospects for this time- and energy-savingmethod. Drying time may be only one-fourththe time required for conventional kiln dry-ing. There are two reasons why energy issaved: (1) The vents are kept closed and (2)the shortened drying time greatly reducesheat loss through the structure and floor.Slight discoloration may bar high-temperature-dried wood from some uses. Themethod, however, may be fully satisfactoryfor drying yellow-poplar and other easilydried woods for concealed uses and for dryingsimilar hardwoods for construction purposes.

Besides major information on the abovedrying processes, this handbook covers spe-cial predrying treatments that reduce defectswhen drying especially refractory or defect-prone woods, a number of other dryingmethods, the proper storage of dried lumber,and economic and energy considerations.Predrying treatments include steaming forcolor promotion or drying rate acceleration,and treatment with the hydroscopic and bulk-ing chemical (polyethylene glycol) for defectreduction. Also included are wax, sodium al-ginate, or salt-paste surface treatment for theprevention of surface checks.

A cardinal rule in drying hardwoods is toreduce the material to the smallest practicalsize before starting to dry it. Drying time ismore than doubled if thickness is doubled.Therefore, it would be better, from the dryingstandpoint, to make 2-, 3-, and 4-inch materialby gluing together the required number ofpieces of dry 1-inch material. This is one of the

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considerations that makes press drying so of these appear likely to replace air and kilnpromising among the special drying methods. drying for the majority of hardwood dryingOther methods discussed are dehumidifier operations but they do have possibilities fordrying, solar drying, high-frequency and mi- some purposes.crowave drying, and solvent seasoning. None

Table 1. —Expected interior relative humidities and recommended moisture content values formost wood items for interior uses in the United States 1

Relative humidity Wood moisture contentAre as

Average Range Average Range

Percent Percent Percent PercentMuch of United States 40 30-55 8 6-10Dry Southwest 2 30 15-50 6 4-9Damp, warm coastal 60 40-70 11 8-13

1 From Wood Handbook, 1974.2 Also applies to areas where interior environment is dry year around.

How Dry isIn the many decades of dealing with the use

of wood, the U.S. Forest Products Laboratoryhas noted that problems often arise when thewood is not dry enough. Too high a moisturecontent can bring about difficulties in drying,manufacturing, and use. Today, with tighterconstruction in homes, with heated manufac-turing plants, and with year-around climatecontrol in offices, the in-use moisture contentof wood products is quite low, lower even thanin the 1950’s and before. To avoid shrinkage,warping, checking, and splitting in the fin-ished product, lumber must be dried to a finalmoisture content (after conditioning) close tothe middle of the range of expected in-usemoisture contents (table 1).

Furthermore, dried wood must be stored,manufactured, and warehoused at or near theexpected moisture content values. Failure to

Dry Enough?dry wood low enough and then keep it this lowas it gets into use may result later in openglue joints, checking and splitting, finishingimperfections, and warping. And these all de-crease profits and waste this natural re-source.

The moisture content for special uses maybe higher; if so, stringent storage precautionsgenerally are not required. Wood to be bent orused in boats should be somewhere between15 and 20 percent moisture content. Afterbending, wood for interior uses should bebrought down to the moisture levels indicatedabove. Wood to be used as framing or supportsin construction should be in the 10 to 19 per-cent moisture content range. Wood to betreated with preservatives before use shouldbe between 20 and 35 percent moisture con-tent.

Literature Cited

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CHAPTER 2STOCK PREPARATION AND STACKING

Before actual drying operations begin, anumber of steps are important in reducingwarp and avoiding other seasoning defects.Unless great care is exercised in early phasesof the seasoning process, subsequent drying

From LogsLog Protection

Logs should be taken from the woods andsawed into lumber as soon as possible, par-ticularly during warm weather. At a smallsawmill where only a few logs are accumu-lated, no other precautions are needed. Atlarger sawmills, a considerable supply of logsis often kept on hand, and some precautionsshould be taken to protect against deteriora-tion.

Logs that have been piled without protec-tion may end check and be attacked by fungiand insects, especially during warm weatherwhen higher temperatures speed up dryingand fungus and insect activity. Spraying withcold water to keep the logs wet, especiallyduring warm dry weather, will reduce endchecking, splitting, staining, and decay. Con-tinuous spraying is the most beneficial, sospray equipment should be as clog-free aspossible. One type is illustrated in figure 2.Chemical sprays effective against fungi andinsects also are available.

Protection of Green Lumber

It is important that lumber be protectedfrom the time it leaves the sawmill until it canbe transported to the seasoning site. Unlessthe weather is cold, freshly sawed lumbermay be attacked by fungi and insects, and issusceptible to checking and splitting if ex-posed to direct sunshine. Dipping the greenlumber in a solution of toxic chemicals canprotect it against attack by fungi or insects.Prompt piling of the lumber for air dryingalso makes attack by fungi and some types of

steps may be ineffective in reducing degradeand costs. The precautions start with the logs.Major points are discussed here, but othersare included in later sections where they arespecifically appropriate.

to Lumberinsects less probable. Protection againststicker stain, interior graying, and otheroxidative discoloration also begins with thegreen lumber (MCMillen 1976).

Protection against insects is accomplishedalong with dipping or spraying to preventblue stain, the most common fungal hazard.Principal chemicals used in the past includedmercurial and chlorinated phenols but envi-ronmental considerations have been chang-ing the picture. Check with your CountyAgricultural Agent or State Agricultural Ex-periment Station for approved chemicals.Manufacturers will supply directions for use.Treatment is imperative during most monthsin the South and has been found beneficialduring the warmer months in many NorthernStates. A general recommendation is to con-sider dipping during any period of a week ormore when a daily mean temperature of 60° For over is expected.

Dipping lumber to prevent fungus attackdoes not protect it against oxidative discol-oration; in fact, the excessive moisture left onthe wood may promote discoloration undersome circumstances. The best preventative isto rapidly dry the surface moisture. Oxidationstain can be minimized in hardwood lumberby keeping the wood in the strictly greenstate until rapid drying can be started, butbulk storage should not be prolonged. If greenor excessively wet sapwood lumber has beenheld more than 2 weeks before stacking, fastdry the lumber surfaces before regular airdrying is started. Such fast drying can be ac-complished by 1 day of accelerated air dryingor 3 days of very openly spaced yard drying. Ifdried with mixed loads of sapwood and

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M 144 876

Figure 2.—Water spraying equipment used effectively on decked logs.

heartwood, or if the sapwood boards haveheartwood on one face, the heartwood shouldbe observed frequently so that slower dryingcan be started as soon as minor checks ap-pear.

One mistake in handling green lumber is tosticker it and then hold it in closely spacedstacks on a temporary storage yard under theassumption that it will be properly piled orkiln dried “soon.” Damage from discoloration,stain, or even decay can be very costly, evenwith dipped lumber, if this storage is inadver-tently prolonged. Spacing equal to that rec-ommended for air drying is advisable duringsuch storage.

Preventing Surface Checksand End Checks

Surface checks can start during exposure of2-inch heartwood of any hardwood or l-inchheartwood of oak, hickory, and beech 4 to di-

4 Wood species are generally listed in this handbook bytheir common names. But, because different names areso often used for the same species, both common andbotanical names are included in Appendix D.

rect sunlight or strong winds during hot ordry weather. Basically, the surface of the ma-terial is drying much faster than the interior.The material can be protected from directsunshine by a roof. This might be over thearea used to sort and accumulate the freshlysawed lumber, or even a temporary roof onany pile that will be exposed an hour or more.

Research has shown that presurfacinglumber to remove all fine saw marks reducessurface checking in oak (McMillen 1969; Rietzand Jenson 1966; Simpson and Baltes 1972;Wengert and Baltes 1971). Although somefield trials have been successful (Cuppett andCraft 1972, Rice 1971) industrial adoption willdepend upon economic considerations. Pres-ently, surface checking in oak and other re-fractory woods can be economically controlledby other precautions described later. Surfac-ing to a uniform thickness, when the lumberhas not been precisely sawed, is beneficial incontrol of warp, a major cause of degrade.

Certainly another of the most commonforms of degrade is end checks. In uncoatedthick stock, end checks that are not even hair-

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line cracks on air-dried material can extend12 inches or more from the end of the piece.This emphasizes the importance of end coat-ings.

Many hardwood manufacturing operationsroutinely trim 3 to 10 inches off the ends ofevery board. This is a 6 to 20 percent loss oflumber for 100-inch lengths!

Almost 6/4 and thicker hardwood stockwould benefit from end coating. Two-inch andlarger squares usually are end coated, andgunstock blanks must be coated.

End coatings may be divided into two class-es: Cold coatings are liquid at ordinary tem-peratures and can be applied without beingheated; and hot coatings are solid at ordinarytemperatures and must be applied hot. Coldcoatings can be applied readily to logs andlumber. Special end coatings of the cold-coating class are available from manufactur-ers and from dry kiln companies. Coldcoatings are usauly applied by brush, al-though they can be sprayed with properequipment. For small-scale use, heavy pastessuch as roofing cement can be used. Sprayablewax emulsions, which have been very effectiveon redwood lumber, gennerally do not provide

sufficient protection on thick hardwoods.Hot coatings (pitch, asphalt, and paraffin)

are well suited for small stock that can beeasily handled. They are low in cost and highin water resistance when applied in a singlecoat. Hot coatings are generally applied bydipping, but they also can be applied by hold-ing the lumber end against a roller that isrotated while partly submerged in the coat-ing. Paraffin is a material with a low soften-ing point; it is suitable only for air drying- orfor temporary protection of the ends ofsquares that will be kiln dried by a kilnschedule that uses a high relative humidityduring the first stages.

Some specialty items, such as walnutgunstock blanks, require a highly water-resistant coating with a high softening point.Without a good coating, end checks can ex-tend themselves through the length of thepiece as honeycomb.

End coatings should be applied as soon aspossible to freshly cut end surfaces; end coat-ings applied after checking has begun usuallydo not prevent deepening of the checks. Coldcoatings should be allowed to dry a few hoursbefore being subjected to kiln temperatures.

SortingFrom the standpoint of stacking and con-

trol of warping, hardwood lumber must besorted by individual nominal thicknesses.Sorting by length and width also simplifiespiling. Sorting on the basis of species alsotends to increase the efficiency of the dryingoperation, because species differ in their dry-ing characteristics. A further sorting on thebasis of heartwood and sapwood would con-tribute to drying efficiency within some indi-vidual species. Further discussion of the mostcommon sorting practices is contained in AirDrying of Lumber.

In the North, where the number of speciesis small and the value of each species may behigh, sorting by individual species is the rule.In the South, especially when removinghardwoods from sites where pine is desired, itmay be expedient to combine species that canbe kiln dried together. However, hickory–pecan, red oak, and white oak should be han-

dled individually and southern lowland oaksshould be separated from upland oaks.

Sorting for quality has long been practicedwith some species, but there are new scien-tific findings explaining the quality differ-ences and why low quality material presentsdrying problems. So-called mineral streak ormineral portions of sugar maple are nowknown to be wound-induced discolorations(Shigo 1965). Such material is very low inpermeability, greatly hindering water re-moval. Some wood of the oaks, aspen, cotton-wood, and several other hardwood specieshave streaks of wood infected with bacteria(Ward et al 1972; Sachs, Ward, and Kinney1974). Efforts should be made to recognizethese abnormal types of wood. When they arepresent in large quantity, they should besorted out so that special kiln schedule andmoisture monitoring measures c a n b eapplied.

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StackingThe importance of proper stacking cannot

be overemphasized. Sloppiness at this pointcannot be compensated for later on. Themajor purposes of stacking lumber in aspecified manner are twofold: (1) To promoteuniform air circulation, which in turn resultsin good drying; and (2) to reduce or eliminatewarp.

Hardwood lumber is most typically piled fordrying in packages with horizontal layers,using stickers as spacers between each layer(fig. 3). If the lumber is first end racked, endpiled, or cribbed for rapid surface drying, itshould. be repiled in flat piles within 3 or 4days. Degrade from warp can be very costly ifthe stock is end racked or cribbed very long.

The length of the lumber package is usuallythe same as the length of the lumber beingcut. When several sizes are being sawn, sev-eral different package lengths may be used.Sometimes 6-, 10-, and 12-foot lengths are putin one sorted-length pile and 8-, 14-, and 16-foot lengths in another. The shortest piecesare piled end to end and the intermediate-length pieces all pulled to one end. Hardwoodssawed in both odd and even lengths should bebox piled (fig. 4). In either method, it is impor-tant to have full-length pieces on the outeredges of each layer and full support undereach end of each board.

Width of the pile is frequently governed bythe requirements of the dry kiln or forced-airdryer. For most modern kilns with air velocitythrough the load of 200 feet per minute ormore, the most common package width is 48

M 135 021

Figure 3.—Stacking stickered package of lumber.

inches. However, in air drying, and in olderkilns with low air velocity and insufficientventing, less air is moving. Thus, the nar-rower the pile, the faster the wood will dry.Hand-stacked piles should not exceed 6 feetwide unless they are provided with verticalflues. See chapter 3 for more comments on pileconstruction.

Hardwood lumber stacking should aim forsquare, level, and straight-sided piles with allstickers and bolsters in perfect alinement.Acceptable piles are the rule with machinestacking, which is widely used in medium- andlarge-sized hardwood mills and concentrationyards. It is also possible to meet qualityspecifications with hand stacking if stickerguides are used with proper care.

Further Warp ControlIn addition to perfect, vertical alinement of of a unit package and reduces the amount of

the stickers, good bolster placement, and good warp. Precision sawing to a uniform thicknesspile roofs, several other considerations help in also will reduce warp.reducing degrade, particularly warp:

1. Basic warp control can be obtained3. The foundation, kiln floor, or kiln trucks

through proper sawing procedures, especiallymust provide a firm, flat bearing surface for

for small logs. Saw especially warp-prone ma-the lumber pile. A crooked or uneven surface

terial, such as city-grown elm, into widewill cause twist, bow, or kink in the lumber

boards.during drying.

2. Presurfacing lumber (planing both faces) 4. Properly sized and placed stickers areor blanking (one face) to a uniform thickness highly important, with uniform thickness abefore stacking increases the usable volume prime requirement. Broken or distorted

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stickers can increasepaired or discarded.

M 144695Figure 4.—Method of box piling for random-length lumber.

warp and should be re-

5. Sticker spacings of 16 to 24 inches aresatisfactory for many hardwoods, provided alltiers of stickers are fully supported and theiralinement is perfectly vertical. Very valuablelumber or especially warp-prone materialmay benefit from spacing as close as 12inches.

6. Several drying firms restrain the tops ofthe loads by spring clamps anchored lower inthe pile, or by added weight, Research showeda load of 50 pounds per square foot wasadequate for aspen 2 by 4’s dried by high tem-perature. More dense woods probably wouldrequire more pressure. Research on 1-inchblackgum, for instance, showed 150 poundsper square foot more effective than 90 poundsper square foot in reducing cupping. Twotypes of hold-down clamps for restraint ofwarp are shown in Air Drying of Lumber. Ifdifficulty is experienced in locating sources of

hold-downs, the utilization specialists in stateforestry or forestry extension offices may beable to help.

7. Rapid natural air drying or equivalentaccelerated air drying may be a necessaryfirst step in drying warp-prone woods such asAmerican elm. Such methods tend to developa large amount of tension set in the outershell, which helps to hold the lumber flat inlater stages of drying (MCMillen 1963). Whenkiln drying lumber green from the saw, useschedule modifications that provide initialconditions of lower temperature and lowerrelative humidity than those generally rec-ommended.

Most of these procedures will require theoperator to spend some money to implementthem. However, as the value of high-qualitylumber rises and quantities lessen, the cost-benefits may just i fy employing thesemethods.

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Literature Cited

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CHAPTER 3AIR DRYING

Where annual mean temperatures are nottoo low and space for a drying yard is not toolimited, air drying is the most economical andenergy-conserving method to get most of thewater out of hardwoods. To achieve that end,however, keep in mind a thought that is beststated colloquially: “Wood can’t help drying,if you help it!”

The green moisture content of U.S.hardwoods ranges from 45 to 160 percent, onthe ovendry basis. The moisture above 30 per-cent is present as water or water vapor in thewood cell lumens (cell cavities) and is called“free water.” This water comes out of thewood very readily in air drying. The rest ofthe water, in the wood-cell walls, is called“bound water.” It comes out more slowly, par-ticularly as the moisture content decreasesbelow 25 percent.

In the past, the moisture content goal forair-dried lumber was 20 percent. One reasonwas to reduce the chance that mold, stain, ordecay would develop during transit, storage,or use of the wood. Another was to reduce theamount of shrinkage that would occur if thewood were used for an exterior purpose with-out further drying. Time to air dry to an abso-lute 20 percent moisture content is undulylong, however, in some regions at some timesin the year. This is especially true for stockthicker than 4/4 (1 inch).

Under proper air drying procedures, thesurface zone will be at or below 20 percentmoisture content when the average is 25 per-cent. There are no substantial difficulties inkiln drying air-dried stock at this moisturelevel. This handbook, therefore, recommendsterminating the air drying of stock that is tobe subsequently kiln dried at the 25 percentaverage moisture content level whenever it isadvantageous economically. Items to be

treated with preservatives also should stopair drying at this level.

On the other hand, lumber to be bent toform, used for outdoor furniture, or used inconstructing unheated barns and garagesshould be air dried to 20 pereent or slightlybelow. Wood to be bent in a hot press ormachined for trim and flooring in boats andbuildings that are heated only occasionallyshould be dried to a lower moisture content(12 to 18 percent). This level can be obtainedby prolonged air drying in a shelter. Materialfor ultimate interior use in heated or de-humidified rooms should, of course, be driedfurther, in a kiln or by heated-room drying.

The general principles of air drying are rea-sonably well understood. Their proper appli-cation can make this method of reducingmoisture content more efficient and profita-ble. During good air drying weather, slightdeviation from best practices is tolerablewithout incurring serious degrade losses orretardation of drying rate. Three or moredays of greatly abnormal weather, however,can be devastating. High humidity, heavyrain, and fog can cause discoloration; strongwinds in combination with low relativehumidity can cause severe checking. There-fore, it is important to adhere to good practiceyear around, irrespective of normal weatherconditions. With such practices, degrade canbe limited to 1 or 2 percent of the lumbervalue.

The Air Drying of Lumber gives a completediscussion of drying practices. Several pointsare expanded in this handbook, however, toemphasize the assumption that faster dryingand conserving more energy and resourcesnow are paramount in air drying. Some ofthese points involve promotion of practicesknown to be helpful but not commonly usedbecause they add some cost.

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Advantages and Limitations ofAir Drying

One real advantage of air-drying lumberover drying by other processes is its low ini-tial cost. Although there are substantial land,installation, and operating costs for air dry-ing, the cost of kiln-drying dense hardwoodsto the moisture content levels achieved in airdrying could be prohibitively high. However,as the value of the wood increases, kiln-drying species like beech, birch, and maplegreen from the saw becomes more feasible.This is especially so in regions having fewerthan 6 months of good air-drying weather.

A second real advantage is substantialenergy saving. Each percent moisture re-moved by air drying saves energy in sub-sequent kiln drying by roughly 50 to 85British thermal units per board foot. For aconventional kiln with a capacity of 50,000board feet, this means 2.5 to 4.25 millionBritish thermal units can be saved for each 1percent moisture lost in air drying. At late1974 prices, this amounted to $3.75 to $6.38(Wengert 1974).

The air-drying process offers the produceror large-scale user a means of carrying aninventory of various species, grades, and sizesof lumber. To meet shipping schedules duringperiods of the year when the sawmill cannotbe operated to capacity, the yard inventory isbuilt up when sawing conditions are favor-able, and the lumber is air dried while beingheld. Air drying also reduces weight andshipping costs. Any degree of drying that canbe achieved, even during short periods of ac-cumulation, is helpful. A variety of dryingprocesses are available at the destination toaccommodate partly air-dried stock. Somehazard of oxidative discoloration, however,may still exist for white sapwood species par-tially dried and then bulk-piled for shipment.

In combination with properly applied anti-stain dip treatments,5 air drying decreasesthe chance that mold, stain, and decay will

5 Check with your County Agricultural Agent or StateAgricultural Experiment Station for approved recom-mendations.

cause degrade, as they could during bulk-piled storage and shipment. Rapid air dryingat low relative humidities produces a largeamount of set that assists in reducing warp. Ifthe stock is properly stacked and protected ina well-laid-out yard, minimum degrade fromchecking will be apparent, even for thickstock of dense woods and 1-inch heartwood ofespecially refractory woods.

Limitations of air drying are generally as-sociated with the weather and the uncon-trollable nature of the process. Productionschedules are at the mercies of changingclimatic conditions—temperature, relativehumidity, rainfall, sunshine, and winds. Dry-ing generally is very slow during the coldwinter months in the northern regions of thecountry. But mean annual values for the var-ious factors are fairly consistent within anyone region from year to year, so general pro-duction schedules and plans can be workedout. Specific technical information and goodjudgment are needed, however, to completedetailed planning.

Lack of absolute control of drying condi-tions poses some hazards of excessive de-grade. Unusually fast drying of the surface,caused by sun or wind and low relativehumidity, can cause surface checking of beechwithin a day, even in the winter in the North.In other times and areas, brief periods of hot,dry winds may increase degrade and volumeloss due to severe surface checking and endsplitting. Warm, rainy, or sultry periods, withlittle air movement, encourage the growth ofblue stain and aggravate chemical (oxidation)stain and can cause excessive losses unlessproper dipping, pile spacing, and pilingmethods are used.

An excessively large inventory is verycostly, especially when interest rates are veryhigh. Substantial deterioration of the lumbercan occur if air drying is prolonged beyondthe time needed to bring the moisture contentdown to 18 percent or less. If poor marketconditions result in the holding time goingbeyond 2 years, extra measures against de-terioration may be advisable.

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Utilizing Air MovementAir circulation is so important that it is

considered broadly here as well as in detaillater in yard layout, foundations, and piling.Air circulation is a principal means of bring-ing into the air-drying yard or shed the heatneeded to evaporate water. Air circulationalso brings heat into the lumber piles as wellas being itself a factor in drying. It is involvedin removing moisture-laden air from the pileand from the drying area.

Because air must move into and throughthe drying area, yard layout is very impor-tant. Adequate alleys and spaces must beprovided between rows and piles. By customin the United States, the “main” alleys for thetransport of lumber to the piles are perpen-dicular to the prevailing wind direction. How-ever, recent research showed that, for sometypes of yards, better circulation is obtainedby having the main alleys parallel the pre-vailing wind direction. Whatever suchrefinements, t h e a i r s p a c e s m u s t b eadequately large and straight and continuousacross the yard. If these principles are ob-served, air flow will be adequate through theyard whenever there is wind, regardless ofwhich direction the piles face.

Except for those piles exposed directly tothe wind, with their orientation perpendicu-lar to the wind direction, no wind blows

through any pile on an air-drying yard.Rather, all air flow across the boards is due tosmall air pressure differences from eddy andaspiration effects and to air density differ-ences. As the water evaporates, the air iscooled in and around all piles. Cooling makesthe air denser, and it tends to flow downward.When the cool dense air is removed from be-neath the piles, fresh air is brought into theirtops. This effect and the consequent drying goon continuously, regardless of wind, unlessthe air around all the boards is saturatedwith water vapor. This is the reason why,from the fast, economical drying viewpoint,high and open pile foundations are very im-portant.

One precaution should be stated about thelocation o f th ick or espec ia l ly check-susceptible material in the yard and themanner in which the yard is filled up. Suchmaterial should not be located on the wind-ward side of the yard. Lower air velocities andincreased relative humidities in the central orleeward areas of the yard modify the severityof drying conditions and reduce the likelihoodof checking. During severe weather, however,it would be bad practice to start refilling anempty yard from the leeward side toward thewindward side, because each pile in turnwould be exposed directly to the wind.

Other Factors Relating to Drying Rate and Defects

The rate at which green lumber will dry between them is related to the permeabilityafter it is placed in the air-drying yard de- of the wood and the diffusivity of the water inpends upon factors that involve the wood it- it. Much of this difference is due to differencesself, the yard, the pile, and other climatic con- in the proportion of sapwood and heartwood.ditions as well as wind. Beech and maple are almost all sapwood; oak

Woodis almost all heartwood. The fibrous parts ofthe oak heartwood, making up most of its

Species structure, are low in permeability and dif-Some lightweight hardwoods (basswood, fusivity.

willow, yellow-poplar) dry rapidly under fa- Generalized estimates of air drying time byvorable air-drying conditions. The heavier species are given later in this chapter. It musthardwoods require longer drying periods. be recognized, however, that some woods areSpecific gravity, then, is a physical property handled commercially in groups of species,of wood that can guide estimations of drying and there are differences between species inrates or overall drying time. But beech and the groups. Pecan, for instance, is regularlysugar maple will dry faster in northern yards made up of sweet pecan (Carya illinoensis),than will northern red oak, even though all water hickory which is often called bitterhave the same specific gravity. The difference pecan (C. aquatica), and one other minor

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Carya species (see Appendix D). The sapwoodand heartwood of sweet pecan and the sap-wood of bitter pecan dry very rapidly, but theheartwood of bitter pecan dries very slowly.

A similar situation exists with the tupelos.The heartwood and sapwood of black tupelo,usually called blackgum (Nyssa silvatica) andthe sapwood of swamp tupelo (N. silvaticavar. biflora) and water tupelo (N. aquatica)dry rapidly, but the heartwood of watertupelo usually dries slowly. The heartwood ofswamp tupelo is intermediate in dryingcharacteristics.

Red oak is made up of several species ofQuercus and white oak of another group ofQuercus species. Southern lowland oaks, bothwhite and red, have drying characteristicssimilar to each other but different from thoseof the upland oaks. They are, therefore, sepa-rated in drying time and kiln schedule tables.Anyone drying the woods specifically men-tioned and experiencing unduly long dryingperiods should consult a county or Stateutilization and marketing forester to see ifsome practical means of sorting the typescould be derived.

Grain PatternsQuartersawed lumber dries more slowly

than flatsawed lumber. In quartersawedlumber, few wood rays—which aid the move-ment of moisture—are exposed on the broadsurfaces of the boards. Flatsawed lumber,which has more rays exposed, is more likely tosurface check than quartersawed lumberunder severe drying conditions.

ThicknessThick stock naturally takes longer to dry

than thin material. One theoretical approachsuggests that drying time, under identical orsimilar drying conditions, is a function of thesquare of the thickness. In actual kiln drying,the effect of thickness is slightly less. Sincethick stock takes longer than one air-dryingseason to reach 25 percent moisture contentin the central and northern parts of theUnited States, actual air-drying times for2-inch stock are three to four times as long asthose for 1-inch. Also, the greater the thick-ness, the greater the tendency for hardwoodsto surface check and end check.

YardSite

An efficient yard for rapid air drying shouldbe on high, well-drained ground with noobstruction to prevailing winds. One south-eastern furniture firm made considerablesavings by building a new 12-acre yard on ahill 3 miles away from three old, overcrowded,and inefficient yards near their plants(Minter 1961). The savings included a 45-percent labor reduction, faster drying, and adecrease in total inventory of 2 million boardfeet.

LayoutAlley orientation and size, row and pile

spacing, and pile size for both hand-built pileand forklift yards are discussed extensively inAir Drying of Lumber, The general functionof alleys and pile spaces in removingmoisture-laden air from the yard has beenwell known. The effect of wind-induced circu-lation within the piles, however, was not un-derstood until wind tunnel-miniature modelpile studies were made using forklift layouts.

White (1963) used two orientations of asingle block of row-type model piles. Artificialsmoke was used to indicate the relativeamount and direction of air movement. Whenthe piles were oriented perpendicular to thewind, much of the air was deflected over thetop of the block. No wind blew directlythrough any pile except those right on thewindward side. Some of the air that flowedbetween the other piles, however, enteredthese piles from the rear and left from theirfronts, sides, and tops. Double eddies at therear of each pile moved air up into the turbu-lent area above the piles. When the piles wereoriented parallel with the wind, more airmoved through the spaces between the pilesand into and out of the piles.

Finighan and Liversidge (1972) used eightdifferent layouts. Two were single largeblocks of row-type yards; the others wereline-type yard variations. The evaporationrate o f a so l id p lug o f moth crysta ls(paradichlorbenzene) located in each modelpile simulated its rate of drying. The rates inall the line-type layouts were 20 to 40 percentgreater than in the row-type layouts. Thesedifferences probably would have been less if agreater amount of “open space” had been

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used in the row-type layouts. The greatestamount of “drying” took place in the line-typelayouts with the greatest amount of openspace-double lines of piles with 25-foot al-leys. The best combination of good average“drying” rate and good uniformity through-out the layout occurred when the model pilesand the main alleys were oriented parallelwith the wind.

For those species and sizes that may bedried as rapidly as possible, without danger ofloss from splitting and checking, the line-typelayout parallel with the prevailing wind ap-pears to be best. For species such as oak thatare subject to checking, the row-type layoutmay be required. The length of rows and thenumber of rows should be limited, however. Ifthe row-type layout is used and if there areproblems of getting lumber of all piles uni-formly dry for shipment or kiln drying in theshortest possible time, v a r i a b l e o rnonuniform (using movable pile foundations)pile spacing in each row should be tried(MCMillen 1964). Such nonuniform spacinghas been widely used for redwood, one of thesoftwoods. In this method, the spacing be-tween piles in the middle of the row is aboutdouble the spacing between the piles near thealleys.

Whatever yard layout style is used, themain and cross alleys and the spaces betweenpiles should be perfectly alined all the wayacross the yard, and free of all obstructions.Surface

The drying efficiency of a yard depends tosome extent on how well the surface isgraded, paved, and drained. If water stands ina yard after a rain, it will decrease the dryingrate. The yard described by Minter (1961) wassurfaced to a 6-inch depth with crushed rock.Blacktop paving also is used extensively. Theyard should also be kept clean. Vegetationand debris, including broken stickers, boards,or pieces of timber from pile foundations,interfere with the movement of air over theground surface.

PilesPiling Methods

The drying rate of lumber is affected by theway the boards are stacked. For instance,lumber dries faster when air spaces are leftbetween the boards of a unit package thanwhen the boards are placed edge to edge. Theair spaces permit greater downward flow of

air within the piles. This is important whenthe outside air is calm or nearly so. Boards inunit packages 4 feet or less in width are usu-ally stacked edge to edge, and the downwardflow occurs in the spaces between the piles.Minimum pile spacing is 2 feet. If the unitpackages are wider than 4 feet, the boards ineach course should be spaced. When random-length, random-width lumber is box-piled,enough space develops within the pile tomake additional board spacing unnecessary.Wide unit packages of even-length lumberand wide hand-stacked piles are often builtwith flues or central chimneys.Pile Foundations

High openly designed pile foundations arenecessary to allow the moist air to pass outreadily from beneath the pile. They also are asignificant factor in obtaining uniform dryingfrom the top to bottom of each pile. Finighanand Liversidge (1972) found that the evapora-tive loss from top packages from wind effectsalways was greater than that from bottompackages. Foundation height for hand-builtpiles should be at least 18 inches above theyard surface. In well-laid-out and well-drained forklift yards, a 12-inch minimum issatisfactory in many regions, but in areas ofhigh rainfall, the 18-inch minimum should beused with this type of stacking. In unpavedyards, weeds and debris should not be allowedto block air passage.

Roofs and ShedsClark and Headlee (1958) showed that pile

roofs saved enough in degrade and dryingtime for 4/4 No. 1 Common and Better red oakto pay for the roofs in five uses. Also, fasterdrying rates and lower final moisture con-tents were achieved by the use of roofs inrainy spring weather. A report on similarAustralian air drying research, reviewed byMCMillen (1964), showed outstanding differ-ences between roofed and nonroofed pilesduring rainy autumn and winter seasons.Hardwoods in nonroofed piles dried to 26 per-cent moisture content in 70 days, then did notdry any further for the next 150 days.

In regions of high rainfall, shed roofs givebetter protection of the lumber from rewet-ting. They can cut customary air drying timesin half or greatly reduce the amount of waterto be evaporated in a kiln after a “standard”length of air drying. An example of the halv-

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ing of drying time is the seasoning of somerailroad cross-ties in the southeastern coastalareas.

Climatological ConditionsThe climate of the area or region in which

the hardwood air-drying yard is locatedgreatly influences the air-drying rate or yardoutput of air-dried lumber. Perhaps the mostinfluential factor is temperature, but relativehumidity and rainfall also are in the picture.In northern lumber-producing regions, thedrying rate is retarded during the wintermonths by the low temperature. In the south-ern part of the country, where the winterdry-bulb temperatures are higher, better dry-ing conditions are expected. But, these highertemperatures may be offset in some localitiesby rains that wet the lumber and extend thedrying time unless the lumber is wellprotected.

A map of different air drying weather re-gions in the Eastern United States is shownin figure 5. There are about the same numberof “good air drying months” throughout eachregion. There is no exact definition of a “goodair drying month.” Such a month, however, isprobably roughly equivalent to a month with22 or more days equivalent to the “effectiveair drying days” in the Upper Midwest (Rietz1972). Research on beech, sugar maple, andred oak by Peck (1954, 1957, 1959) indicatedhardwoods air dry at a moderate rate whenthe daily mean temperatures are over 45° Fand at a considerably better rate when theyare over 60° F. The boundaries of the zones inthe map are largely based on the averagecumulative “growing degree days” fromMarch 1 to mid-October for the years 1971-5(U.S. Department of Agriculture 1975).(“Growing degree days” are computed to a 50° Fbase with daily maximums listed to 86° Fand daily minimums to 50° F.) The northern

M 144115

Figure 5.—Air drying map for the Eastern United States.

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extensions of the Central and Mid-South zonesalong the Atlantic Seaboard are located tocoincide with the periods when mean tempera-tures are over 45° F (U.S. Department of Com-merce 1973).

General confirmation of the map was ob-tained from a large number of personsthroughout the Eastern United States who docommercial air drying. Within the Central,Mid-North, and North zones, a drying yardlocated on an exceptionally good site, withlocal temperature and wind velocity abovethe zone averages, could be expected to havean extra month per year of “good” weather.

Obviously, a “good” air drying day is not“the best.” To use the map in planning themost economical mix of air drying and kilndrying or of accelerated air drying and kilndrying, one should consider that “good” airdrying weather is capable of bringing 4/4 oakand other slow-drying hardwoods to 25 per-cent moisture content in 90 days. The “best”weather will do it in 60 days. During unusu-ally cold winters in the South, 90 days will notbe enough. This specific drying job shouldtake about 120 days during such a winter.

Another important factor is the woodequilibrium moisture content (EMC). 6 A tmean temperatures from 45° to 85° F, EMC isclosely related to relative humidity:

Relative humidity EMC(Pct) (Pct)

88 2080 1673 1465 1254 1042 830 617 4

Wood in contact with 42 percent relativehumidity obviously will dry faster than woodin contact with 88 percent. But even air at 88percent has a good potential for drying woodto 25 percent moisture content, The objectiveof good site selection and preparation, goodyard layout, and good pile design is to get theoutside air to move into the pile to pick upmoisture and then move out again. If the airdoes not move out, saturation occurs and dry-

6 The EMC for a given atmospheric temperature andrelative humidity is the moisture content at which woodin contact with that atmosphere neither gains nor losesmoisture.

ing stops. Where average relative humiditiesare high for long periods, the use of spacesbetween boards, or of flues and chimneys, isnecessary along with high, open pile founda-tions. These will promote downward flow andremove moist air whenever any wind, how-ever slight, occurs.

Conversely, too low a relative humidity,especially in company with high winds, cancause surface checking and end checking.This can happen at any time of year, not onlyduring the “good” months. Danger periodsoften occur in late winter or early springwhen rapid solar heating of air with a lowabsolute humidity causes the relative humid-ity to go to very low values. A good yard man-ager will learn from monthly weather recordswhen such periods of low relative humidityare likely to occur and will pile check-susceptible species in a manner that restrictsair circulation so as to avoid checking.

Rain is not a direct factor in governing dry-ing rates. In low sites, with poor yard surfacesand drainage, rain tends to raise the relativehumidity within the bottom of the piles and tointerfere with downward air flow and moistair removal. Any lumber that is rewet, ofcourse, must be redried. Pile roofs keep mostof the rain out, with the water along the sidesof the pile being only superficial, except forwind-driven rain. Where heavy rains arecommon, sheds with wide overhang should beconsidered.

Sunshine is another indirect factor. Solarradiation heats nearby land areas, exposedareas between the lumber piles, and sur-rounding buildings. Air moving over thesewarmed areas and structures is heated byconvection, and its drying potential increasesboth by raising temperature and lowering re-lative humidity. Black bodies absorb moresolar energy and become hotter than light-colored materials. This characteristic is usedto advantage in air-drying yards by blacktop-ping the roadways and sometimes the wholearea.

In some instances it may be desirable toarrange the main alleys of the yard north andsouth to take greatest advantage of the solarheat. This helps to quickly melt snow in theNorth and to dry out the yard faster in allregions after heavy rains. The sun also cancause too high a temperature and too fastdrying in the tops of piles in the South and

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Table 2. —Estimated time to air dry green 1- and 2-inch eastern hardwood lumber toapproximately 20 percent average moisture content

Estimated time by region 2

Species 1 SizeSouth Mid-South Central Mid-North

Ash

Aspen

Basswood, American

Beech, American

Birch, paper

Birch, sweet; yellow

Butternut

Cherry

Cottonwood, eastern

Elm, American; slippery

Elm, rock cedar; winged

Hackberry, sugarberry

Hickory

Magnolia

Maple, red; silver

Maple, sugar; black

Oak, lowland

Oak, red (upland)

Oak, white (upland)

Pecan

Sweetgum, heartwood (red gum)

Sweetgum, sapwood (sap gum)

Sycamore

Tupelo (and blackgum)

Walnut, black

Willow, black

Yellow-poplar

Inch121212121212121212121212121212121212121212121212121212

Days45 - 70

180 - 210——

40 - 65170 - 20045 - 70

180 - 210——————

45 - 70180 - 21040 - 65

170 - 20040 - 65

170 - 20050 - 80

190 - 23040 - 65

170 - 20050 - 80

190 - 23040 - 75

170 - 22040 - 65

170 - 20045 - 70

180 - 210100 -3280No data60 - 120

240 - 36060 - 120

240 - 36060 - 120

240 - 36050 - 80

190 - 23040 - 65

170 - 20040 - 65

170 - 20060 - 110

210 - 30045 - 70

180 - 21030 - 65

150 - 20040 - 65

170 - 200

Days45 - 75

180 - 220——

40 - 70170 - 21045 - 75

180 - 220——

50 - 85190 - 24040 - 70

170 - 21045 - 75

180 - 22040 - 70

170 - 21040 - 70

170 - 21050 - 85

190 - 24040 - 70

170 - 21050 - 95

190 - 240——

40 - 70170 - 21045 - 75

180 - 220——

55 - 100215 - 300

55 - 100215 - 30065 - 100

215 - 30050 - 95

180 - 24040 - 70

170 - 21040 - 70

170 - 21045 - 90

180 - 22045 - 75

180 - 22035 - 70

160 - 21040 - 70

170 - 210

Days45 - 80

180 - 230—

40 - 75170 - 22045 - 80

180 - 230——

50 - 90190 - 25040 - 75

170 - 22045 - 80

180 - 23040 - 75

170 - 22040 - 75

170 - 22050 - 90

190 - 25040 - 75

170 - 22050 - 90

190 - 250——

40 - 75170 - 22045 - 80

180 - 230——

50 - 90190 - 25050- 90

190 - 25050 - 90

190 - 25050 - 90

190 - 25040 - 75

170 - 22040 - 75

170 - 22045 - 80

180 - 23045 - 80

180 - 23040 - 75

170 - 22040 - 75

170 - 220

Days60 –3 165No data50 -3 120No data40 -3 120No data60 -3 165No data40 -3 120

170 - 22070 -3 165No data60 -3 165No data60 -3 165No data50 -3 120No data50 -3 120No data80 -3 150No data30 -3 120No data60 -3 165No data

——

30 -3 120No data50 -3 165No data

——

60 -3 165No data70 -3 200No data60 -3 165No Data70 -3 200No data60 -3 165No data30 -3 120No data70 -3 165No data70 -3 165No data30 -3 120No data40 -3 120No data

1 Forest Service official tree names; corresponding botanical names are included in Appendix D.2 Regions of approximately equal number of months of “good” air-drying weather in accordance with figure 5.3 To an average moisture content of 25 percent.

-18-

Mid-South during the summer. This can like oak and willow. Pile roofs and closer pile

cause honeycombing and collapse in woods spacing will reduce this form of degrade.

Drying Time and Final Moisture ContentAir-drying times published in Air Drying of

Lumber have been limited to the 1-inchthickness and generalized for the entirerange over which each wood grows. Energyconservation and interest rate fluctuationsmakes estimates for both 1- and 2-inch mate-rial and for specific regions desirable. Suchestimates are given in table 2. They are forflat-sawed lumber in unit package piles 4 feetor narrower in width, or having central flues.

The minimum periods given apply tolumber piled during the best drying weather,generally spring and early summer. One-inchlumber piled too late in the period of bestweather to reach 20 percent that fall, or thatis piled during the fall or early winter, will notreach 20 percent for a very long time. Thus,maximum drying times for 1-inch lumber inthe Mid-North region are indicated to the 25percent moisture level. In fact, none of thetimes to 20 percent are to an absolute 20 per-cent but may be as much as 23 percent in poordrying weather. All of the times for 2-inchlumber may be considered as times necessaryto bring the wood to somewhere between 23and 27 percent moisture content.

These time estimates are somewhatspeculative because they are based onsketchy information. It is believed, however,that they are the best base for calculating thefeasibility of revising an old yard or building anew one with a good site, a good yard layout,the best piling practice with high, open pilefoundations, and a roof on every pile.

An aid that should be very useful in es-timating drying time so as to plan actual dry-ing operations is the “effective air drying daycalendar” developed by Rietz (1972) for theUpper Midwest. Each of the best months isconsidered to have 30 “effective air dryingdays,” and other months lesser amounts asfollows:

Month Effective air-drying days

January 5February 5March 10April 20May 25June 30July 30

August 30September 25October 20November 10December 5

As an example, green 1-inch northern redoak was dried to 20 percent moisture contentin 60 days in a good forklift yard with good aircirculation when stacked in June or earlyJuly. This of course is the best drying weatherin the location considered. If piled in earlyNovember the effective air drying days wouldbe: November 10, December 5, January 5,February 5, March 10, April 20—total 55. Dry-ing to 20 percent probably would be completedby May 5 to 8. This would give a total of 182days. The research by Peck (1959) indicates 20days would be saved by taking down the pilesat 25 percent moisture content. This valuewould be reached in 162 days.

A similar calendar can be devised for anylocation. Rietz did not precisely define an “ef-fective air-drying day,” but it could be as-sumed to be equivalent to the average of allthe days in full months of the late spring andsummer season.

The calendar would be devised as follows:Average the mean monthly temperature

for the late spring and summer months; alsofind out the mean annual relative humidityand the mean annual wind velocity.

Assume each summer month had 30 effec-tive air drying days (EADD). For any one ofthese months that has both mean relativehumidity more than 5 percent above the an-nual mean and a mean wind velocity 4 ormore miles per hour less than the annualmean, deduct 2 days (i.e., count such a monthas having 28 EADD).

The major determinant of the EADD forthe other months is temperature. For each 10° Fthat the mean temperature of any month isbelow the average of the mean temperaturesof the summer months, deduct 5 days. Afterthis deduction is made, deduct 2 days formean relative humidity more than 5 percentabove the annual mean or mean wind velocityof 4 or more miles per hour below the annualmean. If both these latter events occur, de-duct 4 days.

-19-

Mean annual relative humidities are gen-erally between 70 and 75 percent in the east-ern hardwood region and mean air velocities 6to 7 miles per hour.

The above values are suggestions based ongeneral considerations and may be changedas experience is gained in applying this ap-proach.

Once such a calendar is set up, one coulduse the minimum number of days for a cer-tain item in the best months (from dryingtime tables) in conjunction with the “effec-tive” days of the various months to predictwhen that item piled on any specific day of theyear is likely to be dry. For yards located inthe Tennessee Valley, and perhaps otherparts of the Central zone, the TVA air dryingguide should be helpful (Tennessee ValleyAuthority 1974). The charts in this guideshow expected finishing dates for 1-inch and

2-inch hardwood lumber stacked on the 5th,15th, or 25th of each month. No distinction ismade, however, as to species of wood.

The practice of holding lumber 90 days onthe air-drying yard and then shipping it withthe implication that it will safely withstanddrastic kiln drying is faulty on two counts.Ninety days is an unnecessarily long and ex-pensive period for some areas of the countryduring much of the year. On the other hand,when the winter is abnormally cold in thecentral and southern regions, 90 days is notlong enough to bring the average moisturecontent down to 25 percent.

There are advantages to keeping track ofthe moisture of the lumber as it air dries. Agraph of past runs (fig. 6) can provide a goodestimate of how long air drying will take for agiven species, thickness, and time of year. Themethod of using samples described in the Dry

Figure 6.—Air drying curves for 4/4 northern red oak piled at different times of year in southern Wisconsin.

-20-

Kiln Operator’s Manual, with certain modifi- The curves (fig. 6) show differences that cancations, is suitable for use in air drying. The occur in drying time depending upon time ofsample pocket can be made two boards wide, piling in northern locations. They further in-with the sample placed in the inner space and dicate the impracticality of always waitinga dummy board on the outer edge of the until the stock comes all the way to 20 percentpackage. An alternative method places the moisture content. Safe methods for startingsample in the bolster space between the two kiln drying of stock at 25 percent moisturelowest packages and uses some means to pre- content, regardless of kiln type, are describedvent excessive air circulation over the sam- later.pie.

Deterioration of Lumber While Air DryingLosses in lumber value from defects that

develop during air drying are reflected in theoverall drying cost. Air-drying defects may becaused by shrinkage, fungal infection, insectinfestation, or chemical action. Shrinkagecauses surface checking, end checking andsplitting, honeycomb, and warp. Fungus in-fection causes blue or sap stain, mold, anddecay. Insect infestation results in pith flecks,pinholes, and grub holes left in the wood.Chemical reactions cause brown stain andsticker marking. Detailed descriptions are in-cluded in Air Drying of Lumber. These dryingdefects can be controlled by following recom-

mended practices. Particularly helpful are at-tention to proper site, yard layout, drainage,and pile roofing.

If lumber is kept on the yard for an ex-tended period of time, it may appear weath-ered because of an accumulation of sawdust,ashes, and windborne dirt. Minor darkeningof the surface is not detrimental. Excessivedarkening may be a sign that the lumber hasbeen held far beyond the time necessary toreach 20 percent moisture content. After thatperiod, alternate wetting and exposure to sunor hot winds can cause serious weather dam-age.

Air Drying Small Quantities of LumberSmall quantities can be dried enough for

general construction by air drying or for inte-rior use by air drying followed by a shortperiod of drying by another method. Whilethis is relatively simple, the general proce-dures of commercial drying should be fol-lowed, with certain adaptations.

Good lumber cannot be produced from de-teriorated material so the “first steps” de-scribed in chapter 2 are necessary. When thelogs are sawed, insure that enough of eachthickness is produced. See that each thick-ness is sawed to 3/8 inch over final size, plus orminus 1/8 inch. Finished items over 2 inchesthick should be made from 1-inch lumberglued together after drying and planing. Ifpolyethylene glycol treatment is to be used,apply that right away (see chapter 7).

Select a good place for the air drying pile.Species that dry rapidly (table 2) can be piledin the open. Species that take the longesttimes usually need protection from strongwinds to avoid checking. Small piles do notneed to be high above the ground, but the pile

foundations should be open and strong, andshould present a perfect plane for the firstcourse of lumber. A slope of 1 inch per foot ofpile length may be provided to aid drainage ifthe pile is to be wide, but two narrow, flatpiles would be preferable. Sloped piles need tohave their fronts pitched forward 1 inch perfoot of height.

Every board in a course of lumber must beof the same thickness to hold warping to aminimum. Stacking should be done as soon aspossible after sawing, using the generalmethods described in chapter 2. Renew theend coatings on thick lumber that has had itsend coating damaged or trimmed off. Protectthe top of the pile as it is being built and whenit is finished by a roof that will shade the pileand shed rain. Weighting down with concreteblocks or other materials at the rate of 60pounds or more per square foot will be helpfulfor warp-prone species.

Material that has been thoroughly air dried(to less than 20 percent moisture content) willnot be dry enough for interior use, but it can

-21-

be made so by using heated room, de- stock in a commercial kiln load of air-driedhumidifier, or solar drying or by inserting the hardwoods. See chapters 5 and 8.

A Quick Guide for Improving Air DryingEfficiency of Hardwoods

The following summarizes several proce-dures to reduce air-drying time and increaseefficiency for greatest energy savings.

1. When green lumber arrives or is first cut,immediately get it on stickers. Even if it isgoing to the kiln soon, store it where it can airdry (where the wind can blow through thepile). Drying is most rapid the first 3 to 4 days:Don’t miss this opportunity for effective earlydrying.

2. Spread lumber piles out to increase theirexposure to drying winds. Spacings in AirDrying of Lumber are minimum. Lumberpiled too closely increases relative humidityin the surrounding area and slows drying. Putthe driest lumber on the outside edges of theyard (especially on the edges facing into thewind) so that it will dry even faster. However,for species such as oak, beech, and hickory,accelerated air drying (from too wide spacing)

can also increase the risk of degrade fromchecking and honey comb.7

3. Consider using a sticker that is 25/32inch thick or more. In contrast to thin stick-ers, this will permit both increased air flowand drying. Stickers should be of uniformthickness. As an alternative, consider nar-rower piles.

4. Cover the tops of the piles. Why exposelumber that is air drying to rainfall or snow?

5. Keep the yard clear of weeds and debrisso that the bottom layers of the pile will dryas fast as the top layers. The pile foundationsshould be of sturdy, open construction andshould support the lumber at least 12 inchesabove the ground.

7 Species like oak and beech (refractory hardwoods)present special problems. Daily inspection is recom-mended with refractory hardwoods to discover any prob-lems in either air or kiln drying before they can becomeserious.

Literature Cited

-22-

CHAPTER 4ACCELERATED AIR DRYING

Accelerated air drying is the use of low-cost third of the bound water from green wood totechniques involving specially designed reduce the moisture content to the air-driedsheds, fans, and (sometimes) heating equip- range. When heating is used, temperaturesment to accomplish air drying in a shorter generally do not exceed 120° F. Solar dryingtime. It includes yard-fan drying, shed-fan that is used to bring lumber down to 20 per-drying, forced-air drying, low-temperature cent moisture content is low-temperaturekiln drying, and controlled-air drying. It thus forced-air drying, but broader scale solar dry-encompasses any process that accelerates the ing is discussed later in this handbook.evaporation of free water and the first one-

Research BasisForced-air drying and other forms of accel-

erated air drying have been studied exten-sively in the United States and Canada. Themethods are technically sound, and therehave been many practical applications. Muchof the research has been of an empirical na-ture. Several published reports are indicated

later in this chapter. The SoutheasternForest Experiment Station of the Forest Ser-vice, U.S. Department of Agriculture, estab-lished technical data to aid in the design andoperation of forced-air dryers. Data from oneexperiment by Vick (1965b) of that Stationwere used to derive figure 7.

M 144694Figure 7.—Effect of temperature on drying of 1-inch yellow-poplar lumber in a low-temperature forced-air dryer

under 10 percent EMC conditions (after Vick 1965b).

-23-

This figure shows the influence of tempera-ture on drying time at a constant equilibriummoisture content (EMC) condition. The airvelocity was 550 feet per minute, and the loadwidth 8 feet. Yellow-poplar has fast-dryingheartwood but, as can be seen, the sapwooddried somewhat faster. At the 10 percentEMC used, the effect of temperature is sig-nificant. A 100° F temperature dries 4/4 sap-wood to 20 percent moisture content (MC) inthree-fifths of the time required at 80° F. Theoriginal paper also shows the effects of differ-ent EMC’s . Degrade was very l imited(maximum of $1.87 per thousand board feet)except when a 6 percent EMC was used at120° F. The paper also shows the mathemati-cal basis by which the actual results werefound to agree with theoretical expectations.Similar data on 1-inch sweetgum lumber,

tupelo rounds, and mixed hardwood roundsare available (Vick 1965a, 1968a, 1968b).

Information on the economic prospects ofaccelerated air drying has been published bya number of authors (Catterick 1970, Cuppettand Craft 1971, Davenport and Wilson 1969,Gatslick 1962, and Norton and Gatslick 1969).In general, the data indicate that acceleratedair drying has little or no advantage overgood summer air drying. An advantage couldbe anticipated during the 6 to 8 months ofpoor air drying weather in the North andMid-North. The advantage may also be pres-ent in other areas where space for a well-laid-out air-drying yard is limited or whererainfall is heavy for long periods. Means ofcomparing economic advantage or disadvan-tage are described in the last chapter of thishandbook.

Types of Dryers and ProceduresThe shed-fan dryer has a permanent roof

and canvas baffles that can be let down at theends of the dryer (fig. 8). One side wall is per-manent and furnished with a large number offans. The fans always turn in one direction, topul l the a ir through the lumber . Theentering-air side of the dryer is open to theyard. The heat in the ambient air is an im-portant factor in drying the lumber. Theshed-fan dryer works very well in the South.One-inch elm, sap gum, hackberry, andyellow-poplar have been dried from green to29 percent MC and lower in 6 to 7 days (Hel-mers 1959). It also is very effective for softmaple. While this type of dryer has been in-stalled in a few places in the North, it is reallyonly effective there from mid-April untilmid-October.

The yard-fan dryer is similar in construc-tion, operation, and applicability to the shed-fan dryer except that different fan wallconfigurations can be used and the fans maybe portable. The roof also is temporary andportable and may be made of canvas, plywood,or sheet metal panels. One mistake in use ofsome yard-fan dryers is to pile additionalstacks of lumber in front of the fans. Thisgreatly reduces their effectiveness.

The next improvement that can be made indrying lumber with forced air is to add heat. Asolar-heated lumber dryer developed by theForest Service in Puerto Rico with the assist-ance of the U.S. Forest Products Laboratory

takes advantage of heat energy coming fromthe sun every day. This dryer dried 5/4mahogany lumber to 20 percent MC in 12days.

At Jacksonville, Fla., a semi-solar dryermade use of both steam heat from a wood-fired boiler and solar energy to predry mag-nolia lumber (Rucker and Smith 1961). Therapid predrying avoided the extensive oxida-tive graying that had been a costly cause ofdegrade of magnolia. Such a predryer couldbe expected to prevent oxidative discolorationin tupelo sapwood and hackberry.

The steam coils provided heat from 5 until10 a.m. to bring the temperature up. From 10a.m. on, the large, almost flat, black-paintedroof absorbed enough solar energy to keep thetemperature at about 110° F until about 8p.m. The fans were left running overnight.The next morning the steam heat would beused again to bring the temperature up. Thisdryer brought 4/4 magnolia down to 20 per-cent MC or less in 6 days and 8/4 in 14 days.

A combination shed-fan and recirculatingforced-air dryer has been installed in alimited number of instances. Heating the airto lower the EMC and increase the dryingrate is not economical unless the air is recir-culated. During very good outdoor dryingweather, the louvers of this dryer are openedby hydraulic cylinders actuated by humiditycontrols. The overhead fans force the outside

-24-

M 143 544

Figure 8.—Shed-fan track-type accelerated-air dryer.

air through the lumber and out through theother louvers. When the weather is damp orcold, the louvers are closed, a small amount ofheat is used, and the air within the building isrecirculated.

Recirculating dryers that use some heatare of two types: The forced-air dryer thatonly partially controls relative humidity bycontrol of venting, and the low-temperaturekiln that fully controls relative humidity. Aschematic diagram of a typical Appalachianforced-air dryer with some provision for sup-plemental solar heating is shown in figure 9.Both the forced-air dryer and the low tem-perature kiln usually consist of an inexpen-sive structure (fig. 10) plus the necessaryheating and controlling equipment. An ear-lier design (Gatslick 1962) had longer airtravel, an undesirable feature unless verypowerful air-circulating equipment is used.

Temperatures in both types of dryers rangefrom 70° to 120° F. Both may have vents toexhaust excessive moisture, but the ventsgenerally are operated in the closed position.They are opened when the drying scheduleprescribes lowered relative humidity. In the

forced-air dryer, temperature is controlled bya thermostat. Both the dry-bulb temperatureand the wet-bulb temperature are observedwith a hygrometer. In the low-temperaturekiln, a steam spray line supplies humiditywhen needed, and a kiln-type recorder-controller both records the dry- and wet-bulbtemperatures and regulates the heating coilsand the steam spray.

The controlled-air dryer (fig. 11) is a largewarehouse-type building of tight design inwhich large quantities of green lumber can bedried to 20 percent moisture content. A com-mon size would hold 200,000 board feet. Aconvenient constant temperature, such as 80°or 85° F, is maintained throughout the build-ing. A single large door at one end allows thegreen lumber to be brought in, and a similardoor at the other end permits taking out loadedkiln trucks. These doors can be left open dur-ing operations without upsetting interiorconditions. The relative humidity in eachquarter, or 50,000-board foot bay of the build-ing, can be controlled at the desired level (forexample, 50 percent) by power intake andexhaust vents. These vents open when the

-25-

humidity gets too high. This procedure is, es-sentially, a one-step drying schedule.

Unit heaters or heating coils located at ornear each intake vent are turned on when thevents open so that incoming air is heated tothe dryer temperature. Air circulation de-signs evolved over a 5-year period currentlyprovide about 100 feet per minute air velocitythrough the lumber. User-furnished data in-dicate that drying times for common itemsare about 40 percent longer than those offorced-air dryers. The controlled-air dryershave been able to satisfactorily predrymineral-streaked sugar maple. They havefound application in the United States innorthernmost States as well as in Canada.

Care must be used in piling lumber in apackage-loaded dryer so that packages arenot tight against each other or air will notflow freely through the sticker spaces.

Two major types o f s chedules foraccelerated-air drying have been tested ex-tensively. For forced-air dryers, with only

partial control of relative humidity, two-steptemperature schedules are used. For low-temperature kilns with steam spray and au-tomatic recorder-controllers, multiple-stepschedules are more appropriate.

Dry-bulb and wet-bulb temperature controlof drying conditions is discussed in the DryKiln Operator’s Manual, and the reasons forcareful relative humidity control in dryinggreen lumber are given in Air Drying ofLumber.

Forced-air drying schedules adapted fromresearch at the Forest Products MarketingLaboratory, Princeton, W. Va. (Cuppett andCraft 1975) are listed in table 3. In this type ofprocedure, the desired dry-bulb temperaturefor the first step of the schedule is somewherewithin a specific temperature range, depend-ing on the species of wood (table 4). If thedryer has vents, they are kept closed as thedryer is heated up. Both dry-bulb and wet-bulb temperature are observed on theentering-air side of the load. When the differ-

M 144692

Figure 9.—Cross section of a typical forced-air dryer with fans located overhead. (After Cuppett and Craft 1975.)

–26-

M 140 966-7Figure 10.—Low-temperature kiln located at Allegany, N.Y.

ence between the two (the wet-bulb de- the moisture content of the wettest half of thepression) becomes great enough to give the dryingdesired EMC condition, the thermostat is set

samples (chapter 6, Dry KilnOperator’s Manual) falls to the moisture con-

at the dry-bulb temperature prevailing atthat time. This temperature is maintained

tent for starting the second step of theschedule, the thermostat is raised to the tem-

throughout the first schedule step. The wet-bulb depression tends to remain about the

perature prescribed for that step. Generally

same for the first day or two because of mois-the vents could be opened for a few days atthis time to lower the EMC and accelerate

ture coming from the wood. Thereafter, the drying. When the wet-bulb temperature set-wet-bulb temperature falls gradually. When tles to a nearly constant level, the vents

Table 3. —Forced-air dryer schedules 1 for selected eastern hardwoods

Schedule designation

Moisturecontent at

start of step

Percent1 Above MC of

step 22a 45

b 40

FA-1 FA-2 FA-3

Dry-bulb Wet-bulb Dry-bulb Wet-bulb Dry-bulb Wet-bulbtemperature depression temperature depression temperature depression

° F ° F ° F ° F ° F ° F

70-80 3 75-85 4 80-90 7100 (2)

90 (2) 95 (2)

1 Developed by Forest Products Marketing Laboratory, Princeton, W. Va. (Cuppett and Craft 1975). All operate at anair velocity of 450-600 feet per minute except for 6/4 and 8/4 oak and 8/4 beech and hickory, which use 250-350 feet perminute.

2 No control of wet-bulb depression.

-27-

Table 4. —Index of forced-air drying schedules

SizeSpecies

4/4 5/4 3/4 3/4- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Ash FA-3 FA-3 FA-2 FA-1Basswood FA-3 FA-3 FA-3 FA-2Beech FA-2 FA-2 FA-1 FA-1 1

Birch FA-3 FA-3 FA-2 FA-1Buckeye FA-3 FA-3 FA-3 FA-2Butternut FA-3 FA-3 FA-3 FA-2Cherry F A 3 FA-3 FA-2 FA-1Hickory FA-2 FA-2 FA-1 FA-1 1

Maple; red, silver FA-3 FA-3 FA-2 FA-1Maple, sugar FA-3 FA-3 FA-2 FA-1Oak, red FA-2 FA-1 F A - 1 1 FA-11

Oak, white FA-2 FA-1 FA-11 FA-1 1

Walnut, black FA-3 FA-2 FA-1 FA-1Yellow-poplar FA-3 F A 3 FA-2 FA-2

1 Air velocity through load, 250-350 feet per minute.

would be closed again to conserve energy.Drying is continued until the moisture con-tent of the wettest sample is down to the de-sired average moisture content for the wholedryer charge. Vents might be used earlier inthe schedule when drying basswood, buckeye,butternut, or yellow-poplar. Cuppett andCraft (1975) suggest hygrostat-controlledpower venting for these species.

A final schedule consideration for this typeof dryer is air velocity. Two-inch oak, beech,and hickory requires a relatively low airvelocity—250 to 350 feet per minute. Thesame low air velocity probably should be usedfor 2-inch stock of other species for which noschedule has been established, until gradualincreases in velocity show the higher rate canbe used without causing drying defects. All4/4 and 5/4 stock, plus the 6/4 of all speciesexcept the oaks, can use a higher airvelocity-450 to 600 feet per minute.

M 140 966-23

Figure 11.-Inside of a controlled-air dryer.

-28-

I tems and spec ies having the sameschedule designation (table 4) can be driedtogether in the same dryer load, but samplesshould be selected for the thickest, slowestdrying item to determine when to remove thestock from the dryer.

During the first 6 months of development ofa schedule for forced-air drying of a particu-lar item of a particular species, a moisturemeter test (James 1975) should be made of 5percent of the material in packages of lumberfrom various parts of the dryer. This shouldshow that all the boards have core moisturecontent values at or below 30 percent. If not, alonger drying time should be used the nexttime this item is dried.

The schedules are not ironclad rules. A usershould gradually approach a more severeschedule for items that develop no defectswhen drying by the above. For warmer cli-mates, slightly higher dry-bulb temperaturescould be used if the desired wet-bulb depres-sions can be maintained during the first step.Solar radiation may cause the dry-bulb tem-perature to rise above the set point for 6 to 8hours. This should be satisfactory unless thedry-bulb temperature exceeds the thermostatsetting by more than 5° F during step 1. Ifthat should occur, lower the thermostat set-ting 5° F throughout step 1.

Schedules developed industrially for low-temperature kilns are listed in table 5. In thistype of procedure, the desired wet-bulb tem-perature is controlled for all steps. Partialcontrol is maintained of the dry-bulb temper-

ature. As with the forced-air dryer, the ventsare kept closed as the kiln is heated up. Whenboth the dry-bulb and the wet-bulb tempera-tures reach the scheduled set points, the re-corder controller maintains the wet-bulbtemperature at its prescribed level and seesto it that the dry-bulb temperature does notgo below the set point. If solar radiationcauses the dry-bulb temperature to riseslightly above the set point during the day, nochange in settings is made.

These schedules were developed with airvelocity through the load of 350 feet per min-ute and presumably would work equally wellwith air velocities up to 450 feet per minute.The vents would be controlled automaticallyby the recorder-controller and presumablywould stay closed until 10° F or larger wet-bulb depressions are called for. Then thevents might be opened a short time at thestart of each drying step but would auto-matically close most of the time and conserveenergy. Drying condition changes would bemade on the basis of the wettest half of thedrying samples ( chapter 6 , Dry Ki lnOperator’s Manual). Determination of whento terminate a run would be on the same basisas for the forced air dryer.

Table 6 is an index of schedules for the low-temperature kiln (table 5). Items and specieshaving the same schedule designation can bedried together, but samples should beselected for the thickest, slowest drying itemto determine when to remove the stock fromthe dryer. Skillful dry kiln operators can

Table 5. —Low-temperature kiln schedules 1 for selected eastern hardwoods

1 Developed by Robert G. Potter, Potter Lumber Co., Allegany, N. Y. (personal correspondence).

-29-

select the schedule most appropriate forspecies not listed in table 6 and insert thedesignations in the blank places left in thetable for this purpose.

Table 6. —Index of low-temperature kilnschedules

SizeSpecies

4/4 5/4 6/4 8/4---------------------------------------------------------Beech LT-2 LT-2Cherry LT-3 1

Maple; red, silver LT-3 1

Maple; sugar L T - 31

Oak, red LT-2 LT-2 LT-12 LT-13

1 Minimum time on each step 24 hours.2 Minimum time on first step 36 to 48 hours.3 Part-time drying (fans on and kiln under control only

50 percent of time) during step 1.

The Eastern Forest Products Laboratory,Ottawa, Canada, has experimented exten-sively with predrying of sugar maple and yel-low birch prior to final drying at 212° F. Theprocedure was essentially low-temperaturekiln drying. With 4/4 sugar maple, Huffmanand Cech (1969) found that least degradecoincided with the procedure that gave theshortest drying time. This involved a 600-foot-per-minute air velocity through the load.Although no steam spray was used, the kilnwas tight enough so the schedule shown intable 7 prevailed.

In similar research done with yellow birch(Cech and Huffman 1968) no EMC data werepresented. The dryer was run with no sup-plemental humidification. Presumably wet-bulb depressions were similar to thoseindicated in table 7 for maple. Some degradewas observed with the table 7 conditions as

Table 7. —Approximate Canadian low-temperature kiln schedule 1 for 4/4 maple and

birch

Stap Moisture content Dry-bulb Wet-bulbNo. at start of step temperature depression_________________________________________________________

Percent °F °F1 Above 60 100 132 60 100 173 50 100 204 40 100 26

1 Developed by Eastern Forest Products Laboratory,Ottawa, Canada (Huffman and Cech 1969).

well as when using no humidification at 80° Fand with 200 feet per minute air velocities.Similar results were obtained when humidifi-cation was used to provide a constant 14.6percent EMC. Although the least degradewas obtained by the 14.6 percent EMC incombination with 100° F and 600 feet per min-ute, Cech (1973) later concluded that theschedule using 100° F, 600 feet per minute,and no humidification was satisfactory forboth maple and birch.

In studying the benefits of presurfacing oakbefore drying, McMillen (1969, 1972) usedlow-temperature kiln drying schedules thatincreased temperature stepwise from 85° F to105° F. Satisfactory results were obtainedwith white oak and red oak using an air veloc-ity of 575 to 600 feet per minute, Cherrybarkoak, however, surface checked excessivelyeven though the air velocity was only 375 to400 feet per minute. For drying rough-sawedoak, smaller wet-bulb depressions would beneeded. The schedule in table 8 is suggestedfor cautious trial on central and northern re-gion rough-sawed 4/4 red and white oak.

Table 8. —Suggested low-temperature kiln schedule 1 for rough-sawed 4/4 red and white oak

Moisture content atstart of step Dry-bulb Wet-bulb

Step No. temperature depressionRed oak White oak

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Percent °F °F

1234

PercentAbove 50 Above 40 85 5

5 03 5

40 90 833 95 13

2 8 28 105 25

1 Postulated from results with presurfaced oak (McMillen 1969, 1972) and rough-sawed northern red oak (Gatslick1962).

-30-

Advantages and Disadvantages of Accelerated-Air DryingThe main advantage of accelerated-air

dryers over yard drying is in getting lumberout of the weather and providing closer toideal air-drying conditions for longer periods.Such dryers have special advantages in thenorthern part of the country where the air-drying weather is not good throughout theentire year. Drying rate in northern areas islikely to be two to four times as fast as out-door drying. The air velocity through the loadis 50 to 300 feet per minute if the lumber ispiled relatively open. If the lumber is tightlyplaced so most of the air has to go through thelumber pile, velocities can be over 300 feet perminute.

Accelerated-air drying has these advan-tages over ordinary air drying:

1. Drying is faster because of protectionfrom rain and faster air circulation. Thisspeeds up drying, making it possible to reducethe lumber inventory, with very importantsavings in rent, interest, risk, and insurance.

2. The lumber supplier is able to respondquicker to a change in the market because hedoes not have a long period of air drying to getthe lumber dry enough so it can go throughhis dry kilns quickly.

3. Accelerated-air drying gives the oper-ator a chance to reduce shipping weight andthus the shipping charges.

4. It increases the productivity of the pre-sent kilns used only for drying green lumber,and the capital investment is less than build-ing new kilns to do the same job.

5. Accelerated-air drying prevents stickerstain and oxidative or chemical stain in thelumber and protects it from the weather, pre-venting surface discoloration and producing abrighter colored stock with greater market-ability. It also reduces surface checking andcan reduce warping.

Disadvantages may be:1. Moisture content may not be uniform if

the dryer is not properly designed andproperly operated.

2. Sticker staining or other discolorationmay occur in light-colored woods if the rela-tive humidity is allowed to remain high whilethe temperature is in a range where discol-oration goes on at a rapid rate.

3. Costs may be higher than those for airdrying when suitable land is readily availablewith low rent or taxes and when the produceris willing to assume a large share of the risk.

Drying TimesDrying times in forced-air dryers and low-

temperature kilns still depend somewhat onthe weather outside and the temperatureselected by the operator. Equally importantare the dryer design and the quality of stack-ing and loading. The drying times (table 9) arebased on actual forced-air dryers and low-temperature kilns, but should be consideredonly as rough estimates. No two dryers willgive the same performance. Gatslick (1962)

and Vick (1965b, 1968a) give extensive dryingdata for low-temperature kilns that com-pletely control EMC during the early part ofthe drying period. Cuppett and Craft (1971,1975) give results in drying Appalachian oakand several other eastern hardwood speciesin a dryer where the only EMC control is bythe dry-bulb temperature. Drying times incontrolled air dryers are about 40 percentlonger than those shown in table 9.

-31-

Table 9. —Accelerated air drying times for common hardwood items in slightly heated forced-airdryers (low-temperature kilns)

FinalSpecies Thickness Temperature moisture Drying

AspenBeech

BirchCherry

Elm, American

Hickory

Magnolia

Maple, hard

Maple, soft

Oak, Appalachian red

Oak, northern red

Oak, white

Sweetgum, sapwood

WalnutYellow-poplar, sapwood

Yellow-poplar, heartwood

4/44/45/44/44/44/44/48/44/45/44/48/44/45/44/45/45/44/44/44/45/46/48/48/4—

4/44/46/44/44/44/44/4

° F—————————————————7085——————

80100—80

10080

100

content

Percent20252525251820352020202025252020122020202525252020

20201720202020

time

Days7

8-910-12

147-8

1077

16-1822-24

8-915-188-10

12–146-77-8

113119

18-2314-3515-4040-62

551 or 2 days less

than for red oak7

4½14

649

6½

-32-

Literature Cited

-33-

CHAPTER 5CONVENTIONAL KILN DRYING

Kiln drying is carried out in a closedchamber or building in which air is rapidlycirculated over the surface of the wood beingdried. Conventional dry kilns use initial dry-ing temperatures from 100° to 170° F and finaltemperatures from 150° to 200° F. Thesehigher air temperatures and faster circula-tions are the principal means of acceleratingdrying greatly beyond the rates of air dryingand accelerated-air drying. Control of relativehumidity or EMC is necessary to avoidshrinkage-associated defects and to equalizeand condition the wood to the degree of preci-sion needed. Air velocities through the load indrying hardwoods generally are between 200and 450 feet per minute. Temperature and

relative humidity are controlled by semi-automatic dry- and wet-bulb temperaturerecorder-controllers. A typical recorder-controller chart is shown in figure 12.

Two types of conventional kilns are in gen-eral use for hardwoods—package-loaded com-partment and track-loaded compartmentkilns. There are two basic heating systems,steam and hot air (or direct-fired). Well overthree-fourths of the kilns designed to dryhardwoods are steam-heated, internal fanforced circulation, track- and package-loadedkilns. In recent years, the direct-fired kiln,with supplemental steam or water spray forhumidification, has been used occasionally forhardwoods. Considerable modification of kiln

M 88590F

Figure 12.—Instrument chart of conditions used in kiln drying a small load of air-driedblack cherry. (Assumed to have undergone surface moisture regain.)

-34-

temperature schedules would be needed ifsuch kilns, without humidification, were to be used for green hardwoods intended for furni-ture or other high-class uses. More informa-tion about all types of kilns is given in chapter2 of the Dry Kiln Operator’s Manual (1961).Many technical factors, however, are in-volved in dry kiln design; it is recommendedthat individuals considering installation ofkilns for moderate- to large-scale commercialproduction obtain them from regular dry kilnmanufacturers.

The length of air travel through the load ina package-loaded dry kiln and reheating theair between the tracks in track-type kilns arecritical factors in drying green hardwoods. Anair travel of more than 8 feet without reheat-ing requires undue lengthening of the dryingtime to avoid collapse and honeycombing ofthe wood in the middle of the load. The lengthof drying time is one of the most importantcost factors. “Booster” reheat steam coils be-tween tracks overcome this difficulty, butthey must be correctly designed and operatedto avoid surface checking of refractory woodsearly in drying. This hazard occurs from in-creasing dry-bulb temperature too much bydirect radiation. Drying defects cause de-grade, which is another very significant costfactor. Kiln drying defects and methods ofminimizing them are illustrated and discus-sed extensively in the Dry Kiln Operator’sManual.

T h e b e s t p r a c t i c e s f o r k i l n d r y i n ghardwoods now are essentially the same asthose described in the Manual. They involvethe use of recommended kiln schedules basedon the moisture content of the lumber beingdried. This initially involves use of kiln sam-ples, short end-coated boards by which thechanging moisture content of the kiln load isestimated. The details of selecting, preparing,and using kiln samples are fully covered in

the Manual, so they will not be repeated inthis publication. Basic kiln schedules, whichcombine the recommended kiln scheduleswith other recommendations to save energy,to equalize board moisture content, and torelieve drying stress, are outlined later in thischapter.

Improvements in kiln schedules have al-ways been sought to shorten drying time anddecrease cost without sacrificing quality. Thisquest has been intensified now that energyand other costs have risen sharply. At thetime the recommended schedules were de-vised, there was a great deal of variability inthe performance of dry kilns and in their op-eration and care. The schedules, therefore,were purposely conservative. In many casesthey can be accelerated and considerable sav-ings will result. Examples of how the dryingof oak can be accelerated are discussed follow-ing the section on basic kiln schedules. Simi-lar schedule accelerations can be applied toother woods, Two other g r o u p s o fschedules—simplified schedules (applicable to4/4 thickness of a limited number of woods)and special schedules for special purposes-also are discussed under Drying Procedures.This section includes suggestions for predict-ing drying time.

Equalizing and conditioning (stress relief)are two quality-control measures necessaryto complete the seasoning of fine purposehardwoods. The previously recommendedmethods are excessively high in energy de-mands. This chapter contains suggestionsthat considerably reduce these demands.

A number of suggestions for tightening upkiln structures and for carrying out the pre-scribed procedures are discussed underOperational Considerations. As many or moresavings can be made by following thesesuggestions as by adopting the new proce-dures themselves.

-35-

Drying ProceduresBasic Kiln Schedules

The recommended kiln schedules forhardwoods in the Dry Kiln Operator’s Manualwere carefully worked out combinations ofdry-bulb and wet-bulb temperatures based onresearch and experience. They will dry anyhardwood item at a generally satisfactoryrate without causing objectionable drying de-fects. D r y i n g d e f e c t s a r e c a u s e d b yshrinkage-related stresses that are related tothe average moisture content of the stock;changes in kiln conditions during the dryingrun are based on the average moisture con-tent of the wettest half of the kiln samples.

The recommended schedules were pre-sented as guides from which the well-trained,experienced kiln operator was expected to de-rive optimum schedules best suited for hismaterial, equipment, and operations. In all,the Manual indexed 77 hardwood schedules,but the operator had a total array of 672schedules which he could use for changingschedules as much as circumstances and hisexperience dictated.

Because much of the future practice inkiln-drying hardwoods probably will involve

air-dried or partly air-dried stock, it becomesfeasible to group eastern hardwood schedulesinto 11 pairs of basic schedules. All species ineach schedule group originally had identicalor very similar dry-bulb temperatures. Also,since the final stages of all hardwoodschedules involve very large wet-bulb depres-sions, all species in each group had similarintermediate and final wet-bulb tempera-tures. The basic schedules, therefore, are es-sentially the same as the recommendedschedules, so far as air-dried and partly air-dried hardwoods are concerned.

There should be little or no increase in dry-ing time for the basic schedules beyond thatexperienced with the recommended schedule.In a few cases, the basic schedules will resultin longer times than the recommendedschedules when drying green stock. The basicschedules also may be slightly too severe forgreen stock of a few species (see footnotes inthe basic schedule tables). When drying largequantities of green stock, the kiln operator isencouraged to revert to the originally rec-ommended schedule from the Manual.

For the basic kiln schedules, species aregrouped in table 10. An alphabetical index of

Table 10. -Grouping species for basic kiln schedules

Temperature scheduleTable for: SeverityNo. of Species in group

4/4 8/4 schedule

12 T2 Irregular Mild Red and white oak, southern lowland13 T4 T3 Mild Red oak, northern or upland14 T4 T3 Mild White oak, northern or upland15 T6 T3 Mild Apple, dogwood, rock elm, hophorn-

beam, black locust, osage-orange,persimmon, sycamore

16 T6 T3 Moderate American elm, slippery elm, holly,mahogany, black walnut

17 T8 T5 Mild Beech, sugar maple, hickory, pecan18 T8 T5 Moderate Black ash, green ash, white ash, yellow

birch, cherry, cottonwood (wet-streak), hackberry, red maple,silver maple, sassafras, sweetgumheartwood (redgum)

19 T10 T8 Moderate Basswood (light color), paper birch,buckeye, butternut, chestnut, cotton-wood (normal), magnolia, swamp tupelo(swamp blackgum), black willow

20 T11 T10 Moderate Yellow-poplar, cucumbertree21 T12 T11 Moderate Black tupelo (blackgum), sweetgum sap-

22wood (sapgum)

T12 T10 Severe Aspen (sapwood or box), basswood

-36-

the species and their basic schedule tablenumber is presented in table 11. The basicschedules themselves are given in tables 12through 22. The schedule code numbers usedin the Dry Kiln Operator’s Manual for eachspecies are given at the bottom of the basicschedule tables. Recommendations andschedule code numbers for 1- and 2-inchsquares and thick hardwoods are given in ta-bles 12 and 13 of the Manual.

Each of the basic schedulel tables showsspecific temperature and wet-bulb depres-sions during the intermediate and finalstages of drying; these temperatures anddepressions are slightly different from theoriginal Manual recommendations. Specifictemperature and wet-bulb depression valuesare also included for equalizing and condi-tioning. All of these specifics are designed toconserve energy and are essentially what aprudent kiln operator would already be doingto accomplish this end.

The intermediate and final stage wet-bulbtemperatures are close to those that are na-turally attained when the vents are keptclosed and the steam spray is turned off.Operators who do not know how to achievethis mode of operation with their equipmentshould check with their kiln manufacturer.Close control of wet-bulb temperature is notnecessary during the final drying stage whenthe schedule shows depression values of 45° For larger. Just keep the vents closed and thesteam spray off. During equalizing and condi-tioning, however, close control is necessary,and a clean wet-bulb wick is necessary to at-tain this close control. An explanation of theequalizing and conditioning specifics is givenlater in this chapter.

Specific procedures for starting up the kilnand the first day or two of the run when dry-ing air-dried or partly air-dried stock aregiven below. The average moisture content ofair-dried stock should be 25 percent or lowerwith no material over 30 percent. For partlyair-dried stock, no material should be over 50percent moisture content. Specific proceduresfor starting up and drying green lumber aregiven in chapter 10 of the Dry Kiln Operator’sManual. Although the same schedule gener-ally is prescribed for 4/4, 5/4, and 6/4 of eachspecies, the 6/4 will take considerably longerto dry than the 4/4, and therefore the best

practice is to dry each size separately. If 4/4and 5/4 must be dried together, most kilnsamples should be taken from the 5/4 stock.

Table 11. —Alphabetical index of speciesand basic kiln schedule table numbers

Species Table No.

AppleAsh: black, green, whiteAspenBasswood (slight browning)Basswood (light color)BeechBirch, paperBirch, yellowBlackgumBuckeyeButternutCherry, blackChestnutCottonwood, normalCottonwood, wet-streakCucumbertreeDogwoodElm: American, slipperyElm, rockGumHackberryHickoryHolly, AmericanHophornbeam (ironwood)Locust, blackMagnoliaMahoganyMaple: red, silverMaple: sugarOak: red, northern or uplandOak: white, northern or

uplandOak: red or white, southern

lowlandOsage-orangePecanPersimmonPoplarSassafrasSweetbaySweetgum, heartwood

(redgum)Sweetgum, sapwood (sapgum)Sycamore, AmericanTupelo, black (blackgum)Tupelo, swamp (swamp

blackgum)Tupelo, water (tupelo)

Walnut, blackWillow, blackYellow-poplar

151 181 22

2219171918

(see tupelo)1919

1 1819191820151615

(see sweetgum)181716

1 151 15

19161817

1 13

1 14

1 121 151 17

15(see yellow-poplar)

18(see magnolia)

181 211 15

21

1 19(see Dry Kiln

Operator’s Manual)161920

1 See special note in table indicated

-37-

Table 12. —Basic kiln schedules for red and white oak, southern lowland 1

Moisture 4/4, 5/4 (T2-C1) 2 6/4, 8/4 (Irregular)

contentat start Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulb

of step temper- depres- temper- temper- depres- temper-ature sion ature ature sion ature

Percent ° F ° F ° F ° F ° F ° FAbove 40 100 3 97 (Air dry to 25 pct MC)

40 100 4 9635 100 6 9430 110 10 100 105 8 9725 120 25 95 11020

11 99130 40 90 120 15 105

15 150 45 105 130 30 10011 160 50 110 160 50 110

Equalize 173 43 130 173 43 130Condition 180 10 170 180 10 170

1 For all oak species, 6/4 stock usually is dried by the 8/4 schedule.2 The recommended green stock code number is the same.

Table 13. —Basic kiln schedules for red oak, northern or upland 1

ature sion ature

Moisture 4/4, 5/4 (T4-D2) 2 6/4, 8/4 (T3-D1) 2

contentat start Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulbof step temper- depres- temper- temper- depres- temper-

Percent ° F ° F ° F ° F ° F ° FAbove 50 110 4 106 110 3 107

50 110 5 105 110 4 10640 110 8 102 110 6 10435 110 14 96 110 10 10030 120 30 90 120 25 9525 130 40 90 130 40 9020 140 45 95 140 45 9515 180 50 130 160 50 110

Equalize 173 43 130 173 43Condition

130180 10 170 180 10 170

1 For all oak species, 6/4 stock usually is dried by the 8/4 schedule.2 The recommended green stock code numbers are the same.

Table 14. —Basic kiln schedules for white oak, northern or upland 1

atrue sion ature

Moisturecontentat startof step

PercentAbove 40

403530252015

EqualizeCondition

4/4, 5/4 (T4-C2) 2 6/4, 8/4 (T3-C1) 2

Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulbtempera- depres- tempera- tempera- depres- tempera-

ture sion ture ture sion ture

° F ° F ° F ° F ° F ° F110 4 106 110 4 107110 5 105 110 4 106110 8 102 110 6 104120 14 106 120 10 110130 30 100 130 25 105140 45 95 140 40 100180 50 130 160 50 110173 43 130 173 43 130180 10 170 180 10 170

1 For all oak species, 6/4 stock usually is dried by the 8/4 schedule.2 The recommended green stock code numbers are the same.

-38-

Table 15 —Basic kiln schedules for rock elm, dogwood, persimmon, and similar woods 1

Moisturecontent at

start of step

4/4, 5/4, 6/4 (T6-B3) 8/4 (T3-B2)

Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulbtemperature depression temperature temperature depression temperature

Percent ° F ° F ° F ° F ° F ° FAbove 35 120 5 115 110 4 106

35 120 7 113 110 5 10530 130 11 119 120 8 11225 140 19 121 130 14 11620 150 35 115 140 30 11015 180 50 130 160 50 110

Equalize 173 43 130 173 43 130Condition 180 10 170 180 10 170

1 Species included and recommended green stock code numbers:Species Code No. for:

4/4, 5/4, 6/14 8/4Apple T6-C3 T3-C2Dogwood T6-C3 T3-C2Elm, rock T6-B3 T3-B2Hophornbeam T6-B3 T3-B1

(ironwood) 2

Locust, black 2 T6-A3 T3-A1Osage-orange 2 T6-A2 T3-A1Persimmon T6-C3 T3-C2Sycamore 2 T6-D2 Te-D1

2 For 4/4 and 8/4 locust and osage-orange and 8/4 hophornbeammoisture content, use the green stock schedule.

(ironwood) and sycamore stock over 35 percent

Table 16. —Basic kiln schedules for American and slippery elm, holly, and black walnut 1

Moisture 4/4, 5/4, 6/4 (T6-C4) 8/4 (T3-C3)

content atstart of step Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulb

temperature depression temperature temperature depression temperature- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Percent ° F ° F ° F ° F ° F ° FAbove 40 120 7 113 110 5 105

40 120 10 110 110 7 10335 120 15 105 110 10 10030 130 25 105 120 19 10125 140 35 105 130 35 9520 150 40 110 140 40 10015 180 50 130 160 50 110

Equalize 173 43 130 173 43 130Condition 180 10 170 180 10 170

1 Species included and recommended green stock code numbers:Species Code No. for

4/4, 5/4, 6/4 8/4Elm, American T6-D4 T5-D3Elm, slippery T6-D4 T5-D3H o l l y T6-D4 T4-C3Mahogany T6-C4 T4-C3Walnut, black T6-D4 T3-D3

-39-

Table 17. —Basic kiln schedules for beech, sugar maple, and pecan 1

Moisture 4/4, 5/4, 6/4 (T8-C2) 8/4 (T5-C1)

content atstart of step Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulb

temperature depression temperature temperature depression temperature

Percent ° F ° F ° F ° F ° F ° FAbove 40 130 4 126 120 3

40117

130 5 125 120 435

116130 8 122 120 6

30 140114

14 126 13025

10 120150 30 120 140 25

20115

160 40 120 150 3515

115180 50 130 160 50

Equalize110

173 43 130 173 43 130Condition 180 10 170 180 10 170

1 Species included and recommended green stock code numbers.Species Code No. for

4/4, 5/4, 6/4 8/4Beech T8-C2 T5-C1H i c k o r y T8-D3 T6-D1Maple, sugar T8-C3 T5-C2Pecan 2 T8-D3 T6-D1

2 Bitter pecan (water hickory) heartwood is very difficult to kiln dry from green. Stock should be thoroughly air driedfirst.

Table 18. —Basic kiln schedules for white ash, yellow birch, cherry, sweetgum, and similarwoods 1

Moisture 4/4, 5/4 6/4 (T8-C4) 8/4 (T5-C3)

content atstart of step Dry-bulb Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulb

temperature depression temperature temperature depression temperature______________________________________________________________________________________________________________________

Percent ° F ° F ° F ° F ° F ° FAbove 40 130 7 123 120 5

40115

130 10 120 120 735

113130 15 115 120 11 109

30 140 25 115 13025

19 111150 35 115 140 30

20110

160 45 115 150 4015

110180 50 130 160 50 110

Equalize 173 43 130 173 43 130Condition 180 10 170 180 10 170

1 Species included and recommended green stock code numbers:Species Code No. for:

4/4, 5/4, 6/4 8/4Ash, black T8-D4 T5-D3Ash: green, white 2 T8-B4 T5-B3Birch, yellow T8-C4 T5-C3Cherry, black 2 T8-B4 T5-B3Cottonwood (wet-streak) T8-D5 T6-C4Hackberry T8-C4 T6-C3Maple: red, silver T8-D4 T6-C3Sassafras T8-D4 T5-C3Sweetgum (heartwood) T8-C4 T5-C3

2 For green or white ash and cherry stock over 35 percent moisture content, use the green stock schedules.

-40-

Table 19. —Basic kiln schedules for magnolia, paper birch, butternut, normal cottonwood, andsimilar woods 1

Moisture4/4, 5/4 6/4 (T10-D4) 8/4 (T8-D3)

content atDry-bulbstart of step Wet-bulb Wet-bulb Dry-bulb Wet-bulb Wet-bulb

temperature depression temperature temperature depression temperature

PercentAbove 50

50403530252015

EqualizeCondition

° F140140140140150160170180173180

° F7

101525354045504310

° F133130125115115120125130130170

° F130130130130140150160180173180

° F57

1119354045504310

° F125123119111105110115130130170

1 Species included and recommended green stock code numbers:Species Code No. for:

4/4, 5/4, 6/4 8/4Basswood (light color) T9-E7 T7-E6Birch, paper T10-C4 T8-C3Buckeye T10-F4 T8-F3Butternut T10-E4 T8-E3Chestnut T10-E4 T8-E3Cottonwood (normal) T10-F5 T8-F4Magnolia; sweetbay T10-D4 T8-D3Tupelo, swamp (swamp blackgum) 2 T10-E3 T8-D2Willow, black T10-F4 T8-F3

2 For swamp tupelo over 40 percent moisture content, use the green stock schedule. For water tupelo, see Dry KilnOperator’s Manual.

Table 20. —Basic kiln schedules for yellow-poplar and cucumbertree 1

Moisturecontent at

start of step

4/4, 5/4, 6/4 (T11-D4)

Dry-bulb Wet-bulb Wet-bulbtemperature depression temperature

8/4 (T10-D3)

Dry-bulb Wet-bulb Wet-bulbtemperature depression

PercentAbove 50

50403530252015

EqualizeCondition

° F150150150150160160170180173180

° F7

101525354045504310

° F143140135125125120125130130170

° F140140140140150160170180173180

° F57

1119354045504310

temperature

° F135133129121115120125130130170

1 Cucumbertree, although really one of the magnolias, is usually dried with yellow-poplar.

-41-

Table 21. –Basic kiln schedules for black tupelo (black gum) and sweetgum sapwood (sap gum) 1,2

Moisture 4/4, 5/4, 6/4 (T12-E5) 8/4 (T11-D3)

content atstart of step

PercentAbove 60

6050403530252015

EqualizeCondition

Dry-bulb Wet-bulbtemperature depression

° F ° F160 10160 14160 20160 30160 40170 45170 50180 50180 50173 43180 10

Wet-bulb Dry-bulbtemperature temperature

° F ° F150 150146 150140 150130 150120 150125 160120 160130 170130 180130 173170 180

Wet-bulb Wet-bulbdepression temperature

° F ° F5 1455 1457 143

11 13919 13135 12540 12045 12550 13043 13010 170

1 Species included and recommended green stock code numbers:Species Code No. for:

4/4, 5/4, 6/4 8/4Sweetgum sapwood (sap gum) 2 T12-F5 T11-D4Black tupelo (black gum) T12-E5 T11-D3

2 Lower grade sweetgum heartwood (red gum) often is included with sap gum sorts. If stock is more than 15 percentred gum, use schedules in table 18.

Table 22. —Basic kiln schedules for basswood, aspen (sapwood or box lumber) 1

Moisture 4/4, 5/4, 6/4 (T12-E7) 8/4 (T10-E6)

content atstart of step Dry-bulb

temperatureWet-bulb

depression

PercentAbove 60

6050403530252015

EqualizeCondition

° F ° F160 20160 30160 40160 45160 45170 50170 50180 50180 50173 43180 10

Wet-bulb Dry-bulb Wet-bulb Wet-bulbtemperature temperature depression temperature

° F ° F ° F ° F140 140 15 125130 140 20 120120 140 30 110115 140 40 100115 140 45 95120 150 45 105120 160 50 110130 170 50 120130 180 50 130130 173 43 130170 180 10 170

1 Species included and recommended green stock code numbers:Species Code No. for:

4/4, 5/4, 6/4 8/4Aspen (some collapse) 2 T12-E7 T10-E6Basswood (slight browning) T12-E7 T10-E6

2 See special schedules in Appendix C.

- 4 2 -

Specific Procedure for Air-Dried Stock4/4, 5/4, most 6/4

(1) Bring dry-bulb temperature up to thevalue prescribed by schedule for the averagemoisture content (MC) of the controlling 8 kilnsamples, keeping the vents closed and thesteam spray turned off.

(2) After prescribed dry bulb temperaturehas been reached:

(a) If the air-dried stock had not under-gone surface wetting or been exposedfor a considerable period to high rela-tive humidity, just before it was placedin the kiln, set the wet-bulb controller atthe prescribed wet-bulb temperature.Turn on the steam spray only if necessaryto start equalizing.

(b) If there has been surface moisture re-gain, set the wet-bulb controller for a10° F wet-bulb depression and turn onthe steam spray. Let the kiln run 12 to18 hours at this wet-bulb setting, thenchange to the dry- and wet-bulb set-tings prescribed by the schedule.

8/4 (plus 6/4 oak)(1) Bring dry-bulb temperature up to the

value prescribed by the schedule for the av-erage MC of the controlling kiln samples,keeping the vents closed. Use steam sprayonly as needed to keep wet-bulb depressionfrom exceeding 12° F.

(2) After prescribed dry-bulb temperaturehas been reached:

(a) If there has been no surface moistureregain, set the wet-bulb controller atthe prescribed wet-bulb temperature.Turn on the steam spray only if neces-sary.

(b) If there has been surface moisture re-gain, set the wet-bulb controller for an8° F wet-bulb depression and turn onthe steam spray. Let the kiln run 18 to24 hours at this setting. Then set for a12° F depression and run for 18 to 24hours more before changing to the con-ditions prescribed by the schedule.

8 The controlling samples usually are the wettest halfof the entire group of carefully selected kiln samples(chapter 6, Dry Kiln Operator’s Manual).

Specific Procedure for Partly Air-DriedStock

414, 5/4, most 6/4(1) Bring dry-bulb temperature up to the

value prescribed by the schedule for the av-erage MC of the controlling kiln samples.Keep the vents closed. Use steam spray onlyas needed to keep wet-bulb depression fromexceeding 10° F. Do not, however, allow thedepression to become less than 5° F or mois-ture will condense on the lumber.

(2) After the prescribed dry-bulb tempera-ture has been reached, run a minimum of 12hours on each of the first three wet-bulb de-pression steps of the whole schedule, but stillobserve the 5° F minimum wet-bulb depres-sion. Then change to the conditions pre-scribed for the MC of the controlling samples.8/4 (plus 6/4 oak)

(1) Bring dry-bulb temperature up to thevalue prescribed by the schedule for the av-erage MC of the controlling kiln samples.Keep the vents closed. Use steam spray onlyas needed to keep wet-bulb depression fromexceeding 8° F. Do not, however, allow de-pression to become less than 5° F.

(2) After the prescribed dry-bulb tempera-ture has been reached, run a minimum of 18hours on each of the first three wet-bulbdepression steps of the schedule, but still ob-serving the 5° F minimum wet-bulb depres-sion. When the kiln conditions coincide withthose prescribed by the schedule for the aver-age MC of the controlling samples, change tothe moisture content basis of operation.

Procedure for Including SmallAmounts of One Species in

Large Kiln Loads of Other HardwoodsIndividuals who have lumber from a log or

two of their own often wish to use this lumberfor paneling, cabinetry, or fine furniture.Such wood should be carefully air dried firstunder a good pile roof or in an unheated shed.Then, if possible, it should be kiln dried so asto bring the moisture content to about 7 per-cent and relieve drying stress. Such air-driedlumber can be kiln dried with the same thick-ness of air-dried stock of another species inthe same basic kiln schedule group (table 10).The kiln schedule will have to be operated onthe basis of kiln samples representing themajor kind of wood in the kiln, but final mois-

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ture content and stress condition should besatisfactory if the kiln operator uses a properequalizing and conditioning treatment.

Small amounts of fully air-dried lumber (20percent moisture content or less) can be kilndried with air-dried stock of species that takeother basic kiln schedules, but final moisturecontent and stress relief may not be as satis-factory as when dried with a species of its owngroup. If the only hardwood kiln available isdrying green or partly air-dried stock, ar-rangements should be made to put the smallamount in the kiln sometime after it isstarted, when the major stock and the air-dried small amount of material have aboutthe same moisture content.

Small amounts of air-dried stock also canhave their moisture content reduced to theproper level by heated room drying, which isdescribed later. Such drying may leave thewood with unrelieved drying stresses, whichcan cause warping problems unless specialcare is exercised in the woodworking opera-tions.

Kiln Schedule AccelerationBy shortening drying time considerably,

significant energy and cost savings can bemade—a possibility that should be exploredby every commercial kiln operator. A step-by-step approach to kiln schedule accelera-tion for green stock is given on pages 124–126,Dry Kiln Operator’s Manual. Under certaincircumstances, several steps can be com-bined. For instance, the recommendedschedule for red oak (4/4 thickness) is T4–D2.This is essentially the same as the basicschedule for 4/4 in table 13.

If the green moisture content is very high,the first acceleration is to change from T4-D2to schedule T4–E2. Thus, changes in the wet-bulb depression are accelerated. Althoughthe initial depression is the same (4° F), the 5°F depression of the second step is started at60 percent moisture content instead of 50 per-cent. Then the depression goes to 8° F at 50percent, and so on. (The dry bulb value is un-changed.)

If T4–E2 works well, for several charges,with no surface checking, the next changeshould be to schedule T4–E3. In this changefrom E2 to E3, the initial wet-bulb depressionis increased. The starting depression is 5° F

rather than 4° F. Subsequent wet-bulb de-pressions also are increased. In a kiln withwell-calibrated instruments and good con-struction, T4–E3 should work well, but aslight amount of surface checking could occuron the entering air edges of the load.

The 50° F depression prescribed in theManual for the latter stages of drying is onlya guide. Generally the kiln operator shouldturn off the steam spray by a hand shut-offvalve and set the wet-bulb controller so thatthe vents stay closed during the latter half ofthe drying schedule. If the wet-bulb tempera-ture does not come down to the value shownfor each step in the basic schedules, the kilnoperator may want to open the vents for shortperiods only. When dry-bulb temperatures of160° F or higher are reached, however, thevents should be kept closed.

Finally, one temperature acceleration issuggested for this 4/4 thickness only; when allthe samples are below 20 percent, go to thefinal 180° F temperature. The results of all themodifications outlined above are summarizedin table 23. If the oak cited above is from stockthat normally is very wet when green, but hashad some slight air drying, it may come downto 60 percent moisture content very rapidly.In this situation, extend the time on the firstschedule step to a total of 2 days.

Not all the generally recommended kilnschedules are so conservative that they canbe modified as much as the oak schedule. SeeAppendix A for a listing of schedules andcomments for each species. A general modifi-cation that applies to all species when drying8/4 stock is to use a final dry-bulb tempera-ture of 180° F when the average moisture con-tent of the controlling samples reaches 11percent.

The question is sometimes asked: “Whatcan be done when drying seems to stop in themiddle of the kiln run?” Drying will neveractually stop if the EMC of the kiln at-mosphere is lower than the core moisturecontent of the lumber. Slight increases ofdry-bulb temperature during the inter-mediate stages of drying can be made, butgenerally they are not recommended. An in-crease of 5° F can be tolerated, however, bysome woods. In table 23, it might be satisfac-tory to use 115° F dry-bulb temperature at 35percent moisture content. A temperature of

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125° F dry-bulb has been used at 35 percentmoisture content for 8/4 hickory (table 17, DryKiln Operator’s Manual). When a decision ismade to try such increases of temperature atmoisture contents above 30 percent, the kilnoperator should make sure that his kilnrecorder-controller is properly calibrated.Table 23. —Accelerated kiln schedule for

northern or upland red oak 4/4, 5/4 with ahigh initial moisture content (some risk of

slight surface checking)

Moisture Dry-bulb Wet-bulb Depres-content temper- temper- sion

ature ature

Percent ° F ° F ° FAbove 60 110 105 560-50 110 107 750-40 110 99 1140-35 110 91 1935-30 110 (1)

1 3 030-25 120 (1)

1 4025-20 130 (1) 1 5020-18 140 (1) 1 5018–(F–3)2 180 (1) 1 50

1 Vents closed, steam spray shut off, accept whateverdepression occurs.

2 Equalize and condition as in table 13. F is desiredfinal moisture content.

Experimentally, some acceleration of dry-ing of oak has been obtained by automation inconnection with presurfacing and an acceler-ated kiln schedule (Wengert and Baltes 1971).Automation has not been adopted commer-cially. Pilot tests have been made, however,on presurfacing and the accelerated kilnschedule it permits (Cuppett and Craft 1972,Rice 1971). Drying time savings were esti-mated to be 24 percent or higher and kilncapacity was increased 8 to 12 percent. Pre-surfacing does not fit in with currenthardwood processing practice, and therewould, of course, be some added costs for thepresurfacing. Further details are given underSpecial Kiln Schedules for Special Purposes(app. C).

Mixed Species and SimplifiedSchedules

When kiln drying large quantities of valu-able species from the green or partly air-driedcondition, the recommended practice is to dryeach species, in each thickness, separately.When smaller amounts of individual speciesare available, as when clearing southern pine

sites for planting, drying groups of specieswith similar characteristics together is oftenexpedient. General suggestions for dryingmixed kiln charges are given in chapter 10 ofthe Dry Kiln Operator’s Manual.

Frequently, when using easy-drying woodsfor demonstration material in kiln dryingshort courses, several steps of the recom-mended schedule were omitted without caus-ing degrade. This led to development ofsimplified schedules that primarily save sometime for the kiln operator, but they also mayreduce kiln residence time slightly. The easeof drying and the simplicity of the schedulessuggest that several species can be mixed,and substantial industry experience backsthis up. A classification of southern pine-sitehardwoods by similarity of schedules is givenin Appendix B , along with suggestedsimplified schedules I through VI (table B1).

This classification can also be used with thesame species grown on other upland easternhardwood sites, but the kiln operator mustuse judgment in applying it. For instance,commercial sapgum that contains more than15 percent sweetgum heartwood should bedried by the red gum schedules (table 18).Tupelo from flooded or swampy sites shouldbe dried by the water tupelo or swamp tupeloschedules. In commercial practice, these moredifficult-to-dry tupelos generally are air-driedbefore kiln drying. Bitter pecan heartwoodalso is air-dried before kiln drying. Whereonly small quantities of rock elm are avail-able, it can be combined with open-grown(hard type) American elm and dried by thewarp-reducing schedule II. Large quantities,however, should be dried separately using therock elm schedule (table 15). Likewise, largequantities of forest-grown soft-type Americanelm can be dried by schedule I, but smallquantities can be mixed with hard-type elmusing schedule II.

Special Schedules for Special PurposesAspen

Now that aspen is being used for a widerarray of products than crating and roughlumber, consideration must be given tominimizing collapse. This defect is a principalproblem in drying aspen. Special schedulesfor 4/4 and 8/4 No. 2 and 3A Common lumberare given in Appendix C. The uppermostgrades of lumber sawed from the outside of

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larger logs, as well as crating lumber, can stillbe dried by the schedules in table 22. A Cana-dian schedule for faster drying of studs ismentioned in chapter 6 on High-TemperatureDrying.Sugar Maple

Schedules are sometimes needed to keepmaple sapwood the whitest color possible andto dry maple containing mineral streak withthe least amount of honeycombing. The spe-cial schedule for white maple given in appen-dix C has been used successfully on 4/4 and 5/4stock (MCMillen 1976). When drying 4/4 maplethat contains mineral streak or other charac-ter marks, many kiln operators satisfactorilyuse the regular wet-bulb depression schedule,C3, with a lower-than-customary tempera-ture schedule, either T5 or T3. See page 118,Dry Kiln Operator’s Manual for these sepa-rate schedules and the method of their as-sembly.

For 6/4 and 8/4 mineral-streak stock, see thespecial procedure suggested in appendix C.

Fine internal hairline checks that do notshow up in 8/4 and thicker maple until man-ufacture or use have sometimes been a costlyproblem in maple. These are believed to becaused by stresses resulting from surfacemoisture regain and improper redrying ofpreviously kiln-dried stock. The first preven-tative is to store kiln-dried maple in a mannerto prevent regain (see chapter 9 on Storage ofDried Lumber). If this has not been done, seethe special schedules for adjusting moisturecontent of kiln-dried wood.Bacterially Infected Oak

Lumber from northern red oak and blackoak infected with anaerobic heartwood bac-teria is highly susceptible to internal check-ing when dried by normal or accelerated oakschedules. Both honeycombing and ring sep-aration occur (Ward 1972; Ward et al. 1972).Research has shown that material in the ad-vanced stage of infection is especially subjectto surface checking and honeycombing. Suchwood should be sawed into 4/4 rather thanthicker lumber. Good results were obtainedwhen such material was forced air dried to 20percent MC by an 8/4 oak procedure (Cuppettand Craft 1975), and then kiln dried by thebalance of the schedule given in appendix C.Almost as good results were obtained whenkiln drying was started at 25 percent MC. Lowair velocity was used, and the fans were run

only half the time for the first 11 days offorced air drying. Material in an early stageof infection was successfully kiln dried fromthe green condition by the schedule in appen-dix C.Presurfaced 1-Inch Upland

Red and White OakThe suitability of an accelerated kiln

schedule for fully presurfaced upland red andwhite oak was confirmed by research at theU.S. Forest Products Laboratory (MCMillen1969, MCMillen and Baltes 1972, Wengert andBaltes 1971). Drying time can be reduced 25 to50 percent by the fully accelerated scheduleand kiln capacity is increased 8 to 12 percentby the presurfacing. Slight additional ben-efits are obtainable by automation, but fullbenefits of the accelerated schedule are notavailable without automation. The scheduleis not applicable to presurfaced green cher-rybark oak, a common southern oak, but canbe applied to such stock carefully dried byother means to 40 percent moisture content.Successful field tests of the presurfaced oakschedule have been carried out with northernred and mixed Appalachian red oak in Mas-sachusetts and West Virginia (Rice 1971,Cuppett and Craft 1972).

The accelerated schedule, modified for kilnsample operation, is given in Appendix C.This is for oak fully surfaced in the strictlygreen condition to remove all saw marks, notto material that is merely blanked or skipdressed to obtain uniform size. The thicknessafter complete surfacing probably should be33/32 inch to insure kiln dry dimension cut-tings 13/16 inch thick. Rough sawing wouldhave to be to a l-5/32-inch thickness (seeMCMillen (1969) for further suggestions).Warp and Shrinkage Reduction

A number of measures for warp reductionare indicated in chapter 2. A major one is touse rapid natural air drying or accelerated-air drying of properly piled lumber. Researchon red oak has shown that shrinkage is re-duced by using lower temperatures and rapidreduction of relative humidity (MCMil len1963). The effect is uniform from 140° F downto 95° F. From 95° F down to 80° F the effect isgreater because more of the wood is affectedby tension set. Tension set tends to resistshrinkage of the board and thus reduceswarping. This is a major part of the reasonwhy air drying gives the least shrinkage and

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warping. Compression set in the interior ofthe wood is also important, as it tends to in-crease board shrinkage and warping. Thus,the drying of green oak with an initial tem-perature above 110° or 115° F would be ex-pected to produce more than normal warping.

The effects of uniform thickness, goodstacking, and restraint may overcome the ef-fects of temperature differences with 4/4stock. No significant difference was detectedin the warp of 1-inch green sugar maple withinitial temperatures of 110° to 160° F when thesame EMC was used in all runs (Rietz 1969).For thicker stock, in which compression setprobably has more influence, use of lowerthan customary initial temperature and rela-tive humidity probably is helpful.

No series of low-shrinkage, low-warpschedules have been developed. The kilnoperator who has the kiln time available totake slightly longer in drying can experimentwith lower relative humidity schedules byusing slightly larger initial wet-bulb depres-sions. The lower the wet-bulb temperature,the easier it is to achieve a lower dry-bulbtemperature in the kiln. Changes from rec-ommended or basic schedules should be slightat first, and the kiln operator will need toobserve the stock in the kiln frequently to seethat surface checking is not developing. Anysuch experimentation, of course, should bedone only with the advice and consent of themanagement.

Adjusting Moisture Content ofKiln-Dried WoodOnce wood has been kiln dried to a moisture

content suitable for interior purposes, itshould be stored in a heated or dehumidifiedshed or room (see Chapter 9, Storage of DriedLumber). Situations occur, however, whenthe moisture content of the lumber must bechanged: (a) the moisture content is too lowfor use in steam bending, boat construction,or the like, or (b) the wood has not beenproperly stored and must be redried.

If the moisture content is too low, a two-step procedure is advised. The lumber mustbe stickered in properly built piles and thevoid spaces of the kiln baffled. The exactschedule will depend on the moisture contentdesired and the time available. Allow 2 days

for each step for 4/4 stock, longer for thickermaterial or large moisture differences, Forthe first half of the time required, use a kilnEMC equal to the desired moisture content.For the second half use an EMC 3 percenthigher. If enough time is available, use a kilntemperature of 130° or 140° F. For quickerresults, 160° or 180° F can be used. In eithercase, the vents should be closed during kilnwarm-up. Steam spray should be used in-termittently to avoid EMC’s lower than thepresent moisture content or higher than thedesired moisture content during the warm-upperiod. Do not condense moisture on t h elumber. Use the kiln sample procedure tomonitor the moisture pick-up. If the rate istoo slow, use a higher temperature with thesame EMC’s. The moisture level to whichhardwoods can be easily raised is limited toabout 13 percent, for it is difficult to hold anEMC higher than 16 percent in most kilns.

To redry kiln-dried lumber that has beenkept in uncontrolled storage, great care isneeded. Otherwise surface checks that weretightly closed may become permanentlyopened, or internal hairline checks can occur.If the storage period has been short, tightlybundled lumber can be redried in the bundlesbecause most of the moisture pick-up willhave been on the board ends and the surfacesof exposed boards. For longer storage, or forlumber that has been kept outdoors on stick-ers, the lumber must be stickered for redry-ing.

Two temperature steps are suggested forthe redrying operation; the first about 1 daylong at 130° or 140° F. The second should be atthe final temperature of the drying phase ofthe basic schedule (tables 12 to 22) for thespecies and size involved. Do not use steamspray during kiln warm-up. Surface checksalready present may open up, but will closeagain as the wood dries. When the first kilntemperature is reached, set the wet-bulb con-troller to achieve an EMC somewhere be-tween the current moisture content of thestock and the moisture content desired. Forthe final step, set the controller to give anEMC 2 percent below the desired moisturecontent. When the wettest kiln samplereaches the desired moisture content, stopthe drying, No conditioning should be needed.

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Predicting Drying Time

Frequently management is faced with de-termining how long it will take to dry a load orwhen the next change in kiln conditions canbe made.

“How long before the next change” can beeasily answered if the operator will graph themoisture content of the samples as they dry(fig. 13). To make this job easy, special graphpaper can be obtained which has horizontalmarkings in tenths of an inch and verticalmarkings in twelfths. With such paper, it iseasy to show drying time to days and hours.By graphing the moisture content of thesamples as they are weighed, a smooth dryingcurve is obtained. Any portion of this curvecan be extended 24 hours ahead with astraight line to predict the approximate mois-ture content the next day. In fact, when thesame species and thickness is dried re-peatedly, the total drying time can be rea-sonably well estimated.

Early in a kiln operator’s career at a givenlocation, he may find it helpful to developbasic drying curves. He should use his own

sample data for green or nearly green stock ofvarious species and thicknesses. For any sub-sequent charge of lumber dried from a lowerinitial moisture content, the drying curve ofthe first day or two would generally parallelthe line for the first day or two of the greencurve. After that the drying of air-dried orpartly air-dried stock will follow fairly closelythe curve for the green stock.

Since it will not be practical to establishgreen stock drying curves for all thicknessesof lumber right away, approximate curvescan be estimated. These can be set up usingdrying time factors for the most commonthicknesses. One such set of factors was de-veloped by Higgins to predict total dryingtime. Such factors have a theoretical base re-lated to diffusion and other aspects of wooddrying, but these were empirically developedafter several years of experience in commer-cial drying of foreign and domestic woods.They are roughly corroborated by othercommercial drying time data. The drying timefactor for 8/4 stock was set at 1.00 because thebest approximations for other thicknesseswere obtained when the 8/4 drying time data

M 143471

Figure 13.—Black walnut drying curve.

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M 144693

Figure 14.—Method for postulating a drying time scale for a new thickness of a givenspecies to be dried from a known drying curve.

were used as a base. Actual drying times willvary, of course, depending on actual thick-ness, width, percentage of heartwood, andquality of stock.

Until more precise factors become avail-able, the ones developed by Higgins can beused in setting up the preliminary curves toestimate drying time from various moisturelevels. These factors are as follows:

Thickness Drying time factor3/4 0.254/4 .405/4 .556/4 .708/4 1.00

10/4 1.3512/4 1.7514/4 2.2516/4 2.85

The factors for 4/4 and 8/4 indicate that 2½times as long will be required to dry 8/4 mate-rial as 4/4.

To employ these factors in developing a setof drying curves for a species, the kilnoperator should use either his own 8/4 curveor a 4/4 curve obtained when a conservativeschedule was used. Time and moisture con-

tent scales should be selected so that theslope of the curve is between 30° and 48° F. Thefirst step in using a 4/4 curve as a base wouldbe to list the 8/4 drying times directly belowthe 4/4 times. Then compute the drying timesat the same moisture levels for the otherthicknesses. From figure 13, some of the timeswould be as follows:Thickness Factor Drying times-days

4/4 (Given) 2 6 10 148/4 (9) 5 15 25 355/4 0.55 2.75 8.25 13.75 19.256/4 .70 3.50 10.50 17.50 24.50

10/4 1.35 6.75 20.25 33.75 47.25

9 Time for 8/4 = 1.00 ÷ (factor for given thickness)

× given time; in this example, 8/4 time

× given time = 2.5 (given time).

At the start, a separate graph should beused for each thickness. An example for wal-nut 6/4 is shown in figure 14. Write the com-puted drying times for 8/4 and for the selectedthickness on the graph in pencil. Then inter-polate whole number days for the selectedthickness with a pen and erase the pencilvalues. As successive charges are dried and

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drying time improved by satisfactory ac-celerations, new lines can be drawn on thisgraph. When sufficient experience has beengained, and any program of schedule im-provement completed, a new graph can bemade using the average of the actual curves.Ultimately the kiln operator can make up onecurve and a composite time scale for eachthickness on one graph. A crucial point, how-ever, is the fact that hardwoods of varyingquality cannot be dried by time schedules.The kiln operator must revert to the moisturecontent schedule when he is in doubt aboutthe drying characteristics of a particularcharge.

A side benefit of such a finalized dryinggraph is that any subsequent charge of nor-mal stock having a slower drying rate willindicate the kiln may not be operating proper-ly. It may have a malfunctioning instrument,a water- or air-bound steam line, an inopera-tive fan, or some similar fault, and may needmaintenance. A comparison of graphs over aperiod of time should also show any slow lossof efficiency in the kiln.

Equalizing and Conditioning

Frequently in kiln drying, and especiallywith mixed species, sizes, or initial moisturelevels, some lumber in a charge is as dry as

Figure 15.—Method of cutting final moisture content and drying stress tests.

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required while other lumber is still too wet.An equalizing treatment reduces the spreadbetween the wettest and driest boards. Dry-ing stresses and sets (often called case-hardening) are always present at the end ofkiln drying and equalizing. Any lumber thatwill be resawed, ripped, or machinednonuniformly should be conditioned to relievestresses. Failure to do so will result in warp-ing (cupping, crooking, or bowing) during orimmediately after machining and will causedifficulty in boring.

The normal procedures for equalizing anddrying stress relief are described in the lastparts of chapters 8 and 10 of the Dry KilnOperator’s Manual. Figure 15 shows a methodof cutting final moisture content and lon-gitudinal as well as transverse stress tests.The kiln operator’s attention is especiallydrawn to the method of evaluation ofcasehardening in chapter 10 of the Manual. Afinal analysis for freedom from stress cannotbe made until the test prongs have air driedfor 16 to 24 hours, but a significant turningout of the transverse test prongs immediatelyafter they are cut often indicates that thetransverse stresses have been relieved.

It was formerly considered that densehardwood 4/4 lumber required 16 to 24 hoursto condition. Conditioning has high energydemands, so the time should be no longerthan needed to get stress relief. Several low-density imported woods have been success-fully conditioned by equalizing at 1 percentlower moisture content than the recom-mended procedures in the Manual, then con-ditioning 6 hours at the regular EMC(McMillen and Boone 1974). This short-cuthas been successfully used on sugar maplesapwood in kiln-drying demonstrations, tak-ing about 8 hours. Red oak 4/4 reportedly hasbeen conditioned in 13 hours.

If the average moisture content determina-tions are made immediately after the con-ditioning treatment, the moisture contentobtained will be about 1 to 1.5 percent abovethe desired value because of the surface mois-ture regain. After cooling, the average mois-ture content of the lumber should be close tothat desired. The general rules of the condi-tioning procedure do not apply to moisturecontent values above 11 percent. Condition-ing is hard to accomplish at the higher values.

The important factor in conditioning is toobtain the specified EMC. The simplest way isto use an equalizing dry-bulb temperaturethat is lower than the dry-bulb temperaturefor conditioning. At the end of equalizing, theheating coils are shut off. The wet-bulb con-troller is then set up for the conditioningwet-bulb temperature. As the steam sprayraises the wet-bulb temperature, the dry-bulbtemperature also will rise some. When thewet-bulb temperature reaches the desiredlevel, the dry-bulb controller is set to its pre-scribed value. Only one heating coil should beturned on. Both the dry-bulb and the wet-bulb temperatures should reach and remainat the desired levels for conditioning.

The equalizing and conditioning tempera-tures shown in the basic kiln schedules (ta-bles 12–22) are selected to utilize the aboveprocedure and give stress-free wood at a final7 percent moisture content. For other desiredfinal moisture content levels, use tempera-tures to equalize at an EMC 3 percent belowthe desired moisture content and to conditionsat 4 percent above the desired moisture con-tent. If equalizing is expected to be prolonged,use the reduced dry-bulb temperature onlythe last 12 to 18 hours of equalizing.

The conditioning time could be undulylengthened if high-pressure steam is used forconditioning and no special measures weretaken to reach the correct wet-bulb depres-sion. A desuperheater should be used in thesteam line to remove the excess heat. (SeeOperational Considerations.)

Occasionally, the transverse prong test willshow no stress, but the lumber will bow whenresawed. The cause of the bowing is longitu-dinal stress resulting from either longitudi-nal tension set in the surface zones orlongitudinal shrinkage differentials due toreaction wood (tension wood in hardwoods).These stresses are most likely to be unrelievedwhen conditioning temperature or EMC is toolow or when conditioning time is too short. Thelongitudinal stress sticks in figure 15 willshow whether such stresses are present. Ifstresses are a problem, conditioning should beat 180° F or higher. The lumber must havebeen equalized, and the recording instrumentmust be in calibration. If longitudinalstresses are still a problem, the wet-bulb set-ting can be raised 1° F over the recommended

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value. Also, the conditioning period can be If tension wood stresses are very severe, theyextended about 4 hours per inch of thickness. may not yield to any conditioning treatment.

Operational ConsiderationsAn extensive discussion of considerations

for operating a dry kiln is given in chapter 10of the Dry Kiln Operator’s Manual (1961).That chapter presents several aspects of kilnoperation, other than se lec t ion o f theschedule to use, to aid the operator in exercis-ing good judgment. Of particular interest, inview of the greater need to conserve energyand material now than heretofore, are thesuggestions on starting the kiln, restricteduse of steam spray during kiln warm-up, andoperating the kiln after warm-up.

Additional information is given in thishandbook on the important heating, venting,and air circulation equipment, on humiditycontrol with high-pressure steam, and part-time drying.

Heating, Venting, and Air Circulation

Many older kilns were designed for dryingair-dried hardwoods. Usually these kilnscannot dry green hardwoods very effi-ciently—they have inadequate heating,humidification, and circulation systems.Many older kilns have inadequate venting forthe rapid drying of green stock of faster dry-ing species.

It is easy to determine if a kiln is short ofheating capacity—it will take more than 4hours to reach the desired elevated tempera-ture. If there is adequate circulation andother equipment is functioning properly, thekiln manufacturer can add more heating pipeor ducts.

A kiln with inadequate humidification willbe unable to condition lumber adequately—the steam or water spray will be running allthe time but a 10° F depression cannot beobtained. If there is little or no superheat, ifthe vents are closed tightly, if there are noleaks in the structure, and if the spray lineand holes or nozzles are not plugged, a largersteam supply is needed. Increasing only thepressure will increase the superheat and maynot solve the problem.

It is important to have adequate ventingcapacity. Failure to exhaust moist air rapidlywill cause the wet-bulb temperature to behigher than the schedule requires. The resultis slow drying. For some species, like hard

maple, the risk of stain and discoloration isincreased. Restricted use of steam spray dur-ing kiln warm-up is very helpful, but vents areinadequate if desired depressions cannot beeasily obtained a few hours after they are seton the instrument. Venting can be increasedby increasing vent size or by power venting.

Power venting is the discharging of humidair from or the injection of air into a kiln byfans or blowers and suitable duct work in-stalled by the kiln manufacturer. One aim isto increase the venting rate of a kiln when theregular vents cannot conveniently be en-larged. Another aim is to conserve energy. Toconserve energy, care must be taken to placethe power vents so that the humid air is dis-charged before the kiln air passes over theheating coils.

A poor air circulating system is evident by awide range of final moisture content values inthe charge and uneven drying. If air velocitymeasuring instruments are available, poorcirculation will be indicated by velocitiesbelow 200 feet per minute (ft/min) or by dif-ferences between the highest and lowest ve-locities greater than 150 ft/min (measured onthe leaving-air side of the load). The recom-mended kiln schedules are based on a velocitythrough the load of 200 to 400 ft/min, andprobably are as satisfactory with velocities upto 450 ft/min. Any improvement in circulationwill improve heating and venting, however.Increasing the velocities in the early stages ofdrying to 500 ft/min or so will accelerate dry-ing, but will increase the risk of surfacechecking with check-prone species like oak orbeech. This can be compensated for by de-creasing wet-bulb depressions 1° F. Thoroughbaffling to prevent “short circuiting” of airwill greatly improve circulation. Changes infan speed and design usually require advice ofa kiln engineer.

Humidity Control With High-PressureSteam

Some hardwood dry kilns are heated withhigh-pressure steam (80 to 100 pounds persquare inch (lb/in2) or higher). High-pressuresteam is excellent for economy in heating

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coils because of lower initial cost, less finradiation area required, and smaller steamfeed lines and heat control valves than alow-pressure kiln operating at 10 lb/in2. How-ever, the heat in high-pressure steam makesit unsuitable for use in the humidity spraysystem. The steam is so hot and so very drythat it raises the dry-bulb temperature in thekiln excessively when the wet-bulb tempera-ture is raised; consequently, it is impossible tocarry a 4° or 5° F wet-bulb depression duringthe initial stages of some schedules, or to get a10° F wet-bulb depression for conditioning andstress relief at the end of a charge. Modifyingthe conditioning procedure, as described in apreceding section, is helpful if the steam pres-sure is not too high. For steam pressures of 100lb/in2 and over, however, a desuperheater isneeded.

A desuperheater unit supplies low-pressurewet steam, cooled to the saturation point, forthe humidity spray system. A suitablepressure-reducing valve brings steam pres-sure down to approximately 5 lb/in2. This veryhot low-pressure steam is then mixed withwater spray in a suitable mixing chamber,and the extra superheat is absorbed by con-verting some of the water into steam.

Part-Time OperationIn past years some mills and factories gen-

erated their own electricity with steam de-veloped by burning wood waste. Often theyused the exhaust steam to operate the drykilns. The steam was available only duringworking hours, so the kilns were operated ona part-time basis, that is, 8 or 9 hours per day,5 days per week. Other firms burned woodwaste to generate ordinary steam for theirkilns and frequently used coal, oil, or gas dur-ing nonworking hours. Now that fossil fuelcosts have risen greatly, such firms are con-sidering part-time operation for periods ofslack product demand. Such operation alsocould be considered for periods when demand

is higher, if a greater proportion of the woodwere dried from the air-dried instead of thegreen or partly air-dried condition.

Part-time drying is technically feasible o nair-dried stock and can be done successfullywith green or partly air-dried stock whenproper care is used in schedule applicationand operating procedures. Rasmussen (1961)recommended full-time drying during thefirst stages of drying refractory hardwoodswhen excessive checking occurs during part-time operation.

Part-time drying takes almost twice as longto dry an item as does full-time operation.Under the experimental conditions used,Rasmussen and Avanzado (1961) found thatfull-time operation required slightly less totalenergy (4,510 British thermal units per poundof water (Btu/lb H2O) evaporated) than part-time operation, either with fans running allthe time (5,490 Btu/lb H2O) or only duringheat and spray control periods (5,560 Btu/lbH 2O). The material dried was rough 4/4 redoak lumber. Drying quality was the same byall methods. Drying times, from 70 to 7 per-cent moisture content, were 18 days for full-time, 35½ for part-time with fans continuallyrunning, and 46½ days for part-time withrestricted fan use.

In the report on the above described exper-iment, the authors stated that making thekiln schedule more severe for part-time oper-ation would not be expected to reduce dryingtime greatly.

Wolfe (1962, 1963) used temperatures dur-ing the “operating” hours as much as 15° Fabove established kiln schedule temperature.He also used the fans and some ventingduring “off’ hours. He concluded part-timeoperation was economical under regular pro-duction circumstances.

The technology of part-time drying has notbeen well enough established to includeschedules and drying time in this publication.

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Drying TimeThe time required to kiln dry a given

species and thickness depends upon thecharacter of the wood, the type of kiln, andthe kiln schedule used. The time estimatesgiven in table 24 are generally minimumtimes that can be obtained in well-maintainedcommercial kilns with relatively short airtravel. They also are based on the assumptionthat the operator will take some steps to in-crease drying rate. The times will vary a halfto a full day from kiln to kiln. Kilns withlonger air travel, with less than 200 ft/min airvelocity, or in a poor state of maintenance willtake longer in drying green stock. They maynot take much longer than the table valuesfor airdried stock.

By drying to 6 percent average moisturecontent or slightly lower, moisture content

will finalize at about 7 percent after equaliz-ing and conditioning. If a greatly differentmoisture content is desired, proper adjust-ments in conditions must be made, and totaltimes will differ from the table values. Thetime estimates in table 24 are for the dryingof stock for high-quality uses. If considerablymore severe conditions than the basicschedules are used, time will be shortened,but quality may be decreased.

The drying times in the table are for pre-cisely sawed rough green material 11/8 inchesthick, plus or minus 1/8 inch. Miscut lumberwith some of greater thickness will, of course,take longer to dry. For estimates of the effectof other thicknesses on drying time, see thesubsection on Predicting Drying Time underOperational Considerations.

Table 24 .—Approximate kiln-drying times 1 for 4/4 hardwood lumberin conventional internal fan kilns

Species

AppleAsh, blackAsh: green, whiteAspenBasswood (slight browning)Basswood (light color)BeechBirch, paperBirch, yellowBuckeyeButternutCherryChestnutCottonwood, normalCottonwood, wet-streakDogwoodElm: American, slipperyElm: cedar, rockHackberryHickoryHollyHophornbeam, easternLocust, black

Time required to kiln dryfrom

Air-dried 2 Greencondition condition

Days Days4 104 74 103 93 64 95 122 45 123 65 105 104 84 84 105 124 95 134 74 105 126 146 14

Time required to kiln dryfrom

SpeciesAir-dried 2 Greencondition condition

MagnoliaMahoganyMaple: red, silver (soft)Maple: sugar (hard)Oak: red, northern or uplandOak: white, northern or

uplandOak: red or white, southern

lowlandOsage-orangePecanPersimmon, commonSweetgum, heartwood (red

gum)Sweetgum, sapwood (sap

gum)SycamoreTupelo, black (black gum)Tupelo, swampTupelo, waterWalnut, blackWillowYellow-poplar

Days Days4 84 104 75 115 21

5 23

6 (3)6 144 (3)5 12

6 15

4 104 84 85 105 (3)5 114 103 6

1 Approximate times to dry to 6 percent moisture content, prior to equalizing and conditioning, in kilns having airvelocities through the load of 200 to 450 feet per minute.

220 percent moisture content for most woods; 25 percent slow-drying woods like oak, pecan, hickory.3 These items should be air dried before kiln drying,

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Literature Cited

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CHAPTER 6HIGH-TEMPERATURE DRYING

High-temperature kiln drying of wood hasbeen explored for many years. It is similar toconventional kiln drying, but operates attemperatures of 212° F or higher. Technically,high-temperature drying includes two proc-esses. In superheated steam drying, thewet-bulb temperature is maintained at 212° Fand air is excluded. In the other, a mixture ofair and steam is used, and the wet-bulb tem-perature is below 212° F, often with no precisecontrol. Current use of high-temperature dry-ing in the United States involves only thelatter process.

In the last 20 years, high-temperature dry-ing has become acceptable to accelerate thedrying of many softwoods. Most of the newkilns being installed for southern pine lumberin the United States are high-temperaturekilns. What are the prospects for high-temperature drying of hardwoods?

Hardwood high-temperature dryingtechnology has not advanced enough to per-mit recommendations, but the prospects areprobably good enough that anyone planningto install new kilns should consider equip-ment suitable for ultimate use of the process.Drying rates two to five times those of con-ventional methods make high-temperaturedrying very attractive. Because of increaseddrying speed, dry kilns could be smaller thanconventional kilns and still handle the sameannual volume of lumber. Smaller kilns wouldpermit more flexibility in drying (less mixingof species, faster loading and unloading) andrequire less space. Due to faster turnover, in-ventory could be reduced, resulting in savingson interest, insurance, and taxes. Last, but

not least, high-temperature drying would re-quire 25 to 60 percent less energy than con-ventional kiln drying. Because there would besome added capital investment for extra insu-lation, kiln tightness, a larger capacity heat-ing system, and larger capacity fans, directcosts of drying per thousand board feet wouldnot be cut in half, but it has been estimatedthey would decrease by 20 percent or more(Wengert 1972).

High-temperature drying has been com-mercially used in Europe for hardwoods pre-viously air dried. Research in EasternCanada and the United States has indicatedthat the process would be technically feasiblefor air-dried hardwoods of many U.S. species.It is technically applicable to green hard-woods of very permeable species.

Most of the 20 species of hardwoods listed inof high-temperature drying

(Wengert 1972) developed more drying defectsunder high-temperature drying than underconventional temperature drying. The prin-cipal defects were collapse, end checking, andhoneycombing. If dried from the air-driedcondition, such defects were absent or almostso. Another defect that is common to high-temperature drying is discoloration. This isusually a toast-brown color which, with somespecies, is a very thin layer, and with others isnot. While these defects may largely preventthe high-temperature drying from being usedfor highest quality hardwood uses, theyshould not preclude the use of the process forhardwoods suitable for construction purposesand other nonexacting uses.

Basic ConceptsA. In theory the high-temperature kiln dry- sapwoods of low- to medium-density hard-

ing process has three distinct stages when woods are likely to be permeable enoughused on green wood: (1) the constant drying to sustain the first stage very long.rate; (2) the first falling rate; and (3) the sec- When the wood can no longer supply freeond falling rate (Lowery, Krier, and Harm water to the entire surface area, the first fal-1968). In the first stage, moisture moves to- ling rate starts. It continues until the freeward the wood surface predominately by water is gone. The temperature of the wood ismass flow. Rapid evaporation of moisture at between the wet-bulb and the dry-bulb tem-or near the surface keeps the wood tempera- peratures of the kiln. The length of both theture at the wet-bulb temperature. Only the constant and first falling rate periods are

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govered by the permeability of the wood,while the final stage (as well as normal dry-ing) is more influenced by the diffusivity.When the free water is gone, the second fall-ing rate starts. The drying slows con-siderably more, and the wood temperaturerises approximately to the dry-bulb tempera-ture.

B. Both the rate of moisture movementand the rate of evaporation are greatly in-creased by the high temperatures. During thefirst two stages, the rate of drying is deter-mined almost solely by the rate of heat trans-ferred to the wood. Very high air velocitiesare needed to maintain this high rate of heat.transfer as well as to remove the evaporatedwater from the surface.

C. No mater what the relative himidity isin the high-temperature kiln, the kiln condi-tions are severe (EMC cannot exceed 7percent at 230° F). As a result, a high-temperature kiln, after being heated to 212° Fat the start, can be operated without concernfor the wet-bulb temperature until the condi-

tioning. Unless the wood being dried is verywet and very permeable, venting is unneces-sary. This is one of the major energy-savingaspects of the process.

D. After the first stage of drying, steepmoisture gradients can develop; these gra-dients can, in some cases, cause severe dryingstresses and checking. Stresses in uncheckedhigh-temperature-dried wood can be relievedeasily, but stress relief must be done below212° F and with properly equalized material,as is normally required.

E. The equilibrium moisture content ofhigh-temperature-dried wood is somewhatlower than that of conventionally dried wood.

F. The exposure of wet wood to high tem-perature causes both temporary and perma-nent reductions in strength of the wood. Theshortness of the drying time achieved by theuse of high-air velocities in presentday kilnsis an alleviating factor and makes obsoleteresults of some older studies on strength. Thisarea needs further evaluation.

Research and Practice to DateThe first research on high-temperature dry-

ing of hardwoods in the United States, byTiemann (1918), was done with superheatedsteam. Basswood and sweetgum sapwoodwere dried successfully, but other hardwoodswere not. Commercial superheated steamkilns were used for softwoods on the WestCoast about 1918, but they deteriorated sobadly under the severe drying conditions thattheir use was discontinued. Subsequent re-search by Richards (1958) and John L. Hill10

indicated good results when drying a varietyof air-dried southern hardwoods but poor re-sults when drying the same hardwoods fromthe green condition. Their drying equipmenthad no humidity supply. These results andthe likelihood of discoloration, strengthlosses, and other degrading factors discour-aged further research on high-temperaturedrying of hardwoods in the United States atthat time.

Research went on elsewhere, confirmingthe general suitability of the method for air-dried hardwoods and bringing out other fa-vorable aspects of the process. Noteworthy

10 Presented at Forest Products Research Society An-nual Meeting, Madison, Wis., June 1958.

reviews of this research were prepared byKollmann (1961); Lowery, Krier, and Harm(1968); Wengert (1972); and Cech (1973).Kol lman (1961) reported use o f h igh-temperature dryers in many Germanwoodworking plants. These generally usedair-steam mixtures rather than superheatedsteam.

The commercial use of high-temperaturedrying for softwoods began slowly in theUnited States (Kimball and Lowery 1967,Lowery and Kimball 1966). It developedrapidly after tight prefabricated kilns becameavailable and the manufacture of housingstuds became big business. During this de-velopment, rapid heating of the kiln to operat-ing temperature was found advantageousboth to save time and minimize degrade.Meanwhile, research by various inves-tigators, as reviewed by Lowery, Krier, andHarm (1968), indicated some hardwoods couldbe high-temperature-dried from the greencondition if heating up was rapid under satu-rated steam conditions.

A commercial trial of high-temperaturedrying of hardwoods confirmed the applicabil-ity of the process to air-dried sapwood ofsweetgum in which the redgum (heartwood)

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portions are below 25 percent MC.ll The East-ern Forest Products Laboratory of Canadahad done considerable research on high-temperature drying of birch and maple overthe years; Cech (1973) reported a most favor-able procedure was to force-air dry the stockto 20 percent MC, then kiln dry at 212° F.Similar results were obtained with red oakwhen the high-temperature portion was keptat 200° F. Some of the research on birch, how-ever, had shown possibilities for high-temperature drying after very brief airdryingor predrying periods.

In a screening study to determine at whatmoisture level a wide variety of U.S.hardwoods could safely start high-temperature drying, Wengert (1974) heatedthe kiln very rapidly to 200° F. To do this, thesteam spray was used continuously and theheating coils intermittently. Then the tem-perature was increased almost immediatelyto 230° F. Somewhat surprisingly, most of thewood tested tolerated high-temperature dry-ing satisfactorily from the green condition.Red and white oak and sweetgum heartwoodrequired air drying to 25 percent MC, or con-ventional kiln drying to 20 percent MC, beforehigh temperature could be used without caus-ing honeycombing and collapse.

Mackay (1974) designed a high-temperaturedrying procedure for mixed aspen and balsampoplar studs involving a mid-process condi-

11 Presentation by P. Deverick, Fairchild Chair Co.,Lenoir, N. C., to Southeastern Dry Kiln Club, Clyde, N.C.,Nov. 1972.

tioning period. The total time was 4 days andthe studs were dry enough in the outer zonesto pass the 19 percent maximum moisturecontent test using ordinary moisture metertechniques. Follow-up research, however,showed that wet spots still present continuedto dry, producing delayed shrinkage and col-lapse (Mackay 1976).

To solve this problem of delayed shrinkage,several industries drying aspen in the LakeStates have high-temperature-dried for 2days and then gone to an equalization settingat lower temperatures (160° F dry-bulb, 140° Fwet-bulb temperature) until the requiredmoisture contents are obtained. This appearsto be a possible procedure for hardwoods thatdry readily at first but have persistent wetspots.

The success of the experimental high-temperature drying of hardwoods suggeststhat high-temperature drying has potentialfor commercial use in the United States withgreat benefits. Data under semi-commercialconditions are needed, both to determinemaximum safe initial moisture content condi-tions to avoid degrade and to determine pro-cedures acceptable for commercial operation.Also, some method of determining the mois-ture content during drying must be de-veloped. Without such a method, it is likelythat some overdrying or underdrying will re-sult. There also is some need for research toanswer questions on color, strength, andother aspects of quality for the various usesto which hardwoods can be put.

Drying Times and Species PotentialitiesTable 25 lists the U.S. hardwoods for which timates of drying time to 8 percent moisture

high-temperature information is available, content with minimum equalizing and condi-their potential for drying from the green or tioning, and at least one reference for eachthe air-dried condition, some very rough es- wood.

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Table 25 –Eastern hardwood species for which high-temperature kiln-drying potentialhas been indicated, and estimated drying time for 1-inch lumber

Apparent Process RequirementsIf high-temperature drying is to be applied

commercially, the process requirements aresomewhat different from those of softwoods.These requirements should be kept in mindwhen planning to install a high-temperaturekiln for softwoods now and for hardwoodslater on or when installing a hardwood kilnfor conventional drying now which eventuallymay be converted to high-temperature dry-ing.

A. Target dry-bulb temperature should berapidly attained. The usual target is 230° Falthough higher temperatures have beentried.

B. The initial heating medium would de-pend on the material being dried:

(1) Green—steam spray and heated air.(2) Partly air-dried—undetermined at

present.(3) Air-dried—heated air. Dry-bulb and

wet-bulb temperatures should be brought upas close together as possible, with the wet-bulb temperatures raised to 200° F on green

stock. To attain the high wet-bulb tempera-tures needed, vents, wall panels, and doorsmust seal very well.

C. Air temperature should be uniformwhen at high temperatures, with no areabelow boiling, especially on the leaving airside of the load.

D. Air velocity should be high through theload—a minimum of approximately 600 feetper minute with 4-foot-wide loads and of 900feet per minute for wider loads. Research stillin progress is using velocity up to 2,000 feetper minute.

E. Equalizing and conditioning, whennecessary, must be done below 212° F. In thehigh-temperature drying of aspen studs in 96hours (Mackay 1974), an 18-hour period ofhigh humidity at 204° F dry-bulb temperaturewas used before the final 15 hours of drying at250° F. The reader is referred to the chapteron Conventional Kiln Drying for generalequalizing and conditioning procedures.

In view of these process requirements, spe-cial consideration must be given to the kiln.

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The kiln must be able to operate at both nor-mal and high temperatures. Due to the highheat and humidity, masonry structures mayhave a short average l i f e expectancy .Masonry structures would have to be well in-sulated, and the interior wall and ceilingwould have to be well sealed. Cracks will haveto be repaired immediately. Presently avail-able prefabricated kilns with insulatedaluminum-paneled walls and roofs are ex-pected to have long service lives. All corners,joints, floor sills, and door frames must bewell designed and properly installed to pre-vent leakage and undue heat loss. Particularattention should be paid to having tight,well-insulated roofs. Some thought should begiven to a lightweight aggregate to providemore thermally efficient concrete floors.

There must be sufficient heating capacityto rapidly attain target temperatures. Sincethe kiln will require steam for humidification,heat for major operations should be providedby fin-type steam coils. In experimentalhigh-temperature drying of softwoods, extraheat for the heating up period has been pro-vided by a direct firing equipment. Steampressure should be 150 pounds per squareinch.

Air travel length should be 8 feet or less, or16 feet or less in a double-track kiln withbooster coils midway between the loads. Inpackage-loaded kilns, 10 to 12 feet would be

the maximum air travel unless the kiln can beloaded from both sides, with booster coilsalong the middle. To obtain the high air veloc-ities necessary, a direct-connected fan system(rather than a line shaft) is required. Two-speed fan motor operation is desirable whenboth normal and high-temperature dryingwill be done, to save electrical energy. Com-monly available in-kiln fans are designed tooperate safely up to 250° F. Special attentionmust be paid to baffling.

Equal attention should be paid to thehumidification system. Many high-temperature softwood kilns are not equippedwith any humidification system at all, so theyare not suitable for hardwoods without add-ing this feature. A proper system should beable to deliver large quantities of saturatedsteam at low pressure. Steam at high pres-sure suitable for the heating system must gothrough a desuperheater before being usedfor humidification.

If a high-temperature kiln is to be designedfor both hardwoods and softwoods, provisionscan be made for opening the vents on just thehigh-pressure side of the kiln. This helps tomaintain uniform drying conditions. Other-wise, the venting that is necessary when dry-ing a very wet permeable wood will take placethrough leaks, tending to cause the kiln todeteriorate.

Literature Cited

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CHAPTER 7SPECIAL PREDRYING TREATMENTS

Special treatments may be used before or treatments. Under steaming, the postdryingearly in drying to accelerate drying rate, to treatment of collapsed woods also is dis-modify color, or to prevent checks and otherdefects. This chapter deals with steaming, cussed. Presurfacing was discussed earlier

chemical seasoning, polyethylene glycol under acceleration of conventional kiln dry-(PEG) treatment, and surface and other ing.

SteamingTo Accelerate Drying

Early in the study of wood drying at theU.S. Forest Products Laboratory, observa-tions were made that steaming hardwoods be-fore drying sometimes resulted in reductionof drying time. Detrimental effects werenoted, however, when air-dried stock contain-ing surface checks was steamed. The checksdeepened and widened and sometimes be-came bottleneck or honeycomb checks. Thesteaming used at that time was relativelylong. The conclusion was drawn that suchsteaming represented a delay during whichno drying occurred and general use of thepractice was stopped.

More recent studies in a number of schoolsand laboratories have shown a number ofbenefits. The permeability of both softwoodsand hardwoods is increased by short periodsof steaming. Moisture migration rates are in-creased significantly, and drying times arereduced. The development of prefabricatedaluminum kilns has made possible the use ofsteaming in commercial drying operations.

Simpson (1975) reviewed the most signifi-cant research results and investigated theacceleration of drying small specimens ofwood. He tried several species of wood in thegreen condition and steamed at 212° F. Dry-ing rates were increased for northern red oak,cherrybark oak, and sweetgum heartwood.The drying rate at 50 percent moisture con-tent for these small specimens increased 34 to75 percent for the oaks and 11 to 36 percentfor sweetgum heartwood. Steaming timeswere in the range of ½ to 5 hours. For sweet-gum heartwood, the 5-hour period was best.

The drying rate of sweetgum sapwood wasslightly reduced by steaming.

In another study with 1-inch-thick oak,Simpson (1976a) found that the moisture gra-dients, during drying after steaming 4 hoursat 212° F, were smooth curves. The naturalmoisture gradients of the unsteamed controlshad inflections that indicated free watermovement was restricted. Simpson’s workshowed that free water migration from thecenter toward the surface was enhanced bysteaming.

The above results were achieved with satu-rated steam at 212° F, a condition difficult toobtain in commercial kilns. A larger scalestudy used both green and partly air-driedrough 4/4 northern red oak (Simpson 1976b).The lumber was pretreated with nearly satu-rated steam at 185° F for 4 hours. Reductionsin drying time for both classes of lumber wereabout 17 percent. No defects occurred in thegreen lumber nor in one batch of the partlyair-dried material. The other partly air-driedbatch, however, had been severely surfacechecked during air drying. The steaming ap-peared to deepen the surface checks andchange them into bottleneck or honeycombchecks. The change to honeycomb checksconfirms the admonition not to use steamspray during warmup of a kiln charge of fullyair-dried oak. Surface checks may be presentin such oak but not visible.

While the above studies are only explor-atory, they give some prospect for practicallyaccelerating the drying of eastern hardwoodsby presteaming treatments. Additionalstudies are needed to confirm benefits anddetermine limitations, comparative energydemands, and economics.

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To Modify Color

Walnut 12

The most common use of presteamingtreatments is to modify the color of the sap-wood of black walnut. Steaming darkens thesapwood, toning down the contrast between itand the rich brown-colored heartwood andfacilitating the uniform finishing of the wood.It also improves the color of the heartwood,making it and the sapwood more uniform.There is some extraction of coloring matterfrom the heartwood during the process. Theseextractives do not penetrate the sapwood be-neath the surface.

The best steaming results are achieved bytreating green lumber with wet steam in astight a structure as possible at temperaturesthat give the most color in the least time.

12 This section has been prepared by K. C. compton,U.S. Forest Service, State and Private Forestry, Madi-son, Wis.

Nonpressure steaming of walnut is done inspecial vats or buildings with provisions forwet steam from 180° to 215° F. Any structureis suitable so long as it is made of materialsthat will stand up under wet heat up to 215° Fand the acid corrosion of the wood extrac-tives. Steaming times are 24 to 96 hours. Wal-nut should not be steamed in the dry kilnbecause of the time required and the corro-sive effects of steam and volatile extractives.

Three steaming vats or chambers areshown in figures 16 to 18. In figure 16, thefloor is reinforced concrete and the doors aretypical dry-kiln type. The lumber is stackedby forklift trucks on wooden bolsters. Low- tomoderate-pressure steam is introduced byperforated steam pipes in the troughs, whichare filled with water. An alternative is to cir-culate steam in closed pipes submerged in thewater in the troughs. The steam traps, in thiscase, discharge their condensate into thetroughs to keep them full. The capacity of a

Figure 16.—Steaming chamber constructed of concrete block and poured, reinforced concrete: A, Roof is pouredconcrete or prestressed concrete sections; B, apron or loading ramp; C, trough for perforated or imperforated steampipes in water; and D, walls of concrete blocks or poured concrete.

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M 130314

Figure 17.—Steaming vat of wood and sheet aluminum construction (Details courtesy of Hartzell Industries, Inc.,Piqua, Ohio): A, Sheet aluminum lining, with joints sealed with asphaltic materials; B, roof sections of creosotedframe and planks, lined with sheet aluminum; sections are placed by forklift truck as vat is filled; C, trough forperforated steam pipes in water; D, slanted jamb causes door sections to fit securely by weight alone; E, creosotedposts set in ground; and F, creosoted 2- by 8-inch plank wall.

M 130316

Figure 18.—Steaming chamber of poured concrete walls and roof (Details courtesy of Iowa-Missouri Walnut Co., St.Joseph, Mo.): A, drain for condensate; B, steam pipe of 2-inch diameter with ¼-inch perforations; C, concrete blocksturned on sides and imbedded in concrete to hold lumber packages off of floor; and D, apron or loading ramp.

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chamber of the dimensions shown is 40 to 50thousand board feet (M bm) of lumber insolid-pile packages.

In figure 17, the lumber is stacked in the vatin the same manner as in figure 16. The sec-tions of the vat roof are put in place by theforklift as the vat is filled. The front door sec-tions are the same construction as the roofsections. Joints between sections are coveredwith sawdust to reduce steam losses. The ca-pacity of this vat is 16 M bm solid-piledlumber packages.

The walls of the chamber in figure 18 are 10inches thick. The permanent roof is made ofhollow, precast concrete sections coveredwith insulation and built-up roofing. Dry-kilntype insulated doors with double surfaces ofaluminum are used. Steam from a high-pressure boiler is injected directly into thechamber. The capacity of this unit also is16,000 board feet.

In figure 17, all joints of the flashing-weightaluminum sheets were sealed with a high-melt asphalt. Other materials of constructionare possible, including solid wood cribbing,aluminum frame and sheathing, and tile. Alltypes of steam vats or chambers should beprotected by a coating of asphalt or one of thematerials supplied by dry kiln companies forkiln coating. Some walnut steam chambershave trowelled mortar-type coatings similarto those used in patching boiler masonry. Inany event, iron and steel fittings should notbe exposed directly to the steam. Provisionshould be made for gasketing or sealing doorsat tops, sides, and bottoms.

Wet steam is preferred for coloring blackwalnut. Green walnut wood darkens morerapidly and to a greater degree than dry wal-nut, and the more saturated the atmosphere,the less chance for the wood to dry duringsteaming. In some installations, however,high-pressure steam, which is dry and has astrong superheat, is injected directly into thechamber. Apparently drying does not takeplace fast enough to offset the rapid coloringeffect from the higher temperature. Dryingdefects customarily do not develop. Underthis procedure, temperatures in the range of215° to 225° F are attained.

Walnut steaming chambers preferably areequipped with recording thermometers, andsome have temperature controllers also.

There are no fans in steaming chambers.Steaming has also been done commercially

under pressure. Brauner and Conway (1964)developed the optimum conditions experi-mentally. Then they settled on steaming at 6pounds per square inch pressure and 230° Ffor 5 hours. A longer time is needed in thewinter. This procedure not only darkens thesapwood; the heartwood loses its purplishcast and become chocolate brown. Althoughthe coloration is rapid and time saving, thelumber must be cooled in the pressure vesselor end checking and honeycombing occur. Analternative is to take the load out of the retortand cover the wood with a tarp until cool.Millions of board feet of walnut have beensteamed at the Conway plant using a 7- by 8-by 20-foot pressure vessel. At another loca-tion a suitable pressure steamer was made byadapting a preservative treating cylinderdoor to the end of an old tank car.Generally, 25 to 35 boiler horsepower areneeded for a walnut steaming installation.

Steaming walnut may increase its dryingrate, but there is no conclusive proof of this atthis time.Other Woods

European beech lumber and dimensionitems are frequently steamed to give them areddish-brown tone that enhances their colorwhen finished appropriately. Beech steamingis not generally done in the United States, butpresumably it could be, if desired. Steam hasbeen used, however, to give oak a gothicbrown color. Ordinary kiln-drying conditionscan be manipulated to give sugar maple areddish brown color (McMillen 1976) but pre-sumably the same could be done by a shortsteaming period followed by customary dry-ing. Sweetgum sapwood was steamed beforeair drying, at one time, to sterilize the wood,promote rapid air drying, and avoid bluestaining. No record was made of color effectsfrom such steaming.

To Recover Collapsed WoodNormal eastern hardwoods ordinarily do

not collapse badly during kiln drying. Some ofthe western hardwoods do. There is some evi-dence, however, that excessive shrinkage insome oaks involves collapse. Collapse is tech-nically excessive shrinkage and distortion ofindividual wood cells in various zones of the

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wood. In a collapse-prone wood, the external at about 18 percent moisture content, steam-effect of collapse is a “washboard-type” sur- ing them for 12 to 24 hours in a chamber likeface. that used for walnut, and then finishing the

In Australia, where collapse-prone species drying in a kiln with noncollapsed wood.are common, recovery is accomplished by tak- Steaming is done with saturated steam ating the collapsed boards out of a kiln charge 212° F.

Chemical SeasoningChemical seasoning consists of treating

green wood with a hydroscopic chemical andair drying or kiln drying the treated material.The chemical reduces surface checking dur-ing seasoning, rather than speeding the dry-ing.

The objective is to impregnate the outerzone of lumber with chemicals to a depth ofabout one-tenth of the thickness, with thehighest concentration at or near the surface.The chemicals maintain the outer zone at ahigh moisture content during early stages ofdrying. This reduces the shrinkage of theouter zone and lessens the tendency to sur-face check. Some chemicals additionally im-part a certain degree of bulking. Kilnschedules must be modified somewhat tobring the initial relative humidity below therelative humidity in equilibrium with thesaturated chemical solution.

Numerous chemicals have been used(MCMillen 1960). Common salt is cheap andeffective in reducing surface checking. Urea,

which was effective with Douglas-fir, was notas effective as salt on oak. Other chemicalsincluded invert sugar, molasses, diethyleneglycol, and a urea-formaldehyde mixture. Aproprietary salt mixture having corrosion in-hibitors in it was popular for a time.

The proprietary chemical, as well as com-mon salt, can corrode metals and damagedry-kiln equipment, woodworking machinery,and hardware fastened to the treated wood ifthe amount of treating chemical is excessiveor the treatment time too long. Salt-treatedwood, regardless of care in treatment, willcorrode metals in contact with it in regions ofprolonged high humidity such as the Gulf ofMexico and South Atlantic coasts. Salt alsocan reduce the strength of wood and causeproblems in gluing and finishing. Althoughsalt has been and continues to be used suc-cessfully in the seasoning of thick southernhardwoods, considerable care in treatment,drying, and use is recommended.

Polyethylene Glycol ProcessGreen wood heavily treated with PEG

(polyethylene glycol-1000) retains its greendimension during drying and indefinitely;thus the wood is permanently restrained fromshrinking, swelling, or warping regardless ofatmospheric humidity. For maximum dimen-sional stability, PEG must be diffused deeplyinto the wood in amounts of 25 to 30 percent ofthe dry weight of the wood. Two solutionscommonly used are 30 and 50 percent PEG byweight. Heating the solution during treat-

ments speeds up the diffusion, but soakingtimes range from 3 to 30 days. Kiln dryingafter treatment can in many cases be muchmore drastic than normal. The process isespecially helpful for hardwood tree and limbcross sections, thick novelty items, and carv-ings and material with highly irregular grain.Details of the process, which is relatively ex-pensive, have been described by Mitchell(1972).

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Surface Treatments to Prevent CheckingIn addition to presurfacing, there are cer-

tain materials that can be applied to thewood’s surface to retard checking. Chemicalsused successfully are wax, sodium alginate,and a salt paste. These materials either re-tard moisture movement or alter the vaporpressure at the surface. No details are avail-able on use of waxes, but a thick emulsion ofmicrocrystalline wax has been applied to thesides of highly figured gunstocks in Californiato prevent checking.

Sodium AlginateIn Australia, Harrison (1968) investigated

the use of very viscous sodium alginate solu-tions or emulsions as dip treatments on a va-riety of hardwoods up to 2 inches thick. Whenthe lumber is air dried, the alginate dries outto form a porous skin over the surface. It iseffective in preventing checking in all of thewoods tried under severe air drying condi-tions. The method has not been tried in theUnited States but might have some benefitfor thick oak.

Sodium alginate is a dry powder obtainedfrom seaweed and is used in a variety of prod-ucts in the United States. Considerable caremust be used in mixing it to form the 1½percent solution found most effective in Au-stralia. The wood must be still quite green forthe treatment to be effective in preventingchecking. The air-drying piles must be care-fully roofed to keep the alginate from beingwashed off by rain. For 2-inch stock, a dryingshed is necessary to protect the treatingchemical and provide mild conditions for airdrying.

Salt Paste

The U.S. Bicentennial celebration inspiredwidespread interest in seasoning disks or

thick sections of large trees. The disks weredesired for exhibits or usable items on whichthe chronology of important events could beshown. It is very difficult to season such diskssuccessfully with even the least-difficult-to-dry species. The most damaging defect is thelarge V-shaped check that is likely to developbecause tangential shrinkage is usually muchgreater than radial shrinkage. In addition,many small end checks tend to appear overthe entire surface. Polyethylene glycol haspromise, but is slow, costly, and the tempta-tion always exists to terminate treatment be-fore all wood cell walls are fully saturatedwith PEG.

A simple and inexpensive method de-veloped by W. T. Simpson and J. L. Tschernitzof the U.S. Forest Products Laboratory issuccessful enough to merit its inclusion inthis handbook. It is a thick paste modificationof the old “salt seasoning” method, combiningsome bulking of the surface zone with reten-tion of moisture at the disk surface by hy-droscopic action. The bark should be left onthe disk. Before applying the paste, the sur-faces of the green disk are alternativelybrushed with a concentrated table saltsolution—3 pounds salt per gallon of water,with excess salt crystals visible in the solu-tion after thorough mixing. Several hours areallowed for the salt to penetrate by diffusionand gravity.

Then the wood is treated with the paste. Tomake the paste, the concentrated solutionabove is mixed with enough cornstarch to getthe right consistency to build up a thick layeron the disk surface. The addition of severalegg whites acts as a convenient binder to re-duce the flaking of the paste after it dries. Thetreated disks can be air dried in a room withplenty of ventilation or kiln dried using amoderate schedule.

Other PretreatmentsPrecompression thickness compression of 7 to 8.5 percent in a

roller device resulted in greatly reduced col-Cech (1971) has established that dynamic lapse and honeycombing compared with un-

transverse compression of 2-inch yellow birch compressed material. Drying time was only 81/3lumber, before drying by a severe schedule, days compared with a customary 18 days forsignificantly improved drying behavior. The noncompressed material dried by conven-drying was carried out at 215° F. Momentary tional schedules.

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Chech and Pfaff (1975) also used a 7.5 percentprecom pression with both conventional andmild accelerated kiln schedules on 4/4 red oak.Precompression resulted in about a 5 percentsaving in drying time.Precom pression has not been acceptedcommerically, at least not on any large scale,but experimental results to date appear tomerit more study.

Prefreezing

species for which decreased shrinkage hasbeen found is black tupelo (blackgum). Thebest prefreezing temperature for black wal-nut is –100° F, but significant improvement isfound at –10° F, a temperature readily at-tained in commercial freezing equipment. Afreezing time of 24 hours is adequate forthicknesses up to 3 inches. A similar length oftime has been used for thawing.

Although prefreezing would require sub-stantial additional investment, a shorteningof drying time to half of the time required bysome of the industry has been demonstratedfor walnut gunstock blanks (Cooper, Bois, andErickson 1976). Using a slightly acceleratedkiln schedule, 600 prefrozen gunstock blanksdried from green in 103 days had only 2.66percent defective from collapse, checking, andwarp while 6.50 percent of the 400 unfrozenblanks in the pilot test had similar defects.

Thus precompression and prefreezing aretwo treatments that have some promise ofimproving the drying rate of hardwoods with-out decreasing quality.

The freezing of green wood, followed bythawing before the start of drying, is anothertreatement that increases the drying rate anddecreases shrinkage and seasoning defects insome species. Most research on hardwoodshas been done with black walnut (Cooper,Erickson, and Haygreen 1970). The effect onshrinkage appears to be a good indicator ofseasoning improvement, and favorable re-sults have been attained with black cherry,American elm, and white oak (Cooper andBarham 1972). Another eastern hardwood

Literature Cited

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CHAPTER 8OTHER METHODS OF DRYING

Heated-Room DryingIn heated-room drying, a small amount of

heat is used to lower the relative humidity.This also lowers the equilibrium moisturecontent (E MC) to which the wood will come ifleft in the room a long time. This method issuitable only for wood that has been air driedfirst. Green lumber may check and split bythis method. This method does not dry lumberrapidly, but it is suitable for small amounts oflumber.

Before air drying is started, the lumbershould be cut as close as possible to the size itwill have in the product. Allowance must bemade for some shrinkage and warping duringdrying and for a small amount to be removedduring planing and machining. If it is neces-sary to shorten some long pieces of air-driedlumber before heated-room drying is started,the freshly cut ends should be end coated toprevent end checks, splits, and honeycomb.

For reasonably fast heated-room drying,the wood should be exposed to an EMC about2 percent below the moisture content of use.The wood is left in the room just long enoughto come to the desired average moisture con-tent. Then the wood is taken out and stored ina solid pile until it is used. The storage areashould have the same EMC as the area inwhich the wood will be used.

The amount that the temperature must beraised above the average outdoor tempera-ture depends upon the average outdoor rela-tive humidity. Typical values are given intable 26. Do not attempt to use more heatingwith this method.

Any ordinary room or shed can be used andany ordinary means of heating the room

Table 26. —Amount temperature must beraised above average outdoor temperaturefor various equilibrium moisture content

values in heated-room drying

EMC valuedesired

Degrees above average outdoortemperature at—

70 pct RH 75 pct RH

Percent456789

10

°F383123181310

6

° F403325201512

8

80 pct RH

° F42352723171410

should be satisfactory. A slight amount of aircirculation is desirable to achieve tempera-ture uniformity. If the material is relativelysmall-sized, it can be piled in small, stickeredpiles on a strong floor. It also could be stickerpiled on carts that can be pushed in and out ofthe room. Long lumber should be box piled onstrong, raised supports as shown in figure 19.

The lumber should be marked or recordskept as to when it entered the room and whento expect to remove it. Any amount of air-drylumber can be put in or dried lumber removedwithout upsetting conditions. The doorsshould be kept closed as much as possible sothat the average temperature can be keptclose to the desired elevation above averageoutdoor temperature. Variations of the aver-age outdoor temperature from night to dayand small variations from day to day do notaffect the process.

Dehumidifier DryingConsiderable interest has been expressed in Research Laboratory (Pratt 1968) indicated

dehumidifier-type kilns developed in Eng- such dryers were not operated at tempera-land, Norway, Germany, and Italy. These tures over 130° F. Drying of hardwoods to 7dryers have been used successfully for percent moisture content requires use ofhardwoods in Europe where kiln schedules higher temperatures to be practical.are generally conservative. In these loca- The dehumidifier dryer uses electricaltions, the desired final moisture content refrigerator-type equipment in an arrange-usually is not below 9 percent. Technical in- ment favorable to the drying of wood andformation from the British Forest Products other materials. In a closed building or room,

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Figure 19.—Lumber piled

moist air from the lumber is drawn over re-frigerated coils. The air is cooled below itsdew point and part of the moisture is con-densed. The water flows to a tray in the baseof the unit and is drained out of the kiln. Theair circulates past the condenser unit of thesystem and picks up the latent heat of vapori-zation. Then the air is blown by the fansthrough the lumber. In some installations,however, the dehumidifier is located outsidethe drying chamber and the latent heat ben-efit is not realized.

The relative humidity generally is lowerthan can be obtained at the same tempera-ture in a conventional kiln vented to the air.Electrical resistance heaters, hot water coils,or steam coils can be used to augment theheat from the condenser. Brief coil-heatingperiods can be used to defrost the coils, whennecessary, and “interruptions” of drying canbe inserted in the schedule to minimize dry-ing stresses.

M 144690for drying in a heated room.

Advantages claimed by manufacturers, andborne out to some extent by published data,are:

Capital investment is relatively low.Operating costs are not exorbitant.Drying time to 12 percent moisture con-

tent is not unduly prolonged for woodsnormally dried at low temperatures.

The equipment lends itself readily to au-tomatic (time-based) programming sothe dryers can be operated by relativelyunskilled personnel with little dangerof overd rying.

Mixed species and thicknesses can bedried, within certain limitations, in thesame dryer.

Quality of dried material, when properschedules are used, is better than thatof material air dried under somewhatadverse conditions.

The amounts of shrinkage and warpingare low.

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At the low temperatures generally used,personnel can move in and out of the.dryer to insert more material or removedried wood as needed.

Many of these advantages are also found inheated-room drying, but heated-room dryingis best restricted to air-dried material. De-humidifier dryers can be scheduled to safely

dry refractory woods from the green condi-tion. In their 1976 stage of development theyapparently had no provision for conditioningto relieve drying stresses. The relative eco-nomics of careful air drying and heated-roomeconomics of careful air drying and heated-room drying compared with dehumidifier dry-ing have not been investigated.

Solar DryingThe changed world fuel situation has re-

newed interest in use of solar energy to drylumber. Research in small experimental andsemi-commercial solar dryers showed thathardwoods can be satisfactorily dried fromthe green to the air-dried condition. Dryingtimes are about 50 to 75 percent of the air-drying times. Hardwoods can be dried furtherto 10 percent moisture content, but the timerequired is long compared with kiln drying.

Peck (1962), using a wood-framed rectangu-lar dryer with double-layer transparent plas-tic film walls and roof in Madison, Wis., dried4/4 northern red oak from 75 to 20 percentmoisture content in 23 to 105 days starting indifferent months of the year. Drying timesfrom 20 to 10 percent MC were 25 days for aperiod starting in July and 42 days starting inSeptember. A 24-inch fan driven by a5/8-horsepower electric motor circulated theair during daylight hours. The dryer roof,which essentially was the major solarenergy-collecting surface, had 95 square feetof surface; the south wall had 17 square feet.The dryer held 425 board feet of lumber. Av-erage temperatures within the dryer duringdrying operations were 13° to 19° F above out-door temperatures.

Additional research on solar drying of woodhas gone on around the world using equip-ment varying from very small cabinets inIndia to semi-commercial units holding about3,000 board feet in Uganda. Plumbtre (1973)discussed his own research with the Ugandankilns and reviewed world literature on thesubject in a United Nations report.

Starting with a design supplied by theForest Products Laboratory, the Institute ofTropical Forestry, Rio Piedras, Puerto Rico,constructed a solar dryer initially holding2,000 board feet of 1-inch lumber and laterexpanded it to 3,000 board feet (Chudnoff,Maldonado, and Goytia 1966). The wood-framed building, 10 feet wide and 20 feet long,

was oriented east and west. The sloping roofangled 16° from the horizontal, the south wailwas 9¾ feet high, and the north wall 13-1/3feet. Originally the roof, ends, and south sidewere covered with two layers of plastic filmseparated by a 15/8 inch air space. The northside, which contained the loading door, wasplywood. Due to early film deterioration, theroof panels were replaced with single-layerwindow glass.

Sixteen-inch fans circulated the air over ablack-painted heat absorber under the roofand through the lumber. Vents, with louverdampers, provided ventilation. The damperswere kept closed during the early part of thedrying to prevent drying defects. Later in thecycle the vents were opened. At the end ofdrying they were closed again, and a water-spray was used to relieve drying stresses.

In this tropical location, 4/4 mahoganylumber was dried from 50 to 20 percent mois-ture content in 9 days, 5/4 in 13 days, and 8/4in 23 days. Drying from 20 to 12 percent took9, 12, and 28 more days, respectively. Mixed5/4 hardwoods dried from 60 to 20 percentmoisture content in 27 days and from 20 to 12percent in 15 more days. Time of year made nopractical difference in drying time for eitherthe mahogany or the mixed hardwoods.

Johnson (1961) built a small homemadesolar kiln in southwestern Wisconsin to drysmall quantities of hardwood lumber for hisown use. Solar heat was collected by single-thickness windows on the south facing wall ofan A-frame structure. A ventilating slot wasbuilt in the wall below the windows. A wind-powered centrifugal blower drew air up over aheat-absorbing metal sheet behind the glasswindows and then forced the air downthrough the lumber edge-piled on a rack.One-inch cherry and white oak were dried to10 percent moisture content in 2 to 6 weeks.

Johnson has continued to dry lumber in adryer of slightly modified design (Bois 1977).

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The lumber is piled flat. An electric motor isused to drive the fan, but only when the tem-perature is above 85° F. The air is forced downinto a central flue, then horizontally throughthe lumber pile. He estimates that it takes 400hours of sunshine (70 “good” days) to dryhardwood 4/4 lumber from green to 10 percentmoisture content.

While this method of drying is feasible forsmall quantities of lumber for hobby use, dry-ing times are affected greatly by season of theyear and the location. The relative suitabilityfor solar energy collection the year round isonly fair in Northeastern United States fromMaine to central Wisconsin, including NewEngland, New York, and Pennsylvania, andonly good in most of the rest of the EasternUnited States (Crowther 1976). In general, itappears that heated-room drying is a morepractical way of drying small quantities ofhardwoods as long as fuel costs are no morethan double what they are now.

The experimental and semi-commercial

PressPress drying (Hittmeier, Comstock, and

Harm 1968) is defined as the application ofheat to opposite faces of a board by heatedplatens to evaporate moisture from the board.Temperatures range from 250° to 450° F.Thermal contact between the heated platensand the board face is maintained by platenpressure of 25 to 75 pounds per square inch.

During drying, heat is transferred from theplatens to the wood, causing air and vapor inthe wood to expand and water to vaporize. Amixture of vapor and liquid then moves to thesurface of the board where it escapes. Venti-lated cauls located above and below the boardsometimes have been used to help the vaporescape. The platens also hold the board flat

solar dryers first described are essentiallyflat-plate radiation collectors and their majorcollecting surface is the roof. They are rela-tively inefficient (Wengert 1971). Thus theamount of lumber that can be satisfactorilydried in a dryer of this type is limited. Suchdryers do not appear practical for moderate tolarge commercial hardwood operations in theEastern United States.

Solar kilns do, however, have current po-tential in developing tropical countries wherefuel costs are extremely high. Solar kiln de-sign research is continuing with emphasis onseparate solar collectors and heat storageunits. Read, Choda, and Cooper (1974) de-scribe a kiln built in Australia. The U.S.Forest Products Laboratory is developing adesign for the Philippines. Rising fuel costsand improvements in solar energy technologymay ultimately bring solar kilns into generaluse for commercial hardwood drying in theUnited States, but this appears to be some-thing for the future.

Dryingand reduce width shrinkage; however, platenpressure and high temperatures generallylead to greater than normal shrinkage inthickness. The heartwood of some species ofwood tends to darken and develop checks andhoneycombing during press drying. These de-fects, however, may not adversely affect theboard for many applications, and the darkercolor is often more appealing than the origi-nal color.

The process is particularly suitable for½-inch and thinner boards of species highlypermeable to moisture. It is being used forbeech for flooring in Europe and is being con-sidered for a variety of purposes in the UnitedStates.

High-Frequency and Microwave HeatingHigh-frequency heating involves both

technical and cost problems. In operation,wood is placed in a powerful electric field os-cillating at more than 1 million cycles per sec-ond and heated quickly to a temperatureabove the boiling point of water. If there wereno resistance to the movement of free wateror vapor through a wood, it could be driedrapidly. In general, however, wood is not very

permeable. Some species have fairly perme-able sapwood, but others have a tight struc-ture. Moisture movement may be so impededthat high internal pressures and tempera-tures well above the boiling point of water arebuilt up. Local explosions or ruptures mayoccur. In permeable woods, temperaturelevels off slightly above the boiling point aslong as free water is present. When only

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bound water remains, the temperature rises.Prolonged high temperatures weaken woodand lower its resistance to internal pressures.

Apart from technical difficulties, high-frequency heating is generally too expensive.The main costs are for high-frequencygenerators, power tube replacement, andelectricity. A minimum estimate of the elec-tricity costs in drying 10,000 board feet ofgreen beech, birch, or maple sapwood from 70percent to 8 percent moisture content in 24hours follows:

To heat the lumber to drying temperatureand remove the calculated 764 pounds ofwater per hour requires 263 kilowatts. Be-cause a high-frequency generator is onlyabout 50 percent efficient and a certainamount of heat would be lost, the power linewould have to deliver at least 600 kilowatts to

the generator. At 3 cents per kilowatt hour,the cost would be $43.20 per 1,000 board feet oflumber.

In view of the above, high-frequency dielec-tric heating is not practical for the generaldrying of heartwood. It has been used for dry-ing persimmon sapwood golf club heads. Acompany in New England tried the methodfor drying turning squares of birch andmaple, then abandoned it.

Although not for eastern hardwoods, mi-crowave heating has been tried for the season-ing of tanoak baseball bats in Oregon. Theprinciples and constraints are similar to thoseof high-frequency drying. Although thetanoak was believed to be principally sap-wood, degrade was high (20 to 30 pct) andthere was considerable collapse; thus themethod was abandoned.

Solvent SeasoningThe solvent seasoning process involves sub- While this method has not been applied to

jetting the wood to a spray or continuous im- eastern hardwoods, extensive research hasmersion of hot acetone, or a similar solvent been done in California on drying tanoakmiscible with water, for a number of hoursuntil most of the water is extracted from the sapwood by this method. One-inch lumber has

wood. Then the solvent is removed bv steam- been dried in 30 hours. A few boards suffered

ing or vacuuming. Additional water is re- streaks of collapse, probably because of themoved at the same time. presence of heartwood.

Minor Special MethodsWood can be rapidly dried in an oily liquid

maintained at a temperature high enough toboil off the water. A complicated variation ofthe method is called azeotropic drying. Re-cent research (Eckelman and Galezewski1970, and Huffman, Pfaff, and Shah 1972) hasdemonstrated that the process does not havegood prospects for drying hardwoods unless aconsiderably reduced pressure (partial vac-uum) is used. Although the simple boiling-in-oil process is not suitable for hardwoodsbecause of checking, a review of the details(MCMillen 1961) would be valuable to anyoneconsidering such use.

Vapor drying exposes the wood to the va-pors of a boiling organic chemical such as in apressure-treating cylinder and removed themixed chemical and water vapors (MCMillen

1961). This method has had considerable usein drying oak, hickory, and other hardwoodrailroad crossties before preservative treat-ment. It would also be suitable for crossingplank and car decking, but is not suitable forhardwoods to be used at low moisture contentvalues for fine purposes.

Infrared radiation has been proposed in thepast for drying wood, but the depth of raypenetration is slight and the method is notsuitable for drying hardwood lumber. Vac-uum drying and freeze-vacuum drying are ofno practical value for hardwoods because anyheat available is quickly used up in evapora-tion. Patents have been granted on combiningheated-platen or high-frequency heating withvacuum drying, but have not been commer-cially applied in the United States.

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Literature Cited

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CHAPTER 9STORAGE OF DRIED LUMBER

Lumber dried to a moisture content of 10percent or less, and items manufactured fromit, will regain moisture if stored for extendedperiods under conditions of high relativehumidity. Excessive regain of moisture fre-quently results in (1) swelling of whole piecesor of certain parts, such as the ends of the

Air-DriedWhen lumber is as dry as can be expected

on the air-drying yard and at least below 20percent moisture content, it should be storedwhere it is no longer exposed to wind, sun,and rain. T’he deteriorating effect of weather-ing continues as long as lumber is left out-doors. When stickered lumber is taken downand solid stacked, it takes up less space and ashed can provide the needed protection. Whenyard space is needed for air drying morelumber, and no adequate shed is available,less favorable drying yard areas may be setaside for bulk storage of fully air-driedlumber. Such yard-stored lumber must befully protected, however.

If good, rain-tight pile roofs with adequateprojections were used during air drying or ifthe lumber was air dried in a shed and the airdrying space is not needed, the lumber doesnot need to be solid piled. The lumber will notgo above 20 percent moisture content againunless it is rewet by rain. Dry, stickeredlumber should be inspected regularly, how-ever, to avoid insect attack in areas of infes-tation.

Kiln-DriedAfter lumber has been kiln dried, it is often

placed in a storage shed or room, unstickered,until ready for use. The ends of the boards, ifnot end coated, will start to pick up moistureimmediately. If storage periods are short (upto 1 month) and the storage building is kept

pieces; (2) warping of items, glued-up panelsfor example; and (3) wood or glueline failuresin solid-piled items where the moisture regainis confined to the ends. Hence, improper stor-age can negate many achievements of properdrying.

LumberDry lumber that is solid piled outdoors

should be wrapped with tarpaulins or en-closed in prefabricated waterproof-papershrouds. Rain wetting of any dried material isdetrimental and tends to increase the degreeof checking. Surface layers can take on acompression set that will cause any checks tostay open when the wood is finally kiln dried.If the amount of rain wetting is great enough,the moisture content can go above 20 percentand the wood again would be subject to stainand decay. Rain that penetrates into a solidpile does not evaporate easily.

Foundations in the storage area shouldprovide good support and ground clearancejust as in the original air-drying pile. Supportis still essential to prevent pile sagging.Ground clearance is not only needed to pro-vide room for forklift operation but also forventilation to keep moisture regain in thebottom layers to a minimum. If the watertable in the ground is high, a ground cover toprovide a barrier to moisture vapor move-ment out of the soil may be desirable, particu-larly if the storage period is extended.

Lumber(EMC) condition. In some arid or semiarid re-gions of the United States, such as Arizona,New Mexico, and southern California, un-heated dry storage will hold dried lumber at alow EMC. However, in the rest of the country,unheated dry storage will not prevent kiln-

closed most of the time, little change in mois- dried lumber from gradually increasing inture content will occur. However, in many moisture content.cases dry lumber is stored for several months The major effects of different wood storageor longer in buildings not designed to main- conditions on kiln-dried wood are discussedtain a fixed equilibrium moisture content below.

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Open ShedsAn open shed that consists of a roof with no

side or end walls, or a shed open on one ormore sides, can be a dry storage shed for air-dried lumber awaiting sale, kiln drying, or usein the air-dried condition. However, kiln-driedand well-conditioned lumber destined for fur-niture or other high-grade use will deteri-orate in moisture quality if stored in such ashed.

Unheated Closed ShedsKiln-dried lumber stored in an unheated

closed shed will ordinarily absorb some mois-ture. Moisture is absorbed because the mois-ture content of the wood is usually lower thanthe EMC corresponding to the atmospherewithin the shed. Outside weather conditionsheavily affect moisture absorption by lumberstored in an unheated shed. While appreci-able moisture gains in such sheds cannot betolerated for uses such as furniture, this typeof storage is being successfully used in someareas for less demanding end uses. If storageis in an unheated shed, use the lumberpromptly.

The unheated shed should be located on adry, well-drained site with a concrete or as-phalt floor if possible. Also, the shed should beventilated by adjustable openings in the roofand walls. Never use an earthen floor if theshed stands on a low, damp site. Shed doorsshould be closed at night and as much as pos-sible on rainy or foggy days.

Heated Sheds

The efficiency of a closed shed in maintain-ing a low moisture content in lumber is en-hanced if the air can be heated as required tomaintain the desired EMC. Only a smallamount of heat is needed to raise the temper-ature enough above the average outdoortemperature to keep the EMC in the 6 to 11percent range. The following temperaturecrease values are typical:

Amount aboveDesired EMC outside temperature

(Percent) (° F)6 257 208 159 12

10 811 5

in-

Heat can be supplied to a closed storageshed by steam coils, radiators, or unit heat-ers. The system need not have a large capac-ity, but it should be arranged so that thetemperature throughout the shed is rea-sonably uniform. A minimum temperature of35° F is suggested for a steam-heated storagearea, to prevent the return lines and trapsfrom freezing. The heat supply may be con-trolled by an ordinary thermostat or by amore precise automatic device.

To use a thermostat, estimate or employWeather Service data to obtain the expectedmean outdoor temperature for the next 4 to 7days. Then set the thermostat the number ofdegrees higher that is needed to maintain thedesired EMC. Good air circulation is needed tomaintain temperature and EMC uniformity.This requires the use of fans.

If the heat is to be controlled automatically,either a hygrostat or a differential thermo-stat may be used. A hygrostat maintains agiven EMC by turning the heat on or off asneeded. As the relative humidity in the shedincreases, the hydroscopic element absorbsmoisture and swells. The swelling activates amechanism that turns on the heat. When therelative humidity decreases, the process isreversed.

Differential thermostats can be preset tomaintain temperature in the shed that is aspecific amount above the outside tempera-ture. The differential thermostat is very de-pendable, economical, easily installed, andthe maintenance cost is low.

Dehumidified Rooms

Electric dehumidifiers can be used in small,well-enclosed spaces. Some temperature con-trol should be maintained, but it does nothave to be precise. EMC tables should be usedto select the proper relative humidity for thedesired average moisture content. The de-humidifier can be turned on and off by aproperly calibrated humidistat. De-humidifiers are not as economical as heatedstorage sheds for large quantities of lumber,but large storage sheds have been de-humidified for corrosion control on precisemetal parts during war time.

-75-

CHAPTER 10ECONOMICS AND ENERGY

The cost of drying hardwoods normally in-cludes the cost of stacking, handling, air dry-ing, and kiln drying. Degrade cost should beassigned to each of the drying phases, butoften these costs are overlooked.

As wood drying involves the evaporation oflarge quantities of water, energy can be avery important element of total cost. If fuel isto be conserved, drying procedures haveother implications besides cost. Traditionally,the lowest cost and the lowest energy usagein hardwood lumber production has beenachieved by combining air drying and kilndrying. Accelerated-air drying is an attrac-tive alternative to air drying when produc-

tion time is at a premium, when the goodair-drying season is short, when good land foran air-drying site is not available, or whenclose inventory control is necessary. The cur-rent high cost of energy makes these alterna-tive approaches worth considering.

The information in this chapter cannotserve as a complete analysis of the entire costand energy situation, but it is intended toprovide guidance for those who wish to makea current analysis of their own operation.Professional accountants and engineers canprovide a more thorough analysis. Stateforestry personnel and extension offices areoccasionally equipped to provide assistance.

Typical Costs for Various Drying ProceduresComparative air drying and low tempera-

ture forced-air drying costs were given byCatterick (1970) for northeastern hardwoods,based on a mill with a production of 2 millionboard feet per year. These values have beenaugmented and updated to reflect January1976 costs and have incorporated data ofCuppett and Craft (1971). Kiln drying costs, incustom drying, as of 1970 also have beenaugmented and updated to January 1976.These updated comparative costs are as fol-lows:

Air-Drying Costs($ per M bm 13)

Average drying time3 months 6 months

Land, taxes,maintenance, $1.05 (16 $1.20 (11etc. pct) pct)

Interest (8pct) on 4.00 (63 8.00 (74inventory pct) pct)

Taxes on .35 (5 .65 (6lumber pct) pct)

Insurance 1.00 (16 1.00 (9pct) pct)

TOTAL $6.40 per $10.85 perM bm M bm

Low-Temperature-Drying Cost($ per M bm)

Average drying time10 days 20 days

Land, taxes,maintenance,building, $3.50 (21 $4.50 (23boiler pct) pct)

Interest (8pct) on inven-tory includes10-day .90 (6 1.30 (7storage) pct) pct)

Taxes on .05 (0 .10 (1lumber pct) pct)

Insurance .10 (1 .15 (1pct) pct)

Fuel and 11.00 (67 13.00 (66electricity pct) pct)

Labor .75 (5 .75 (4pct) pct)

TOTAL $16.30 per $19.80 perM bm M bm

These low-temperature costs apply towinter-time operation in north to central lo-cations. Fuel costs would be lower duringsummer operation.

13 Thousand board feet.

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Kiln-Drying Cost 14

($ per M bm)Kiln drying time

5-7 days 10-15 daysEquipment depre- $1.40 (6 $2.80 (8

ciation pct) pct)Insurance .80 (3 .80 (2

pct) pct)Maintenance 1.95 (8 1.95 (6

pct) pct)Supplies .70 (3 .70 (2

pct) pct)Sticker re- .50 (2 .50 (1

placement pct) pct)Steam and 8.40 (35 16.80 (48

electricity pct) pct)Labor 7.95 (33 7.95 (23

pct) pct)Land and 1.30 (5 2.00 (6

building pct) pct)Interest (8

pct) on inven-tory (includes10 day .80 (3 1.50 (4storage) pct) pct)

TOTAL $23.80 per $35.00 perM bm M bm

14 Presentation by E. M. Conway, Conway Corp., GrandRapids, Mich., to Joint Meeting Midwest andWisconsin-Michigan Wood Seasoning Association, Anti-go, Wis., May 1970.

General company selling and administra-tive expenses and degrade costs are not in-cluded in the above data. Degrade costs arediscussed in a following section.

Handling and stacking costs are often in-cluded with drying costs, but they are notincluded in the estimates above. Conway hasestimated handling and stacking costs in ahardwood air-drying and kiln-drying opera-tion.

Handling Cost($ per M bm)

Labor $14.10 (76 pct)Equipment depreciation 1.50 (8 pct)Equipment maintenance 1.25 (7 pct)Land and building .70 (4 pct)Insurance .40 (2 pct)Miscellaneous .50 (3 pct)

TOTAL $18.45 per M bmIt should be emphasized that drying costs

and energy usage can be controlled, however,by following the technical information andsuggestions listed in the various previous sec-tions of this handbook and in the Dry KilnOperator ’ s Manual and Air Drying o fLumber. Usually, the biggest cost in energysavings can be made by air-drying ratherthan kiln drying green lumber.

Degrade Costs and CausesNot included in the previous costs is de-

grade, and degrade can be the biggest cost ofall, especially in air drying. Cuppett (1966)studied degrade in Appalachian air dryingyards. He reported an average loss, adjustedto 1976, of over $16 per 1,000 board feet, al-though at some mills the losses were con-trolled to less than $3 per 1,000 board feet.Hanks and Peirsol (1975) estimated air dryingdegrade losses for 10 hardwoods dried undergood conditions. Degrade values ranged from0 to 5 percent of the lumber value. A recentstudy in Pennsylvania (Bean and Spoerke,1973) has shown that, under careful air dry-ing procedures and terminating the dryingwhen the wood was at 20 percent moisturecontent, degrade for 4/4 oak was between 1and 2 percent of the lumber value and for 8/4oak, 3 to 4 percent. With proper proceduresand attention degrade can be controlled andalmost eliminated.

In all of these studies, checks were the mostcommon defect, followed by splits, and then

warp. Cuppett (1965) has analyzed the majorcauses of degrade with the first listing themost common cause.

1.2.3.4.

1.2.3.

1.2.3.4.5.

1.2.3.4.5.6.

ChecksLack of roofing.Board edges exposed at bunk spaces.Stickers not flush with ends of boards.Too rapid drying due to excessive exposure of lumberstacks.

splitsToo few stickers.Lack of roofing or poor roofing.Stickers not flush with ends of boards.

WarpPoor sticker alinement.Poor bunk alinement.Lack of sufficient stickers.Foundation out of level.Thick and thin lumber in same course in stack.

StainNo chemical dip.Use of green stickers.Use of wide stickers.Base of piles too low.Grass and weeds growing between stacks.Poor yard orientation or location.

-77-

Often surface checks are not noted untilafter the lumber has been air dried, kilndried, and undergone some manufacturing.For a company trying to identify the cause(did it occur in air drying or kiln drying?) aclose examination of the checks with a 10 ×magnifying glass may show the checks to befull of dirt particles. Such evidence is good butnot a completely reliable indication that thechecks were opened during air drying. Cleanchecks are almost always attributed to kiln-drying difficulties, usually too dry initial con-ditions. Although the surface checks inhardwoods normally close as drying is com-pleted, exposure to very high relativehumidities, saturated steam, or water willcause the checks to stay open after redrying.Dry wood that is wetted also may check as itdries again.

A certain amount of drying degrade is un-avoidable and can be attributed to inherent

characteristics of the wood itself. Cuppett(1975) has established rough guidelines on theminimum amount of degrade for severalhardwoods in the Appalachian region:

Species

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

White and red oakWhite and red oakPoplar, basswood,

and cucumberPoplar, basswood,

and cucumberMaple and beechMaple and beech

Thickness

4/4 and 5/46/4 and 8/4

4/4 and 5/4

6/4 and 8/44/4 and 5/46/4 and 8/4

If drying degrade exceeds allowable limits,chapter 9 in the Dry Kiln Operator’s Manualand chapter 6 in Air Drying of Lumber dis-cuss the basic causes and remedies for dryingdefects.

Two recent reports have looked at dryingcosts in detail. Engalichev and Eddy (1970)have prepared a computer program to evalu-ate the cash flow situation for purchasing anew kiln, a low-temperature kiln, or a forced-air dryer. Goulet and Ouimet (1968) haveassembled a cost accounting approach for cal-culating the cost of drying-air, accelerated-air, kiln, or various combinations of thesemethods. Goulet and Ouimet’s approach hasbeen adapted and is included in this section,in order to permit an operator to assess dry-ing costs for his own operation. If, in workingwith this method, more wood is air dried an-nually than is kiln dried (or vice versa), theanalysis must be run twice, once for air dry-ing and once for kiln drying. Note that inusing this general approach for specific cases,not all items will have charges or costs. Alsonote that in the example many charges will beonly a few pennies per thousand board feet,and therefore it is not necessary to establishthese minor charges with a great deal of ac-curacy. Degrade, labor, and energy are majorexpenses and care should be exercised to de-termine these costs accurately.

To use the method presented here, thenumerical values for the 66 items must beascertained and inserted in the blanks of the

Allowablelosses in per-cent of air-dry lumber

value

1.02.0

.5

1.02.03.0

A Cost Accounting Approachsection, “Basis for Calculations.” Then thesenumbers are added, multiplied, and divided asshown in the section headed “Calculations” inorder to obtain the charges per thousandboard feet.

MethodBasis for CalculationsA. Direct investments (total costs)

Buildings, sheds, etc.Kiln, auxiliary equipment

(including boiler)StickersPile roofsPile bases, bolsters

Total direct investment(lines 1 thru 5)

B. Amortization period for directinvestments (years)Buildings, sheds, etc.Kiln, auxiliary equipmentStickersPile roofsPile bases, bolsters

C. Quantity of wood dried annually(M bm)(If all air-dried wood is not

kiln-dried, two entirelyseparate calculations shouldbe made)

D. Annual interest rate (percent asdecimal)

(1)

(2)(3)(4)(5)

(6)

(7)(8)(9)(10)(11)

(12)

(13)

-78-

E.

F.

G.

H.

I.J.

K.

L.

M.

N.O.

Drying yard investments (Total P .Q.

R.

S.

T.

U.

V.

W.X.

Y.

Z.AA

Forklift time (M bm/hr)Hourly forklift cost (include

machine cost, labor, and fringebenefits) ($/hr)

(52)costs)Storage sheds for stickers,

dried lumber, etc.(should not duplicate itemNo. 1)

Permanent road construction,rail access, etc.

Temporary road construction(includes drying alleys)

FencesLighting systemsDrainage systemsSprinkler systems

Total (lines 14 thru 20)Amortization period for drying

yard investmentsStorage sheds for stickers, driedlumber, etc.Permanent road construction,

rail access, etc.Temporary road construction

(includes drying alleys)FencesLighting systemsDrainage systemsSprinkler systems

Maintenance and repair of yard,percent of line 21 ($/yr)Plus snow removal ($/yr)Plus yard cleaning ($/yr)Plus annual lighting expense

(bulbs and electricity) ($/yr)Land area (ft2)

Air drying area (include spacebetween the piles)

Road area (refers to line 15,roads)

Area for buildings, kiln, boiler,etc.Total (lines 33 thru 35)

Land value ($/ft2)Taxable values ($)

Land (line 36 times line 37)Buildings (line 1)Kilns, equipment (line 2)Fences, lighting, drainage,

sprinklers (line 17 + 18 + 19 +20)Total (lines 38 thru 41)

Insurable values ($)Direct investments (line 6)Storage sheds (line 14)Fences (line 17)Wood (line 62 times line 64)

Total (lines 43 thru 46)Tax rate to be applied to line 42

(percent as decimal)Insurance rate to be applied to

line 47 (percent as decimal)Stacking time (M bm/hr)Hourly stacking cost (include

machinery, labor, and fringebenefits ($/hr)

(53)

(54)

(14)Time spent each day observing

and running kilns, boilers, etc.(hr)

Hourly wage rate and fringebenefits for kiln operator and

(15)

(16)(17)(18)(19)(20)(21)

auxiliary equipment includingboiler ($/hr)

Maintenance of kilns and boiler, apercentage of line 2 (in dollars)

Annual office costs attributed todrying (in dollars)

Energy costs (boiler costs shouldbe included in lines 2 and 56, nothere)Annual fuel consumption

(gal. or 1,000 ft3)Fuel cost ($/gal. or 1/1,000 ft3)

(55)

(56)

(57)

(22)(58)(59)(23)

Annual electrical usageattributed to drying (kWh)

Electrical cost ($/kWh)Average price of lumber ($/M bm)Average drying degrade, based

on lumber value (percent asdecimal)

Average daily volume of lumberon yard and in kilns on anygiven day (M bm)

Total capacity of kilnsAverage length of kiln run

(24)(25)(26)(27)(28)

(60)(61)(62)

(63)(29)(30)(31) (64)

(65)(32)

(include loading and unloadingtime) (66)

(33)Calculations

Values are computed in dollars per 1,000board feet (numbers are line numbers fromthe previous section):

(34)

(35)(36)(37)

(38)(39)(40)

(41)(42)

(43)(44)(45)(46)(47)

(48)

(49)(50)

(51)

-79-

Total of all = ($/M bm)

Example

The following represents a hypotheticalhardwood air and kiln drying operation in anorthern climate and should provideguidelines for the expected data necessary toperform the calculations.

Data

Calculations $M bm

TOTAL

-80-

Energy ConsiderationsEnergy usage in kiln drying is significant.

The energy used in the kiln can be reduced byfull air drying and by efficient kiln operation.The energy saving by air drying is illustratedin table 27 where generalized energy usage isgiven.

A recent Maine extension bulletin (Shot-tafer and Shuler, 1974) presented a method ofestimating the individual components ofenergy usage in kiln drying. In the examplepresented in this extension bulletin, energyusage is broken down for a 25,000 board-footcharge of 4/4 sugar maple, drying from 70 to10 percent moisture content in 14 days in aprefabricated, aluminum, insulated kiln.

Energy Use Million Btu’s per25,000 board feet

Heating wood and residual water 4.3Water evaporation 49.8Venting 18.0Building Heat Losses 35.7

(The building heat losses, in turn, can be broken downinto four components: through the walls, 29 ‘pct; throughthe roof, 7 pct; through the door, 12 pct; and through thefloor, 52 pct.)

The energy used for heating the wood andfor evaporating the water, in the above list-ing, cannot be reduced (except through airdrying; air drying will actually reduce energyuse in all four items). In a properly operating

and adjusted kiln, vent losses can be reducedsomewhat, but not greatly. However, savingscan be made in building losses—for example,the floor can be insulated or the kiln can beenclosed in another building (but vented tothe outside). With the enclosure, the heat lostthrough the kiln walls can be utilized to heatthe building that encloses the kiln.

Kiln wall insulation can also be increased toreduce wall losses. If the insulation and wallthickness is increased from 2 to 4 inches in analuminum prefab-type wall, the savings inthe above example would be about 5 millionBtu’s per 25,000 board feet, or approximately$250 annually. However, thicker walls andadded insulation would add several thousanddollars to the building’s cost.

The following listing (Wengert, 1974) shouldbe helpful in increasing the efficient use ofenergy during kiln drying.

1. Use as much air drying or forced-air dry-ing as possible—preferably drying to 25 per-cent moisture content or less.

2. Do not use steam spray or water spray inthe kiln except during the conditioning. Letthe moisture coming out of the wood build upthe humidity to the desired level. Steam mayhave to be used, however, when very smallwet-bulb depressions are required. (Dry KilnOperator’s Manual, p. 160-162.)

Table 27. -Energy consumption and cost estimates in kiln drying hardwood lumber 1,2

Total energy cost 5 for kiln drying 4

Energy to Total energy usedFrom kiln dry per M bm 4 Per M bm Per 30 M bm kiln charge

per M bm 3

Good Intermediate Good Intermediate Good Intermediate

Million Million MillionBtu Btu Btu Dollars Dollars Dollars Dollars

Air-dried(22-7=15 pct) 0.518 1.294 2.587 1.90 3.80 58 115

Partly air-dried(37-7=30 pct) 1.035 2.587 5.175 3.80 7.60 115

Green228

(47–7=40 pct) 1.380 3.450 6.900 5.05 10.10 153 305

1 Assumptions: Av. sp. gr. of wood—0.55water/1 pct MC/M bm of rough, 4/4 lumber—33.95 lbEnergy (Btu) to evaporate 1 pct MC/M bm—34,561

2 Does not include electrical energy for fans.3 100 pct efficiency of kiln.4 Kiln efficiencies of 40 pct for “Good” and 20 pct for “Intermediate;” if 50 pct efficiency is maximum possible in

well-insulated, virtually perfect condition, steam-heated kiln, these estimates seem practical and are borne outgenerally by industry experience.

5 Rate of $1.50 per million Btu.

-81-

3. Repair and talk all leaks, cracks, andholes in the kiln structure and doors to pre-vent unnecessary venting and loss of heat.Make sure the doors close tightly, especiallyat the top. Temporarily plug any leaks aroundthe doors with rags, and order new gaskets,shimming strips, or hangers if necessary. In atrack kiln, use sawdust-filled burlap bags toplug leaks around tracks. Adjust and repairthe vents so that, when they are closed, theyclose tightly.

4. For brick or cinder block kilns, maintainthe moisture vapor-resistant kiln coating inthe best possible condition. This will preventthe walls and the roofs from absorbing water.Dry walls conduct less heat to the outside.

5. For outdoor aluminum kilns only, paintthe exterior walls and roof a dark color toincrease the wall temperature by solar heatand reduce heat loss from the kiln. Check toinsure that weep holes are open, not plugged.(Painting would be disastrous on permeablewalls like brick or cinder block.)

6. In many kilns, more heat is lost throughthe roof than through the walls. Much of thisloss is due to wet insulation. To reduce heatlosses, consider installing a new roof or re-pairing an old one. Add additional insulationif necessary. Make sure the interior vaporbarrier or coating is intact (see suggestion 4).

7. Install or repair baffling to obtain a high,uniform air velocity through the lumber andprevent short circuiting the air travel. Thispays off in saving energy. Reverse air circula-tion only every 6 hours.

8. Research has shown that in the earlystages of drying, high air velocities (morethan 600 ft/min) can accelerate drying. In thelate stages, low velocities (250 ft/min) are aseffective as high velocities and use lessenergy. Therefore, arrange to adjust fanspeeds if possible during a run.

9. Have the recorder-controller calibratedand checked for efficient operation. The kilnshould not oscillate between periods of vent-

ing and steam spraying and should not ventand steam at the same time. (Dry KilnOperator’s Manual, p. 67-73.)

10. Check the remainder of the equipment.Are traps working? Do traps eject mostly hotwater with little, if any, steam? Do valvesclose tightly? Are heating coils free of debris?Is valve packing tight? Is there adequatewater for the wet bulb?

11. Accurately determine the moisturecontent of the wood you are drying. Do notwaste energy by overdrying or by taking toolong because your samples do not representthe load. Try to plan your loads so that, whenthey are sufficiently dry, someone will beavailable to shut off the kiln (and, if possible,to unload it, reload it, and start it again). Donot allow a kiln load of dry lumber to continueto run overnight or through a weekend.

12. Unload and reload the kiln as fast aspossible. Avoid doing this until the air tem-perature has warmed up from the morninglow—do not cool the kiln unnecessarily.

13. In a battery of adjacent kilns, avoidhaving one kiln being unloaded or loadedwhile the adjacent kiln is at 180° F or otherhigh temperature.

14. During nonuse periods, close all valvestightly and keep kiln doors closed. Use a smallamount of heat, if necessary, to prevent freez-ing of streamlines and waterlines.

15. Use accelerated schedules where possi-ble. Check the chapter on Conventional KilnDrying for accelerating schedules withminimum risk. The higher the temperaturefor drying, the more efficiently energy is used.

16. If possible, reduce the length of timeused for conditioning; some low-densityhardwoods can be conditioned in 6 hours.

17. Finally, check with the manufacturer ofyour equipment and find out if you can lowersteam pressures or reduce gas or oil flow ratesduring periods of constant dry-bulb tempera-ture. Also have the manufacturer check theburner for top efficiency.

–82–

Literature Cited

-83-

APPENDIX A—SPECIES, THICKNESSES, DRYING SCHEDULES,AND COMMENTS

Common name Thickness Kiln schedule1 Conservative Otherschedule 2

Apple 4/4, 5/4, 6/4 T6-C3 No8/4 T3-C3 No

Ash, black 4/4, 5/4, 6/4 T8-D4 Yes

8/4 T5-D3 YesAsh: green, white 4/4, 5/4, 6/4 T8-B4 Yes

8/4Aspen 4/4, 5/4, 6/4

Basswood

Beech

Birch, paper

Birch, yellow

Buckeye, yellow

Butternut

Cherry, black

Chestnut

Cottonwood: nor-m alwet streak

Dogwood

Elm: American,slippery

8/44/4, 5/4, 6/4

8/44/4, 5/4, 6/4

8/41-in. squares2-in. squares4/4, 5/4, 6/48/41-in. squares

2-in. squares

4/4, 5/4, 6/48/41-in. squares2-in. squares4/4, 5/4, 6/48/44/4, 5/4, 6/48/44/4, 5/4, 6/48/44/4, 5/4, 6/48/44/4, 5/4, 6/48/44/4, 5/4, 6/48/44/4, 5/4, 6148/4Shuttles4/4, 5/4, 6/4

T5-B3T12-E7

T10-E6T12-E7

T10-E6T8-C2

T5-C1T8-C3T5-C2

T10-C4T8-C3

T10-C6

T8-C4

T8-C4T5-C3T8-C5T5-C4

T10-F4T8-F3

T10-E4T8-E3T8-B4T5-B3

T10-E4T8-F4

T10-F5T8-F4T8-D5T6-C4T6-C3T3-C2T3-B2T6-D4

YesNo

NoYes

YesNo

NoNoNoYesYesYes

Yes

YesYesYesYesYesYesYesYesYesYesYesYesYesYesNoNoYesYesYesYes

——

See appendix Bfor mixed greenand black.

—Possible accel-

eration for 4/4,5/4 green ash isT8-C4. Seeappendix B.

Use for No. 1 Com-mon and Betterand for cratinglumber; middlegrades—seeappendix C.

See appendix C.Use T9–E7 for white

color.—

See special report;degrade controlimportant.

Do.Do.Do.——

Use T5–C6 for whitecolor.

Use T5–C4 for whitecolor.

———————————————————

American elm,soft type, useT8-D4; air dryhard type first,then T2–D5.

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APPENDIX A–SPECIES, THICKNESSES, DRYING SCHEDULES,AND COMMENTS-Cont.

Common name Thickness Kiln schedule 1 Conservative Otherschedule 2

8/4Elm: rock, ced-

ar, wingedHackberry

Hickory

T5-D3T6-B3T6-C3T8-C4T6-C3T8-D3

YesYesYesYesYesYes

4/4, 5/4, 6/48/44/4, 5/4, 6/48/4414, 5/46/4 —

—————

See table 17,Dry Kiln Opera-tor’s Manual.

Do.Do.——————

For cucumbertree,use yellow-pop-lar schedule,

Use T1-C5 forwhiter color orsee appendix C.

8/4Handle stock4/4, 5/4, 6/48/44/4, 5/4, 6/48/44/4, 5/4, 6/48/44/4, 5/4, 6/48/4

— ——

T6-D4T4-C3T6-B3T3-B1T6-A3T3-A1

T10-D4T8-D3

—YesYesNoNoNoNoYes

Holly

Hophornbeam(ironwood)

Locust, black

Magnolia

Maple, sugar(hard)

4/4, 5/4, 6/4 T8-C3 Yes

8/41-in. squares2-in. squares4/4, 5/46/4, 8/4

T5-C2T8-C4T5-C3T2-C1

YesYesYesYes

——

See appendix B.Air dry to 20 pct

MC.See appendix B.

Oak, lowlandsouthern (redand white)

Oak, uplandred

Oak, uplandwhite

Osage-orange

4/4, 5/46/4, 8/44/4, 5/46/4, 8/44/4, 5/4, 6/48/44/4, 5/4, 6/4

T4-D2T3-D1T4-C2T3-C1T6-A2T3-A1T8-D3

YesYesYesYesNoNoYes

See appendix B.

——

Bitter pecan shouldbe air driedfirst.

——————

For sap gum lumbercontaining largeamounts of heart-wood, use red gumschedules.

——————

Pecan

8/44/4, 5/4, 6/48/4Golf club headsShuttles4/4, 5/4, 6/44/4, 5/4, 6/48/4

T6-D1T6-C3T3-C2T3-C2T3-B2T8-D4

T12-F5T1l-D4

YesPersimmon —

—YesYesYesYesYes

SassafrasSweetgum: sapwood

(sap gum)

1-in. squares2-in. squares4/4, 5/4, 6/48/44/4, 5/4, 6/48/4

T12-F6T11-D5

T8-C4T5-C3T6-D2T3-D1

YesYesYesYesYesYes

heartwood(red gum)

Sycamore

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APPENDIX A—SPECIES, THICKNESSES, DRYING SCHEDULES,AND COMMENTS—Cont.

Common name Thickness Kiln schedule1 Conservativeschedule 2

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Tupelo, black 4/4, 5/4, 6/4 T12-E5 Yes

8/4 T11-D3 YesTupelo, swamp 4/4, 5/4, 6/4 T10-E3 Yes

8/4 T8-D2 YesTupelo, water 4/4, 5/4, 6/4 T6-H2 No

8/4Walnut, black 4/4, 5/4, 6/4 T6-D4 Yes

8/4 T3-D3 YesGunstock T3-D4 Yes

blanks

Willow, black 4/4, 5/4, 6/4 T10-F4 Yes

8/4 T8-F3 YesYellow-poplar 4/4, 5/4, 6/4 T11-D4 Yes

8/4 T10-D3 Yes

Other

See table 17, DryKiln Operator’sManual.

Do.See table 17, Dry

Kiln Operator’sManual.

Do.See tables 16 and

17 Dry Kiln Op-erator’s Man-ual.

Air dry first.Steam only green

stock.—

End coating requir-ed; T5–D4 pos-sible accelera-tion.

Air dry buttmaterial

Do.—

1 See page 119 of Dry Kiln Operator’s Manual.2 “Yes” indicates schedule probably can be accelerated by following procedures on pages 124–126 of Dry Kiln

Operator’s Manual; “No” suggests the recommended kiln schedule probably is close to optimum. All 8/4 thicknessconservative schedules can be modified to 180° F dry bulb at 11 pct moisture content or below.

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APPENDIX B—MIXED DRYING OF SOMESIMPLIFIED SCHEDULES

SPECIES BY

Eighteen southern hardwoods growing onupland sites can be grouped for kiln drying bysimplified schedules:

Simplified kilnSpecies schedule No. for

4/4 lumber 1

E ASY-DRYING W O O D S

AshBlack IIIGreen, white III

ElmAmerican (soft type)2,slippery I, IIIAmerican (hard type) 2 I

Cedar: rock, winged IHackberry IIIHickory: pecan 3 II, IMaple: red, silver IIIMagnolia: sweetbay IVSweetgum

Sapwood (not over 15pct heart) VHeartwood (red gum) III

Tupelo: black (blackgum), tupelo V

Yellow-poplar (poplar) IVD IFFICULT W OODS

OakRed oak VIWhite oak VI

In some cases the same schedule is shownfor woods that have greatly different greenmoisture content values. In these cases, twocolumns of moisture content schedule changepoints are shown. The dry-bulb and wet-bulbtemperature settings for these woods aregoverned by the average moisture content ofthe wettest half of the kiln sample of thespecies or group of species that are slowest inreaching the designated moisture contentlevel in its own series.

The simplified kiln schedules I through VIare listed in table B1.

1 Where a wood has two different simplified scheduleslisted, see the distinctions in the schedule tables.

2 Soft type is forest grown, has narrow growth rings,specific gravity (green volume basis) of 0.40-0.48, andmay be sold as gray elm; hard type is open growth, haswide rings, and specific gravity of 0.44-0.52.

3 Divided into bitter pecan (water hickory, C a r y aaquatica) and sweet pecan (pecan, C. illinoensis; andnutmeg hickory, C. myristicaeformis).

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Table B1 –Simplified kiln schedules I through VI for drying 4/4 materialof mixed species

Moisture contentDry-bulb Wet-bulb

Lower Higher temperature temperature______________________________________________________________________________________________________________________

Pct Pct ° F ° FI. American Elm (Hard Type), Rock Elm, Hickory

and Bitter Pecan HeartwoodRock, elm Hard -type American

winged elm elm, hickory, pecan

Above 35 Above 50 12035 50 12030 35 13025 25 14018 18 180

II. Hickory, Sweet Pecan, and Bitter Pecan SapwoodAbove 50 125

50 12540 13030 15020 180

III. Black, Green, and White Ash; American Elm(Soft Type); Hackberry; Red and Silver Maple; and

Sweetgum HeartwoodGreen and white Black ash; soft-ash; hackberry; type Americansweetgum elm; red andheartwood silver maple

115113100105130

120118114112130

Above 40403218

WhiteAbove 40

403430252015

Above 50 13050 13035 14018 180

IV. Magnolia and Yellow-PoplarAbove 50 140

50 14035 15018 180

V. Black Gum, Upland Tupelo, and Sweetgum SapwoodAbove 60 160

60 16040 16025 17018 180

VI. Upland Red and White OakRed

Above 50 11050 11040 11032 12025 13020 14015 180

123118

95130

133128120130

150145130130130

106105102106100

95130

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APPENDIX C—SPECIAL KILN SCHEDULESFOR SPECIAL PURPOSES

Most kiln operators have their own specialkiln schedules for particular circumstances.Some of these take into account specific con-ditions for their unique situation. But suchspecial situations are almost infinite. Alimited number of special hardwood schedulesis given on pages 127 to 129 of the Dry KilnOperator’s Manual, Some additional specialschedules are included here for these speciesand purposes:

Aspen—to minimize collapse in 4/4 to 8/4 material.Maple-to retain the whitest color possible in 4/4 and

5/4 stock, and to dry 6/4 and 8/4 material with mineralstreak with little honeycombing.

Upland red oak—to minimize honeycomb and degradein bacterially infected 4/4 stock, and for presurfacingto accelerate drying of 4/4 material.

These have been given the designation of“C” for Appendix C, “S” for special scheduleand a number. Such a combination should setit off from the basic and simplified schedules:

Table CS1. –Aspen, low-to moderate-collapse kiln schedules

4/4, 5/4, 6/4 stock 8/4 stockMoisturecontentat startof step

Dry-bulbtempera-

ture

Wet-bulbtempera-

ture

Dry-bulbtempera-

ture

Wet-bulbtempera-

ture

P c tAbove 70

70605040302520

21 23 8

F110110115120130150150

11 8 0180180173180

° F100100100100105110110135130130130170

° F140140140140140

1150170170180200200—

° F133130125120110100120120130140150—

Equalize 4

Condidtion

1Operate with vents closed, no steam spray until equalizing.2For 8/4, continue until very wettest sample is 8 pct moisture content.3For 8/4, target is 3¼ to 4¼ pct moisture content; time on this step about 5 dyas.4For 8/4, time on this step about 2 days.

Table CS2. –Maple, whitest color 4/4 and 5/4 schedules

4/4 and 5/4 stock initialmoisture content 50 percent

or lower

4/4 and 5/4 stock, initialmoisture content 51 percent

or higher

Moisturecontentat startof step

Dry-bulbtempera-

ture

Wet-bulbtempera-

ture

Moisturecontentat startof step

Dry-bulbtempera-

ture

Wet-bulbtempera-

ture

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

P c t ° F ° F PctAbove 40

above 28282420161310

105108108108115125160170

959590858080

40353026201612

° F105108108108108115125160

° F95955085808080

105105154Conditioning Conditioning 170 154

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Table CS3. –Maple, 1 mineral streak stock 6/4 and 8/4 schedule for minimum honeycombing

Moisture content 2

Average forwettest half

of kilnsamples

Darkest zonefin wettestsample

Dry-bulbtempera-

ture

Wet-bulbtempera-

ture

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

P c t P c tAbove 40

4035302520

° F110110110120125130140150160180

° F106105102106

9 58 59 5

100110130

30252015

1 This schedule also should be suitable for mineral-streaked yellow birch.2 Kiln samples should be 2 feet longer than normal so that three or four intermediate moisture content tests (page105, Dry Kiln Operator's Manual) can be made. For green stock, start with normal kiln sample procedure. For air-driedstock, cut both an average section and a "darkest zone" section at the start. Cut out the darkest, wettest appearingportion of the latter section with a bandsaw. Weigh and ovendry this portion sseparately to determine when tempera-ture of 140° F and higher cn be used. After the final drying condition has run 1 day, revert the full-size kiln samplemethod to start equalizing and conditioning.

Table CS4. —Upland red oak, bacterially infected 4/4 schedule for minimizing honeycomb anddegrade

Moisture content Dry-bulb Wet-bulbat start of step temperature temperature

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Pct ° F ° FAbove 52 105 102

52 105 10145 105 9940 105 9735 110 9530 115 8525 120 8520 130 8515 150 10011 170 120

Equalize 170 127Condition 175 165

1 Material with advanced infection should be air dried in a sheltered location or forced air dried by a mild 8/4procedure until the wettest half of the infected kiln samples have a moisture content of 22 percent or lower. Kiln dryingthen can be started using 120° F dry-bulb and 110° F wet-bulb temperatures for 16 to 24 hours. Then the above schedulecan be followed in accordance with the moisture content of the controlling samples. Material in the early stages ofinfection can be dried by this schedule from the green, partly air dried, or air dried state in accordance with the generalprocedures in chapter 5.

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Table CS5. —Presurfaced upland oak 4/4 schedule 1

Moisture content for: Dry-bulb Wet-bulb Wet-bulbtemperature depression temperature

White oak 2 Red oak 2

Pct Pct ° F ° F ° FBASIS—AVERAGE MOISTURE CONTENT, ALL SAMPLES

Above 42 Above 53 115 4 11142 53 115 5 11037 43 115 8 10733 37 115 14 101

BASIS—AVERAGE MOISTURE CONTENT, WETTEST HALF OF SAMPLES35 35 120 35 8530 30 125 40 8527 27 130 45 8521 21 140 50 9017 17 180 50 130

1 Accelerations achieved by presurfacing depend partly on 400 ft/min air velocity through the load.2 When there is a mixture of red and white oak in the kiln, operate on the basis of the moisture content of the samples

of the predominating species. If either the red oak or the white oak is not strictly green, operate on the basis of thesamples of the species closest to the green moisture content.

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APPENDIX D—LUMBER NAMES, TREE SPECIES, SEASONINGTYPES, AND BOTANICAL NAMES OF WOODS

MENTIONED IN THIS HANDBOOKCommercial name

for lumber

Ash:BlackW bite

Aspen (popple)

Basswood

BeechBirch:

Soft (white)

Yellow

BlackgumBuckeye

ButternutCherryChestnutCottonwood

CucumberDogwoodElm:

Rock

Soft (American)

GumHackberry

Hickory

HollyIronwood (hophorn-

beam)Locust

Magnolia

Maple:Hard

soft

NormalWet streak

Seasoning Official common treesubtype name

Apple

Black ashBlue ashGreen ashWhite ashBigtooth aspen

Quaking aspenAmerican basswoodWhite basswoodBeech

Gray birchPaper birchRiver birchSweet birchYellow birch(see Tupelo)Ohio buckeyeYellow buckeyeButternutBlack cherryChestnutBalsam poplarEastern cottonwoodSwamp cottonwoodCucumbertreeFlowering dogwood

Cedar elmRock elmSeptember elmWinged elmAmerican elmSlippery elm(See sweetgum)HackberrySugarberryMockernut hickoryPignut hickoryShagbark hickoryShellbark hickoryAmerican hollyEastern hophornbeam

Black locustHoneylocustSouthern magnoliaSweetbay(Also see cucumbertree)

Black mapleSugar mapleRed mapleSilver maple

Hard typeSoft type (gray)

Botanical name

Malus spp

Fraxinus nigraF. quadrangulataF. pennsylvanicaF. americanaPopulus grandident-

ataP. tremdoidesTilia americanaT. heterophyllaFagus gvandifolia

Betula populifoliaB. papyriferaB. nigraB. lentaB. alleghaniensis

Aesculus glabraA. octandraJuglans cinereaPrunus serotinaCastanea dentataPopulus balsamiferaP. deltoidesP. heterophyllaMagnolia acuminataCornus florida

Ulmus crassifoliaU. thomasiiU. serotinaU. alataU. americanaU. rubra

Celtis occidentalsC. laevigataCarya tomentosaC. glabraC. ovataC. laciniosaIlex opacaOstrya virginiana

Robinia pseudoacaciaGleditsia triacanthosMagnolia grandifioraM. virginiana

Acer nigrumA. saccharumA. rubrumA. saccharinum

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Commercial name Seasoning Official common tree Botanical namefor lumber subtype name

Oak:Red Northern or upland

Southern lowland

White Northern or upland

Southern lowland

Osage-orangePecan:

Sweet pecan

Bitter pecan

PersimmonPoplarSassafrasSweetbaySweetgum Red gum

Sap gum 3

SycamoreTupelo, black (black

S a p w o o dHeartwood

gum)TupeloWalnutWillowYellow-poplar

Black oakBlackjack oakCherrybark oak1

Northern pin oakNorthern red oakPin oakScarlet oakShumard oak1

Southern red oak

Cherrybark oak2

Laurel oakNuttall oakShumard oak2

Water oakWillow oakBur oakChestnut oakChinkapin oakPost oakSwamp white oakWhite oakOvercup oakSwamp chestnut oakOsage-orange

PecanNutmeg hickoryWater hickory

Common persimmon(See yellow-poplar)SassafrasSweetbaySweetgum (heartwood)Sweetgum (sapwood)American sycamoreBlack tupeloSwamp tupeloWater tupeloBlack walnutBlack willowYellow-poplar

Quercus velutinaQ. marilandicaQ. falcata var. pago-

daefoliaQ. ellipsoidalisQ. rubraQ. palustrisQ. coccineaQ. shumardiiQ. falcata var. fal-

cataQ. falcata var. pago-

daefoliaQ. laurifoliaQ. nuttalliiQ. shumardiiQ. nigraQ. phellosQ. macrocarpaQ. primusQ. mushlenbergiiQ. stellataQ. bicolorQ. albaQ. lyrataQ. michanxiiMaclura pomifera

Carya illinoensisC. myristicaeformisC. aquatica

Diospyros virginiana

Sassafras al bidumMagnolia virginianaLiquidambar styraciflua

Do.Platanus occidentalsNyssa sylvaticaN. sylvatica var. bifloraN. aquaticaJuglans nigraSalix nigraLiriodendron tulip-

ifera

1 When grown on upland sites.2 When grown on lowland (wetter) sites.3 If sap gum grade contains more than 15 pct heartwood, dry with red gum schedules.

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Accelerated air drying. ––The use of equip-ment and procedures to accelerate air dry-ing. In this handbook it includes yard fandrying, shed fan drying, forced air drying,low temperature kiln drying, and con-trolled air drying.

Active drying period. —In air drying, theperiod or season of the year when condi-tions are most favorable for drying thewood at the highest rate.

Air drying. —The process of drying greenlumber or other wood products by exposureto prevailing natural atmospheric condi-tions outdoors or in an unheated shed.Air drying calendar. A table showing the

number of effective air drying days eachmonth of the year in a specific area.

Air drying efficiency. Operation of the airdrying process to reduce green wood tothe air-dried moisture content level withleast cost in energy, time, and money.

Effective air drying day (EADD). A daywith drying potential equivalent to theaverage of all the days in full months ofthe late spring-summer season.

Good air drying month. Thirty consecutivedays in which daily mean temperaturesare over 45° F. This is roughly equivalentto 22 or more EADDS.

Alleys. —In air drying, the passageways be-tween rows or lines of piles of lumber orother wood products on a yard.Cross alley. The passageways that connect

main alleys and lie at right angles to thepiled lumber.

Main alley. The roads in a yard for thetransport of lumber and other woodproducts.

Baffle. —In forced air or kiln drying, a canvas,metal, or wood barrier used for deflecting,checking, or otherwise directing the flow ofair.

Blank. —A piece of wood cut to a specified sizeand shape from which a finished product orpattern is made, e.g., gunstocks, bowlingpins.

Board. —Yard lumber that is less than 2inches thick and 2 or more inches wide; aterm usually applied to 1-inch thick stock ofall widths and lengths.

Boiling in oil. —A special process for dryingwood; the rough green wood products are

submerged in an open hot bath of a water-repelling liquid such as petroleum oil,c reosote , or molten w a x , o r per-chloroethylene in ozeotropic drying, all ofwhich have a boiling point considerablyabove that of water.

Bolster. ––A piece of wood, generally a nomi-nal 4 inches in cross section, placed be-tween stickered packages of lumber orother wood products to provide space forthe entry and exit of the forks of a lifttruck. (The bolster should be placed inalinement with a tier of stickers and a sup-porting foundation member.)

Bound water. ––In wood technology, moisturethat is intimately associated with the finerwood elements of the cell wall by adsorp-tion and held with sufficient force to reducethe vapor pressure.

Bow. —A form of warp in which a board de-viates from flatness lengthwise but notacross the faces.

British themal unit. —The amount of heatrequired to raise the temperature of 1pound of water at its maximum density,1° F.

Bulk piling. —In handling lumber and otherwood items, the stacking onto dollies or pal-lets, into unit packages or into bins withoutvertical spaces or stickers between thelayers for air circulation.

Casehardening. —A condition of stress and setin dry wood in which the outer fibers areunder compressive stress and the innerfibers under tensile stress, the stressespersisting when the wood is uniformly dry.

Cell. —In wood anatomy, a general term forthe minute units of wood structure havingdistinct cell walls and cell cavities includ-ing wood fibers, vessel segments, and otherelements of diverse structure and function.In dense woods the fibers are thick walledand make up the major part of whole zonesof wood. These fibrous zones are slow dry-ing.

Cellulose. —The carbohydrate that is theprincipal constituent of wood and forms theframework of the wood cells.

Check. ––A separation of the wood fiberswithin or on a log, timber, lumber, or otherwood product resulting from tensionstresses set up during drying, usually the

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early stages of drying. Surface checksoccur on flat faces of boards; end checks onends of logs, boards, or dimension parts.

Chemical seasoning. —The application of ahydroscopic chemical, (e.g., sodiumchloride) to green wood, for the purpose ofreducing defects, mainly surface checks,during drying. The chemical may beapplied by soaking, dipping, spraying withaqueous solutions, or by spreading with thedry chemical and bulk piling.

Collapse. —Flattening or buckling of the woodcells during drying resulting in excessiveor uneven shrinkage plus a corrugated sur-face.

Conditioning. ––In kiln drying, a process forrelieving the stresses present in the woodat the end of drying. Consists of subjectingthe stock while still in the kiln to a fairlyhigh dry-bulb temperature and an equilib-rium moisture content condition 3 to 4 per-cent above the desired average moisturecontent for the stock. The process should beof sufficient durat ion to eliminatecasehardening through the reduction ofcompression and tension sets.

Course. —A single layer of lumber or otherwood products of the same thickness in astickered pile, package, or kiln truckload.

Crook. —A form of warp in which a board de-viates edgewise from a straight line fromend to end.

Cup. —A form of board warp in which there isa deviation from a straight line across thewidth.

Debarking. —Removing the bark from a log.Debris. ––In air drying, rubbish such as bro-

ken stickers, fragments of boards, brokendown foundations, dried weeds, etc., thatrestrict air movement. In kiln drying, bro-ken stickers, fragments of wood, and otherrubbish that interfere with air circulationand increase the fire hazard.

Decay. —The softening, weakening, or totaldecomposition of wood substance by fungi.

Defect. ––Any irregularity or imperfection in atree, log, bolt, lumber, or other wood prod-uct that reduces the volume of useablewood or lowers its durability, strength, orutility value. Defects may result fromknots and other growth conditions and ab-normalities; from insect or fungus attack;from milling, drying, machining, or otherprocessing procedures.

Degrade. —Generally, in lumber and otherforest products, the result of any processthat lowers their value for any purpose.Drying degrade. A drop in lumber grade

and volume due to drying defects.Machine degrade. The downward change of

grade or value of wood items due to de-fects developed during machining.

Density. ––The weight of wood per unit vol-ume, usually expressed in pounds per cubicfoot or grams per cubic centimeter. Aschanges in moisture content of wood affectits weight and volume, it is necessary tospecify the conditions of wood at the timedensity is determined.

Depression, wet-bulb. ––The difference be-tween the dry- and wet-bulb temperatures.

Desuperheater. —A device for removing fromsteam the heat beyond that required forsaturation at a given pressure.

Diamonding. ––A form of warp; the changingof the cross section of a square-sawn wooditem to diamond-shaped, during drying.This occurs where the growth rings passthrough diagonal corners and is caused bythe difference between tangential and ra-dial shrinkage.

Diffusion. —Spontaneous movement of waterthrough wood from points of high concen-tration to points of low concentration.

Diffusivity. ––The measure of rate of moisturemovement through wood as a result of dif-ferences in moisture content.

Dimensional stabililization. —Through specialtreatment, reduction in the normal swel-ling and shrinking of wood.

Discoloration. ––Change in the color of wooddue to fungal and chemical stains, weather-ing, or heat treatment.

Dryer. —Air moving and directing equipmentused to accelerate the air drying of wood.Controlled-air dryer. A warehouse-like

building with overhead fans and layoutdesigned for gentle recirculation of airthrough the lumber piles, having con-trolled vents and unit heaters to main-tain a desired temperature and relativehumidity.

Forced-air dryer. An inexpensive buildingwith reversible fans, tight baffling, and asource of heat, providing high air velocitythrough the lumber piles, thermostati-cally controlled temperature in the rangeof 70°–120° F, and partial relative humid-

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ity control by limited venting.Low-temperature kiln. Similar to a forced-

air dryer having sources of heat andhumidity, providing high air velocitythrough the piles, and dry- and wet-bulbtemperature control by a recorder-controller.

Semi-solar dryer. A low-temperature kiln inwhich part of the heat is from solarenergy and part from an artificial heatsource.

Shed-fan dryer. A shed with permanentroof, permanent or temporary end walls,and a permanent side wall with enoughfans to draw yard air through the lumberpiles at a high velocity.

Solar dryer. A forced-air dryer or low-temperature kiln in which only solarenergy is used for heat.

Yard-fan dryer. Similar in operation to ashed-fan dryer, but roof and end wallsare temporary and made of canvas,plywood, or sheet metal. Some suchdryers have portable fans used toexhaust air from an enclosed centralplenum space between two yard piles.

Dry kiln. —A room, chamber, or tunnel inwhich the temperature and relative humid-ity of air circulated through parcels oflumber, veneer, and other wood productscan be controlled to govern drying condi-tions.

Dryer or kiln concepts. —Important ideas inwood drying technique.Air travel. The distance air has to move

from the enter ing-a ir s ide to theleaving-air side of the load.

Air velocity. The velocity of air as it leavesthe sticker spaces on the leaving-air sideof the load.

Entering-air side of load. Where the air en-ters the lumber pile when the fans areturning in a specific direction. The air onthis side of the load must be controlled inaccordance with the drying schedule toprevent drying defects.

Leaving-air side of the load. The side of theload from which the air is exhausted orreturned to the heat source and fans.This air is always cooler and more humidthan the entering air because of the cool-ing effect of moisture evaporating fromthe lumber.

Drying. —The process of removing moisturefrom wood to improve its serviceability inuse.Drying or kiln schedule. The prescribed

schedule of dry-bulb temperature andwet-bulb temperature or relative humid-ity used in drying a load of lumber. Thehumidity aspect is sometimes expressedin terms of wet-bulb depression or EMC.In kiln drying, air velocity is an impor-tant aspect.

Drying rates. The loss of moisture fromlumber or other wood products per unit oftime. Generally expressed in percentageof moisture content lost per hour or perday.

Drying record. A daily or weekly tabulationof dryer or kiln operation including sam-ple weight, MC, and temperaturereadings or recorder-controller charts.

Drying shed. In air drying, an unheatedbuilding for drying lumber and otherwood products. The building of variousdesigns may be open on all sides, orclosed.

Drying stress. The force per unit area thatoccurs in some zones of drying wood dueto uneven shrinkage in response to nor-mal moisture gradients and to set thatdevelops in wood.

Edge piling. ––In air drying, stacking of woodproducts on edge, e.g., 2 by 4’s, so that thebroad face of the item is vertical; usuallydone to restrain crook. In kiln drying,stacking of lumber on edge for drying inkilns with vertical air circulation.

End coating. ––A coating of moisture-resistantmaterial applied to the end-grain surfacesof green wood such as logs, timbers, boards,squares, etc., to retard end drying and con-sequent checking and splitting or to pre-vent moisture loss from the ends of thedrying samples.

End pile. ––In air drying, stacking of greenlumber on end, and inclined, in a long fairlynarrow row, the layers separated by stick-ers. In lumber storage, placement of wooditems on end in suitable bins, a practice atmills and retail yards. In kiln drying, plac-ing lumber on kiln trucks with their lengthparallel to the length of the kiln.

End racking. —In air drying, placing of boardson end against a support so as to form a pile

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in the shape of an “X” or an inverted “V.”Equalization and conditioning. —In kiln dry-

ing, the process of increasing the equilib-rium moisture content condition in thefinal stages of drying lumber and other millproducts to—(1) reduce the moisture con-tent range between boards, (2) flatten themoisture content gradient within boards,and (3) relieve drying stresses. Usuallyequalization and conditioning are twoseparate stages in final kiln drying.

Equilibrium moisture content. —The moisturecontent at which wood neither gains norloses moisture when surrounded by air at agiven relative humidity and temperature.EMC is frequently used to indicate poten-tial of an atmosphere to bring wood to aspecific MC during a drying operation.

Extractives. ––Substances in wood, not an in-tegral part of the cellular structure, thatcan be removed by solution in hot or coldwater, ether, benzene, or other solventsthat do not react chemically with woodsubstances.

Fiber-saturation point. —The stage in the dry-ing or wetting of wood at which the cell lallsare saturated with water (bound water)and the cell cavities are free of water. It isusually taken as approximately 30 percentmoisture content, based on the weightwhen ovendry.

Flat pile. ––In air drying and kiln drying,stacking of stock so that the broad face ofthe item is horizontal. In kiln drying, thestickered loads are level.

Flue. —In stacking lumber or other woodproducts for air drying, a vertical space, 6inches or less in width in the center of thepile and extending the length of the pile,intended to facilitate circulation of airwithin the pile.

Foundation. —Structural supports for anair-drying pile, designed to prevent warp-ing and facilitate air circulation under thepile.

Free water. —In wood technology, water thatis held in the lumens or the grosser capil-lary structure of the wood.

Fungi. —Low forms of plants consistingmostly of microscopic threads (hyphae)that traverse wood in all directions,dissolving materials out of the cell wallsthat they use for their own growth.

Grain. —The direction, size, arrangement,appearance, or quality of the fibers inlumber or other wood products. When usedwith qualifying adjectives, it has specialmeanings concerning the direction of thefibers or the direction or size of the growthrings.Across the grain. The direction (or plane) at

right angles to the length of the fibersand other longitudinal elements of thewood.

Along the grain. The direction (or plane)parallel to the length of the fibers andother longitudinal elements in the wood.

Cross grain. Wood in which the fibers de-viate from a line parallel to the sides ofthe piece. Cross grain may be eitherdiagonal or spiral grain or a combinationof the two.

Edge grain. Wood that has been sawed orsplit so that the wide surfaces extend ap-proximately at right angles to the annualgrowth rings. Lumber is considerededge-grained when the rings form anangle of 45° to 90° with the wide surface ofthe piece.

End grain. The ends of logs or timbers, di-mension, boards, and other wood prod-ucts that are cut perpendicular to thefiber direction.

Flat grain. Lumber or other wood productssawed or split in a plane approximatelyperpendicular to the radius of the log.Lumber is considered flat-grained whenthe annual growth rings make an angleof less than 45° with the surface of thepiece.

Interlocked grain. Lumber or other woodproducts in which fibers are inclined inone direction in a number of rings of an-nual growth, then gradually reverse, andare inclined in an opposite direction insucceeding growth rings. This pattern isreversed repeatedly.

Raised grain. Lumber and other woodproducts in which a roughened conditionof the surface after planing, particularlysoftwood, is due to either the earlywoodor the latewood projecting above thegeneral level of the surface.

Slope of grain. In lumber and other woodproducts, the degree of cross grain. It isthe ratio between a 1-inch deviation of

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the grain from the long axis of a pieceand the distance along the edge withinwhich this deviation occurs.

Straight grain. Wood in which the fibersand other longitudinal elements runparallel to the axis of a piece.

Green volume. —Cubic content of green wood.Growth ring. —A layer of wood (as an annual

ring) produced during a single period ofgrowth.

Hardwoods. —Generally one of the botanicalgroups of trees that have broad leaves incontrast to the conifers or softwoods. Theterm has no reference to the actual hard-ness of the wood.

Heartwood. —The inner layer of a woody stemwholly composed of nonliving cells andusually differentiated from the outer en-veloping layer (sapwood) by its darkercolor. It is usually more decay resistantthan sapwood and more difficult to dry.

Heated room drying. —Drying air-dried woodin a room or small building, using justenough heat to raise the temperatureslightly above outdoor air temperature andprovide an EMC 2 percent below the de-sired final EMC. Mild air circulation pro-motes temperature uniformity.

High-frequency dielectric heating. ––The use ofan electric field oscillating at frequencies of1 to 30 million cycles per second to heatwood. The electric energy is applied to woodbetween metal plates as electrodes.

High-temperature drying. —In kiln dryingwood, use of dry-bulb temperatures of 212°F. or more.

Honey combing. —In lumber and other woodproducts, separation of the fibers in the in-terior of the piece, usually along the woodrays. The failures often are not visible onthe surfaces, although they can be the ex-tensions of surface and end checks.

Humidity. —The moisture content of air.Relative humidity. Under ordinary tem-

peratures and pressures, it is) the ratio ofthe weight of water vapor in a given unitof air compared with the weight whichthe same unit of air is capable of contain-ing when fully saturated at the sametemperature. More generally, it is theratio of the vapor pressure of water in agiven space compared with the vaporpressure at saturation for the same dry-bulb temperature.

Hygroscopicity. —The property of a sub-stance, such as wood, which permits it toabsorb and lose moisture readily.

Hygrostat. —A device for automatically reg-ulating the EMC of the air.

Hysteresis. —The tendency of dried woodexposed to any specified temperature andrelative humidity conditions to reachequilibrium at a lower moisture contentwhen absorbing moisture from a drier con-dition than when losing moisture from awetter condition.

Infrared drying. —A special process in dryingwood by direct radiation from high-intensity sources such as heat lamps andradiant gas burners.

Kiln. —A chamber or tunnel used for dryingand conditioning lumber, veneer, and otherlood products in which the temperatureand relative humidity of the circulated aircan be varied.Conventional kiln. Such kilns use initial

temperatures from 100° to 17W F andfinal temperatures from 150° to 200° F.Control of EMC is necessary to avoidshrinkage-associated defec ts and toequalize and condition the wood at theend of drying. Air velocities generally arebetween 200 and 450 feet per minute.

High temperature kiln. A kiln for dryinglumber and other wood products oper-ated at dry-bulb temperatures above212° F.

Kiln charge. In kiln drying, the totalamount of lumber or wood items to bedried in a dry kiln.

Kiln dried. Lumber or other wood itemsthat were dried in a closed chamber inwhich temperature and relative humid-ity of the circulated air can be controlled.

Kiln leakage. The undesirable loss of heatand vapor from a kiln through andaround doors and ventilators or throughcracks in the walls and roof.

Kiln operator. In kiln drying, the supervisoror person responsible for the perfor-mance of dry kilns and related equip-ment.

Kiln run. The term applied to the drying ofa single charge of lumber or other woodproduct.

Kiln sample. A length cut from a sampleboard and placed in the kiln charge sothat it may be removed for examination,

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weighing, or testing.Kiln schedule. In kiln drying, the prescribed

schedule of dry-bulb and wet-bulb tem-peratures used in drying a kiln charge oflumber or other wood products.

Low temperature kiln. Forced air drying ina moderately tight building equipped toproduce air movement through the loadsand recirculate the air over heat and/orhumidity sources, with dry- and wet-bulbcontrols to maintain small to moderatewet-bulb depressions in the temperatureranges between 85° and 120° F.

Package-loaded kiln. A trackless compart-ment kiln for drying packages of stick-ered lumber or other wood products. Thedryer usually has large doors that can beopened so that the kiln charge can beplaced in or removed from the dryer byforklift trucks. It is usually a forced-aircirculation kiln with fans mounted over-head or at the side.

Progressive kiln. A dry kiln in which thetotal charge of lumber is not dried as asingle unit but as several units, such askiln-truckloads that move progressivelythrough the dryer. The kiln is designed sothat the temperature is lower and therelative humidity higher at the enteringend than at the discharge end.

Reversible circulation kiln. A dry kiln inwhich the direction of air circulationthrough the stickered loads of lumber orother wood products can be reversed atdesired intervals.

Track-loaded kiln. A kiln, with doors at oneor both ends, for which loads are built upon kiln trucks outside the kiln and movedin and out of the kiln on tracks.

Knot. —That portion of a branch or limb thathas been surrounded by subsequentgrowth of the wood of the trunk or otherportions of the tree. As a knot appears onthe sawed surface, it is merely a section ofthe entire knot, its shape depending uponthe direction of the cut.

Layout. —On an air-drying yard, layout refersto arrangement and orientation of alleys,row and line spacings, pile sizes and spac-ings, and foundation placement.

Losses, drying. —In drying lumber and otherwood products, the reduction in volume andgrade quality that can be attributed to thedrying process.

Lumber. —The product of the sawmill andplaning mill not further manufacturedthan by sawing, resawing, passinglengthwise through a standard planingmachine, cross cutting to length, andmat thing.Boards. Yard lumber less than 2 inches

thick and 1 or more inches wide.Common lumber. A classification of medium

and low-grade hardwood lumber and/orsoftwood lumber suitable for generalconstruction and manufacturing but notsuitable for finish grade.

Dimension lumber. As applied to hardwoodlumber, a term loosely used but generallyreferring to small squares or pieces ofrectangular cross section used for furni-ture and like purposes (small dimension).

Dressed lumber. The dimensions of lumberafter drying and surfacing with a planingmachine.

Finish lumber. A collective term for uppergrades of lumber suitable for natural or

stained finishes.Flooring lumber. Generally, a grade of

either hardwood or softwood boards thathave been found to produce maximumquantity of flooring of the desired qual-ity.

Nominal size. As applied to timber orlumber, the size other than the actualsize, by which it is known and sold in themarket.

Rough lumber. Lumber as it comes from thesaw.

Structural lumber. Lumber that is nomi-nally 2 or more inches thick and nomi-nally 4 or more inches wide, intended foruse where working stresses are required.The grading of structural lumber isbased on the strength of the piece andthe use of the entire piece, e.g., stud,joist, beam, plank, girder, rafters, fram-ing, etc.

Yard lumber. Lumber of all sizes and pat-terns that is intended for general build-ing purposes, The grading of yard lumberis based on the intended use of the par-ticular grade and is applied to each piecewith reference to its size and lengthwhen graded, without consideration tofurther manufacture.

Lumen. —In wood anatomy, the cell cavity.Microwave heating. —Heating a material

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using electromagnetic energy alternatingat a frequency from 915 megahertz to22,125 megahertz.

Mineral streak. —An olive to greenish-black orbrown discoloration in hardwoods, particu-larly hard maples, due to wound-inducedbacterial, chemical, or fungal action. Nar-row streaks often contain accumulations ofmineral matter.

Moisture content. —The amount of water con-tained in the wood, usually expressed as apercentage of the weight of the ovendrywood.Average moisture content. The moisture

content, in percent, of a single sectionrepresentative of a larger piece of woodor the average of all the moisture contentdeterminations made on a board or otherwood item or of a number of determina-tions made on a lot of lumber or otherwood products.

Shipping dry. Lumber partially dried toprevent stain or mold in brief periods oftransit, preferably with the outer 1/8 inchdried to 25 percent MC or below.

Moisture meter. —An instrument used forrapid determination of the moisture con-tent of wood by electrical means.

Moisture movement. —The transfer of mois-ture from one point to another within woodor other materials.

Mold. —A fungus growth on lumber or otherwood products at or near the surface and,therefore, not typically resulting in deepdiscolorations. Mold is usually ash green todeep green in color, although black and yel-low are common.

Final moisture content. The average mois-ture content of the lumber or other woodproduct at the end of the drying process.

Initial moisture content. The moisture con-tent of the wood at the start of the dryingprocess.

Movement. —The alternate swelling andshrinkage that occurs in dried wood due tomoisture content changes caused by varia-tions in the surrounding atmospheric con-ditions.

In-use moisture content. The moisture con-tent that wood items attain in the envi-ronmental conditions of usage.

Range. The difference in moisture contentbetween the driest and wettest boards ina shipment, lot, kiln charge, etc., or be-tween representative samples of the lot.

Moisture gradient. In lumber drying, thedistribution in moisture content withinthe wood. During drying, the differencesare between the low moisture content ofthe relatively dry surface layers and thehigher moisture content at the center ofthe piece.

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Out-of-round. —A form or warp, the ellipticalshape assumed by turned circular greenwood items upon drying due to the differ-ence in tangential and radial shrinkage.

Overhang. —The ends of boards that are un-supported by stickers and extend beyondthe ends of most boards in an air dryingpile, kiln truckload, or unit handling pack-age.

Moisture content classes.-Air dried. Wood having an average mois-

ture content of 25 percent or lower, withno material over 30 percent.

Green. Freshly sawed wood or wood thatessentially has received no formal dry-ing.

Kiln dried. Dried in a kiln or by some otherr e f i n e d m e t h o d t o a n a v e r a g e M Cspecified or understood to be suitable fora certain use, such an average generallybeing 10 percent or below for hardwoods.

Ovendry. —A term used to describe wood thathas been dried in a ventilated oven at 212°to 221° F. (100 to 105° C) until there was nofurther significant loss in weight.

Ovendry weight. —Weight of wood when allthe water has been driven off by heating inan oven at 212 to 221° F. (100 to 105° C).

Ovendrying. —Drying wood specimens to con-stant weight in an oven maintained at 212°to 221° F. (100 to 105° C).

Part-time drying. —In kiln drying, discon-tinuous operation of the dry kilns, usuallynecessitated by an interrupted steam, fuel,or power supply.

Permeability. —The ease with which a fluidflows through a porous material (wood) inresponse to pressure.

Pile. —In air drying, stacking lumber layer by

Kiln-dried lumber can be specified to befree of drying stresses.

Partly air dried. Wood with an average MCbetween 25 and 45 percent, with no mate-rial over 50 percent.

layer, separated by stickers or self snicker-ing, on a supporting foundation (handstacked). Also, stickered unit packages bylift truck or crane, one above the other on afoundation and separated by bolsters.Box pile. A method of flat stacking random-

length lumber for air drying or kiln dry-ing. Full-length boards are placed in theouter edges of each layer and shorterboards in between are a l ternatedlengthwise to produce square-end piles,unit packages, or kiln truckloads.

Crib pile. Stacking lumber to form a hollowtriangle with their faces in contact at thecorners.

Hand-stacked pile. Lumber and other woodproducts stacked by hand, course bycourse, on suitable foundations to buildthe pile.

Machine-stacked pile. Unit packages ofstickered lumber or other wood productsstacked by mechanical means onto a pilefoundation and one above another tobuild a pile of packages.

Open pile. Spacing the boards or otherrough-sawn wood products in stickeredlayers and in wide piles or packages in-corporating flues in addition to boardspacing.

Pile roof. A cover on top of the pile to pro-tect the upper layers from exposure tothe degrading influences of sun, rain, andsnow. The sides and ends of the roof mayproject beyond the pile to provide addedprotection.

Pile spacing. The distance between indi-vidual piles in the row.

Random-length pile. Stacking lumber ofvarious lengths in the same pile or pack-age. The pile or package is usually squareat one end with the long length at theother end unsupported by stickers.

Self-stickered pile. Stacking in which thestock is used as stickers to separate thelayers. In crib stacking, the boards are incontact at the three corners. In level orsloped stacking of softwood boards anddimension, stock is used for stickers andthe pile width is the same as the length.Hardwood dimension stock and railroadties are often self stickered.

Pit. ––In wood anatomy, a recess in the secon-dary wall of a cell where a thin membrane

may permit liquids to pass from one cell toanother.

Pitch. ––The mixture of rosin and turpentineor other volatiles produced in the rosincanals of pines and other conifers.

Predryng. —A wood drying process carriedout in special equipment before kiln drying.

Predryng treatments. ––Special measures be-fore or early in drying to accelerate dryingrate, to modify color, or to prevent checksand other drying defects.Blanking. Surfacing one face of a rough-

sawed board to achieve uniform thick-ness and reduce warping.

Chemical. Impregnating the outer zone ofgreen lumber with hydroscopic andsometimes bulking chemicals to aboutone-tenth the thickness to reduce surfacechecking during drying. Involves controlof relative humidity at a value below theRH that is in equilibrium with the satu-rated chemical solution.

Polyethylene glycol. Deeply impregnatinggreen lumber with polyethylene glycol1000 (PEG) to retain the green dimensionduring drying and indefinitely thereaf-ter, minimizing shrinkage, swelling, andwarp.

Precompression. Momentarily compressinggreen lumber transversely about 7.5% topermit drying by a severe schedule andimprove drying behavior.

Prefreezing. The freezing and subsequentthawing of green wood before drying toincrease dry ing rate and decreaseshrinkage and seasoning defects.

Presurfacing. Surfacing both broad faces ofgreen rough-sawed boards to permit dry-ing by a schedule more severe than theprescribed schedule for rough lumber,achieving faster drying and fewer dryingdefects.

Steaming. Subjecting green wood to satu-rated steam at or close to 212° F to accel-erate drying, achieve a desirable color, orboth.

Surface treatment. Applying a salt orsodium alginate paste to the surface ofgreen wood to help prevent checking asthe wood is dried by other means.

Press drying. —The application of heat to op-posite faces of a board by heated platens toevaporate moisture from the board, using

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temperatures between 250° and 450° F andplaten pressures between 25 and 75 poundsper square inch.

Radial surface. —A longitudinal surface orplane extending wholly or in part from thepith to the bark.

Recorder-controller. —An instrument thatcontinuously records dry- and wet-bulbtemperatures of circulated air and regu-lates these temperatures in a dryer or kilnby activating automatic heat and steamspray valves.

Redry. ––In kiln or veneer drying, a processwhereby dried material found to be at amoisture content level higher than desiredis returned to the dryer for additional dry-ing.

Refractory. —In wood, implies difficulty inprocessing or manufacturing by ordinarymethods; resistance to the penetration ofpreservatives, difficulty in drying, or diffi-culty in working.

Ring failure. ––A separation of wood along thegrain and parallel to the annual rings,either within or between the rings.

Rot.—Brown rot. In wood, any decay that concen-

trates on attacking the cellulose ratherthan lignin, producing a light to darkbrown friable residue.

Dry rot. A term loosely applied to any dry,crumbly rot but especially to that which,when in an advanced stage, permits thewood to be crushed easily to a dry pow-der. The term is actually a misnomer forany decay, since all fungi require con-siderable moisture for growth.

White rot. In wood, any decay attackingboth the cellulose and lignin, producing agenerally whitish residue that may bespongy or stringy or occur in pockets.

Rounds. —Pieces of green wood dried in theform of cylinders. For example, ashbaseball bat stock is often kiln dried asrough turned rounds rather than squares.

Sap. ––The moisture in green wood, contain-ing nutrients and other chemicals in solu-tion.

Sapwood. ––In wood anatomy, the outer layersof the stem that in the living tree containliving cells and reserve materials, e.g.,starch. The sapwood is generally lighter incolor than the heartwood.

Season. —To dry lumber and other wood itemsto the desired final moisture content andstress condition for its intended use.

Set. ––A semipermanent deformation in woodcaused by tensile and compressive stressesduring drying.Compression set. Set, occurring during

compression, that tends to give the wooda smaller than normal dimension afterdrying, usually found in the interior ofwood items during the later stages of dry-ing but sometimes in the outer layersafter extended conditioning or rewetting.Also caused by external restraint duringrewetting of dried wood.

Tension set. Set, occurring during tension,that tends to give wood a larger thannormal dimension after drying; usuallyoccurring in the outer layers during theearly stages of drying.

Shrinkage. —The contraction of wood fiberscaused by drying below the fiber saturationpoint. Shrinkage—radial, tangential andvolumetric—is usually expressed as a per-centage of the dimension of the wood whengreen.Longitudinal shrinkage. Shrinkage of wood

along the grain.Radial shrinkage. Shrinkage across the

grain, in a radial-transverse direction.Tangential shrinkage. Shrinkage across the

grain, in a tangential-transverse direc-tion.

Volumetric shrinkage. Shrinkage of wood involume.

Sinker. —A log which sinks in water.Sinker stock. —Lumber or other sawmill

products sawed from sinker logs. The greenmoisture content is very high and the dry-ing rate can be low.

Softwood. —Generally, one of the botanicalgroups of trees that, in most cases, haveneedlelike or scalelike leaves; the conifers;also, the wood produced by such trees. Theterm has no reference to the actual hard-ness of the wood.

Spacing.––Board spacing. The spacing of boards or

other rough-sawn wood products, in acourse of a pile, kiln truckload, or stick-ered unit package.

Pile spacing. Spacing between the piles inthe row or line of piles in the yard.

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Row spacing. Spaces between rows of pilesin the yard.

Sticker spacing. Distance between adjacentstickers in a pile, kiln truckload, or astickered unit package of lumber.

Species. —A group of individual plants of aparticular kind; that is, a group of indi-viduals sharing many of the same charac-teristics. It is a category of classificationlower than the genus but higher than thevariety.

Specific gravity. ––In wood technology, theratio of the ovendry weight of a piece ofwood to the weight of a volume of water at4° C. (39° F.), equal to the volume of thewood sample. Specific gravity of wood isusually based on the green volume andovendry weight.

Stacking. ––Constructing packages, piles, orkiln truck loads of lumber whose courses orlayers are separated by stickers to facilitatedrying the lumber.

Stain. —A discoloration in wood that may becaused by micro-organisms, metal or chem-icals. The term also applies to materialsused to impart colors to wood.Blue. A bluish or grayish discoloration in

the sapwood caused by the growth of cer-tain dark-colored fungi.

Chemical. A general term including allstains that are due to color changes ofthe chemicals normally present in thewood.

Iron-tannate. A surface stain, bluish-blackin color, on oak and other tannin-bearingwoods following contact of the wet woodwith iron, or with water in which iron isdissolved.

Sticker. —A wooden strip, or its substitute,placed between courses of lumber or otherwood products, in a pile, unit packages orkiln truckload, at right angles to the longaxis of the stock, to permit air to circulatebetween the layers.Dry sticker. A sticker that has been made

from lumber that was dried prior tosticker manufacture.

Green sticker. A sticker cut from greenlumber or made by processing slabs andedgings into stickers.

Sticker marking. Indentation or compres-sion of the lumber or other wood productby the sticker when the superimposedload is too great for the sticker bearing

area. Also, sometimes light areas underthe sticker as the rest of the boardbrowns.

Sticker alinement. The placing of stickers ina pile, unit package, or kiln truckload oflumber or other wood products so thatthey form vertical tiers.

Stock stickers. In air drying lumber, use ofboards being piled (hand-stacked) asstickers. Usually restricted to softwoodsbeing air dried in semi-arid and arid re-gions.

Storage. —Bulk or stickered piling of air- orkiln-dried wood products with protectionfrom the weather in accordance with thedesired level of moisture content; protec-tion might be tarpaulins, or open, closed, orclosed and heated sheds.

Sun shields. ––In air drying, plywood panels,boards, or some other type of shield placedto protect the ends of piles from direct sun.The purpose is to retard end drying, thusminimize end-checking and end-splitting.

Swelling. — Increase in the dimensions ofwood due to increased moisture content.Swelling occurs tangentially, radially, andto a lesser extent, longitudinally.

Tangential surface. —Surface tangent to thegrowth rings; a tangential section is a lon-gitudinal section through a log perpendicu-lar to a radius. Flat-grained lumber issawed tangentially.

Temperature. —May be defined as the condi-tion of a body which determines the trans-fer of heat to or from other bodies; it is ameasure of the thermal potential of a body.Dry-bulb temperature. Temperature of air

in a yard or drying apparatus indicatedby any temperature-measuring devicewith its sensitive element or bulb uncov-ered.

Mean monthly temperature. In air drying,the average dry-bulb temperature over aperiod of a month. Usually obtained fromWeather Service records.

Wet-bulb temperature. The temperatureindicated by any temperature-measuringdevice the sensitive element of which iscovered by a water-saturated cloth(wet-bulb wick).

Tension failure. ––The pulling apart or ruptur-ing of wood fibers, as a result of tensilestresses.

Tension wood. ––In wood anatomy, reaction

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wood formed typically on the upper sides ofbranches and leaning hardwood trees, andcharacterized anatomically by little or nosignification and by the presence of aninternal gelatinous layer in the fibers. Ith a s a n abnormally high longitudinalshrinkage. The machined surface tends tobe fibrous or woolly, especially when green.

Tier. —In air drying or kiln drying, a stack ofpackages of lumber or other wood productsin vertical alinement. Also refers to stickeralinement from layer to layer.

Transverse. —The directions in wood at rightangles to the wood fibers or across thegrain. A transverse section is a sectionthrough a tree or timber at right angles tothe pith.

Unit package. —Lumber or other wood prod-ucts that have been assembled into a parcelfor handling by a crane, carrier, or forklifttruck.

Vapor barrier. —In kiln drying, a materialwith a high resistance to vapor movementthat is applied to dry kiln surfaces to pre-vent moisture migration.

Vapor drying. —Drying wood by subjecting itto the hot vapors produced by boiling anorganic chemical such as xylene.

Veneer. —A thin layer or sheet of wood pro-duced on a lathe, slicer, or saw and is com-monly referred to as rotary, sliced, orsawed veneer.

Vent. —In kiln drying, an opening in the kilnroof or wall that can be opened and closedto control the wet-bulb temperature withinthe kiln.

Volume, green. —The volume of wood deter-mined from measurements made while thewood moisture content is above the fibersaturation point (i.e., above 30 pct moisturecontent; the green state).

Wane. —Bark, or the lack of wood from anycause, on any edge of a piece of square-edged lumber.

Warp. —Distortion in lumber and other woodproducts causing departure from its origi-nal plane, usually developed during drying.

Warp includes cup, bow, crook, twist, out-of-round, kinks, and diamonding, or anycombination thereof.

Warp restraint. —In drying lumber and otherwood products the application of externalloads to a pile, package, or kiln truckload ofproducts to prevent or reduce warp.

Weight of wood. —The weight of wood dependson its specific gravity and its moisture con-tent. Weight includes the ovendry woodand the moisture it holds. It is expressed aspounds per cubic foot at a certain moisturecontent or weight per 1,000 board feet at aspecified moisture content.

Wetwood. —Wood with abnormally high watercontent and a translucent or water-soakedappearance. This condition develops only inliving trees and does not originate throughsoaking logs or lumber in water.

Wood. —The tissues of the stem, branches, androots of a woody plant lying between the

pith and cambium, serving for water con-duction, mechanical strength, and foodstorage, and characterized by the presenceof tracheids or vessels.Diffuse-porous wood. Wood in which the

pores are of fairly uniform or of onlygradually changing size and distributionthroughout the annual growth ring.

Nonporous wood. Wood devoid of pores orvessels; characteristic of conifers.

Porous wood. Wood with vessels; typical ofhardwoods as opposed to conifers.

Reaction wood. Wood with more or less dis-tinctive anatomical characteristics,formed in parts of leaning or crookedstems and in branches. In hardwoods thisconsists of tension wood and in conifers ofcompression wood.

Residue. In the woodworking industry, thatportion of the wood developed as bark,slabs, edgings, trim, sawdust, and shav-ings.

Ring-porous wood. Wood in which the poresof the earlywood are distinctly largerthan those of the latewood and form awell-defined zone or ring.

✩ U.S. GOVERNMENT PRINTING OFFICE: 1978 O—258—680

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