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  • Abstract This report describes how lumber can be air-dried most effectively under outdoor conditions and illustrates the principles and procedures of air-drying lumber that were developed through field investigations and observations of industrial practices. Particular emphasis is placed on the yarding of lumber in unit packages. Included are topics such as why lumber is dried, advantages and limitations of the drying process, properties of wood in relation to drying, layout of the drying yard, piling methods, causes and reme- dies of air-drying defects, and protection of air-dried lumber.

    Keywords: drying lumber, air dry, wood structure, wood shrinkage, drying rate, wood defects

    October 1999

    Forest Products Laboratory. 1999. Air drying of lumber. Gen. Tech. Rep. FPL–GTR–117. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 62 p.

    A limited number of free copies of this publication are available to the public from the Forest Products Laboratory, One Gifford Pinchot Drive, Madison, WI 53705–2398. Laboratory publications are sent to hundreds of libraries in the United States and elsewhere.

    The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin.

    The United States Department of Agriculture (USDA) prohibits discrimina- tion in all its programs and activities on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, or marital or familial status. (Not all prohibited bases apply to all pro- grams.) Persons with disabilities who require alternative means for commu- nication of program information (braille, large print, audiotape, etc.) should contact the USDA’s TARGET Center at (202) 720–2600 (voice and TDD). To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326-W, Whitten Building, 14th and Independence Avenue, SW, Washington, DC 20250–9410, or call (202) 720–5964 (voice and TDD). USDA is an equal opportunity provider and employer.

    Preface This manual is a revision of the 1971 edition of Air Drying of Lumber: A Guide to Industry Practice by Raymond C. Rietz and Rufus H. Page. A major contributor to the 1971 edition was Edward C. Peck, formerly a wood drying specialist at the Forest Products Laboratory.

    The major reason for this revison is the continued interest in and requests for the manual, even almost 30 years after initial publication. Because of this continued interest, we main- tained the same basic framework in the revision. Our main objective was to update with new or more current informa- tion where necessary, keeping in mind that our audience will range from small industry (perhaps new to lumber drying) to larger industry (perhaps having substantial experience in air drying). Involved in this revision were

    William T. Simpson, Research Forest Technologist,

    John L. Tschernitz, Chemical Engineer (retired), and

    James J. Fuller, Research Forest Products Technologist,

    of the Forest Products Laboratory.

    Acknowledgments Special recognition is given to Edward C. Peck, formerly a Wood Drying Specialist at the USDA Forest Service, Forest Products Laboratory, for his research and related contribu- tions that formed the basis for most of this publication. We also appreciate the cooperation of lumber-producing indus- tries in providing photographs of their operations.

  • Air Drying of Lumber

    USDA Forest Service Forest Products Laboratory

    Madison, Wisconsin

  • Contents Page

    Introduction ......................................................................... 1

    1. Why Dry Lumber?.......................................................... 3

    Drying Methods .............................................................. 3

    Objectives ....................................................................... 4

    Advantages and Limitations............................................ 6

    2. Wood Properties and Moisture....................................... 7

    Structure.......................................................................... 7

    Moisture Content .......................................................... 10

    Methods to Determine Moisture Content...................... 10

    Moisture Movement...................................................... 12

    Equilibrium Moisture Content ...................................... 14

    Fiber Saturation Point ................................................... 14

    Shrinkage ...................................................................... 14

    Weight .......................................................................... 18

    Color ............................................................................. 18

    3. Air-Drying Process ....................................................... 21

    Utilizing Air Movement................................................ 21

    Factors That Influence Drying Rate.............................. 21

    Drying Time and Final Moisture Content ..................... 23

    Deterioration of Lumber ............................................... 25

    4. Air-Drying Yard ........................................................... 27

    Yard Layout and Orientation ........................................ 27

    Yard Transportation Methods....................................... 29

    Pile Foundations ........................................................... 29

    Shed Drying .................................................................. 31

    Yard Operation and Maintenance ................................. 32

    Page

    5. Piling Methods For Air Drying..................................... 33

    Sorting Lumber ............................................................. 33

    Sorting Equipment ........................................................ 34

    Stacking Lumber in Packages ....................................... 34

    Stacking Random Length Lumber ................................ 38

    Special Piling Methods ................................................. 38

    Stickering Lumber ........................................................ 39

    6. Air-Drying Defects—Causes and Remedies................. 43

    Chemical Reaction........................................................ 43

    Fungal Infection............................................................ 43

    Insect Infestation........................................................... 44

    Shrinkage ...................................................................... 44

    Warp Reduction............................................................ 48

    Stacking, Package Piling, and Pile Protection .............. 49

    7. Protection of Air-Dried Lumber ................................... 51

    Outdoors ....................................................................... 51

    Indoors.......................................................................... 51

    In Transit ...................................................................... 53

    Appendix. Summarized Guide for Air-Drying Practices .. 55

    Glossary............................................................................. 58

  • Introduction

    Trees contain a considerable amount of water. Most of this water must be evaporated before the lumber obtained from a tree can be converted into consumer products. The lumber from which most wood products are manufactured must be dried.

    Of all the methods to remove large quantities of water from wood, air drying has the least capital costs, especially in the early stages of drying. However, air drying offers little opportunity to control the drying process.

    Drying wood by allowing natural forces to evaporate the water is not a new idea. Over the years, people learned how to utilize these forces effectively. However, this information accumulated slowly, and many individuals are aware of only part of the technology. This publication assembles informa- tion on air drying learned through research and information that has evolved from many years of experience.

    This report gives plant managers, yard supervisors, lumber handlers, and others in related areas an overview of the principles of air drying. It will also assist them in analyzing their air-drying practices. Drying wood by any means is expensive, so operators are constantly on the alert to reduce costs. Application of general air-drying principles may lead to changes in yarding methods that will result in faster drying of lumber and improved quality.

    The results of efficient air drying benefit consumers of forest products and are of key importance in utilization of the

    Nation’s forest resource. Efficient air drying helps to con- serve supplies of wood, thereby the timber resource, by reducing loss of product from drying degrade. Air drying also reduces the need to burn fuels for energy to dry lumber, thus conserving these fuels and reducing atmospheric emis- sio

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