Stabilizing and BendingStabilizing and BendingStabilizing and BendingStabilizing and BendingStabilizing and Bendingof Wof Wof Wof Wof Wood for the Hobbyistood for the Hobbyistood for the Hobbyistood for the Hobbyistood for the HobbyistStabilizing and BendingStabilizing and BendingStabilizing and BendingStabilizing and BendingStabilizing and Bendingof Wof Wof Wof Wof Wood for the Hobbyistood for the Hobbyistood for the Hobbyistood for the Hobbyistood for the Hobbyist

Lumber scraps and cross-sections of angle cutssawed from small logs or large limbs can be made intoattractive items, but only if the wood is first treated withpolymers and then properly seasoned, or if the wood isinitially seasoned following special techniques. Thesetreatments and techniques can result in profitable uses of�low grade� logs and oddly shaped scraps that mightotherwise be discarded. This publication describes sometechniques of stabilizing and bending wood for hobby-ists.

Freshly cut cross-sections normally check badlyand develop typical pie-shaped or v-shaped cracks duringthe drying process. This is because wood shrinks twice asmuch in the tangential direction (parallel to the annualrings) as in the radial direction perpendicular to theannual rings). The internal stresses that result from suchdifferential shrinkage invariably cause serious checks andsplits as the wood loses moisture and comes to moisturecontent (MC) in a heated or air-conditioned wood shop.These are some of the techniques for drying wood cross-sections. These techniques can also be used for lumberscraps.

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Stabilizing and BendingStabilizing and BendingStabilizing and BendingStabilizing and BendingStabilizing and Bendingof Wof Wof Wof Wof Wood for the Hobbyistood for the Hobbyistood for the Hobbyistood for the Hobbyistood for the Hobbyist

PEG-1000 Chemical Treatment

One method recommended often in recent yearsis stabilization of wood with a bulking agent such aspolyethylene glycol (PEG). PEG treatment requiressoaking the wood for a long time. This tends to darkenthe sapwood. Also, PEG is hygroscopic and raises the

MC of the wood;under humidconditions, thechemical maycause the finishedboard surface todevelop discol-ored streaks. Toavoid theseproblems, youmust dry the diskswithout chemicals.

Manyhobbyists usePEG as a bulkingagent that greatlyreduces thedimensionalchanges of greenwood. PEG is awhite, wax-likechemical thatresembles paraf-fin, is solid atroom tempera-ture, has an

average molecular weight of 1000 and dissolvesreadily in warm water. PEG melts at 104 degreesFahrenheit, is nontoxic, noncorrosive, odorless,colorless and has a very high firing point (580degrees Fahrenheit). It is chemically related toantifreeze.

The PEG treatment, which physically bulks thewood cell walls (fibers), prevents shrinkage and thusprevents the development of destructive stresses. Greenwood heavily treated with PEG retains its �green dimen-sions� indefinitely, and thus is permanently restrainedfrom shrinking, swelling or warping, regardless of humid-ity changes. The detailed treatment procedures are asfollows:

Figure 1. (Right)Unusual piecesshould notautomatically bediscarded. (Below)This clock is anexample of value-added production ofwaste wood. Thewood was treatedwith PEG andcoated withpolyurethane.

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1. Choose a treating tank: This is used for soaking thewood disks or slabs. It can be made of glass or anynonmetallic substance. A large plastic garbage can thathas been lined with a thick plastic bag can also be usedfor the soaking tank. The treating tank needs a cover tohelp conserve heat and water. Some water will evaporateand change the concentration of the PEG solution, butthis will be balanced as the PEG is absorbed by the wood.It is also desirable to cover the sides, bottom, and espe-cially the top, with fiberglass insulation to conserve heat.

2. Preparing PEG solutions: Wood may be soaked ineither a 30% or a 50% PEG solution. Soaking times arelonger in the 30% solution. To prepare seven quarts of30% solution, dissolve 4.46 pounds of PEG-1000 in fivequarts of water. The specific gravity of the solution (at 60degrees Fahrenheit) will be 1.05. Check the specificgravity of the solution, using a hydrometer that is similarto the one used to check the concentration of antifreezein the radiator of a car. These solutions can be reused.Between uses, store them in glass containers. One to 2%borax added to the solution will prevent the growth ofsome bacteria and fungi, but not mildew and mold.

3. Put fresh wood disks in solution: Stack wood withthin slats separating the pieces, so that circulation of thesolution is complete. Weight down the wood with a rockto assure complete immersion. Only green wood (withMC above the FSP or approximately 30% MC) can betreated effectively by the PEG method.

4. Heating: For faster results, equip treating tanks withelectric heating elements (with a built-in thermostat) ofthe type used in a water heater. A small pump may beused periodically to circulate the PEG solution. Hand-stirring will do the job of the pump, but regularity isimportant.

5. Soaking time: This depends on solution strength andtemperature, wood species and wood thickness. Table 1shows the treating time necessary for walnut disks toobtain the desired 25% to 30% concentration of PEG inthe finished product.

Woods of lower density, such as soft maple,cottonwood, willow and pine, require only one-half totwo-thirds the time suggested for walnut. Heavy woods,

such as hard maple, oak, apple, beech and birch, require asoaking time two or three times longer than does walnut.Very hard woods, such as mesquite and the burls of allspecies, must be treated at high temperatures for longerperiods.

Wood knots,cherry wood and theheartwood of somespecies resist absorptionof PEG and requirespecial attention. Forexample, soaking cherryat a temperature above110 degrees Fahrenheitwill heart-check orhoneycomb the wood.For treating 1-in. thickcherry disks, allow atleast 45 days in a 50%solution at or below 110degrees Fahrenheit.

Soaking schedules for other types of wood canbe developed by experimentation. At first, soak the woodfor the time recommended for walnut. Then dry thewood in an oven at a temperature of about 200 degreesFahrenheit. If no defects develop, the soaking time isprobably sufficient.

6. Drying: Pile the treated wood using dry stickers. Thepile should be covered and kept in a well-ventilated room,preferably a heated area. PEG-treated wood cross-sections up to 4 inches thick and 40 inches in diameterwill dry in six to eight weeks if they are kept in a heatedroom at low humidity. Treated wood kept outside must becompletely protected from water, and the drying will takesomewhat longer than indoors. Since treated woodshrinks very little during drying, it is not necessary thatthe wood be completely dry before you start workingwith it. It is only necessary that the top 1/4 inch of thesurface be dry enough to sand well.

7. Gluing: White glue or polyvinyl acetate does not workvery well on PEG-treated wood. Only urea resins, epoxiesand resorcinols are satisfactory. For joints that requirestrength, a solvent should be used to cut the wax andexpose the wood fibers. The wood surface should bescrubbed and washed with alcohol.

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Table 1. Treatment schedule using PEG-1000.

Solution strength and Soaking time for wood Soaking time fortemperature up to 9 in. in diameter wood more than 9 in.(degrees and 1-1.5 in. thick in diameter and 2-3 in.Fahrenheit) (Days) thick (Days)

30%, (70) 20 6030%, (140) 7 3050%, (70) 15 4550%, (140) 3 14

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8. Finishing: PEG-treated wood may be finished afterthe surface of the wood has been sanded and shaped.Common single-component polyurethane varnishes givesatisfactory results. Conventional wood finishes do notwork well on wood treated with PEG. If high MC createsproblems, use a base of four to five coats of the mois-ture-resistant polyurethane resin varnishes. Danish oilpenetrating finish works well over PEG-treated wood,although many other oil-type clear finishes cannot beused. The Danish oil finish can be used for PEG-treatedbark.

9. Treating defective pieces:Untreated wooden objectspurchased in other parts of thecountry may check and splitbadly when introduced to ourair-conditioned and heatedenvironment. When suchdefects are noted, soak thepiece in water and treat withPEG at once. If the checksclose completely, they willremain closed, and no furtherdefects will develop.

Salt Paste Method

This is a simple andinexpensive way to treat a woodcross-section. The top and bottom of the green disk arealternately brushed with a concentrated table salt paste,then the disk is dried. To mix the salt paste, add thre epounds of salt to one gallon of water. Mix thoroughly.Salt crystals will be visible in the solution even aftermixing. Allow the solution to stand for several hours,then mix in enough cornstarch to achieve a pastelikeconsistency that will build up a thick layer on the disksurfaces. Finally, mix in several egg whites (this willreduce flaking of the paste after it dries) and apply thepaste to both faces of the cross-sections. The salt pasteshould have the consistency of oil paint or cake batter.Air dry the disks in a well-ventilated or heated room.

Drying Disks Without Chemical Treatment

No method of drying large cross-sections ofwood will guarantee that cracks will not appear. The mostcommon defect is large edge-to-center �V� cracks

Figure 2. This cross-section of oakwood was improperly dried and hasdeveloped �V� cracks because of thedifferent shrinkage rates of the wood.

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(figure 2). Also, disks will tend to show many short crackson the face parallel to the wood rays. The rays in somespecies are visible and look like spokes in a wheel thatoriginate in the center of the disk and extend out in a�radial� manner to the bark. Sometimes cutting the slabsat an angle to the axis of the tree reduces the tendencyof the disk to �V� check but will not affect the potentialof radial checks to develop.

One simple way of drying untreated 1-inch disksinvolves three drying stages. Designed to minimizechances for the formation of checks, they are:

1 . First Stage - Tape plastic film over the inner section(heartwood) of each side of the green disk. The outerportion (sapwood) of the section is allowed to dry to30% MC at a relative humidity of 65% to 75%. This fastdrying will avoid discoloration, stains and separation ofthe bark. About three to four days are needed. The plasticfilm will prevent heart checks.

2. Second Stage - The pitch area can now be exposed toan environment of 75% to 80% relative humidity (RH)and allowed to begin to dry. Let the MC of heartwoodand sapwood both decrease to 25%. Maintain a moderatedrying rate to avoid surface checks. At least three daysare required for this stage.

3. Third Stage - Slow drying at a decreased rate is accom-plished by placing the disks in containers with tinyopenings, such as plastic bags with small punctures. Theroom humidity should be adjusted to 40% RH. In thecontainer, the evaporating moisture provides the properatmosphere with 80% to 90% RH. Little vapor escapes,and the wood dries at a decreasing rate, reaching the MCof the room after four weeks. While heaters can be usedto raise the temperature and shorten the final stage,higher temperatures make it more difficult to maintainthe necessary humidity. The container should be insu-lated to prevent condensation at cool spots.

Chemical-free drying can also be accomplished withoutplastic bags. The following guidelines can help you andwill reduce the likelihood of checking.

1 . Use temperature close to room temperature for theearly stages of drying.2. Use high relative humidity in the early stages of drying.

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3. Thin disks (2 in. or less) are less likely to crack thanthick disks, but tend to dish in drying.4. Thick disks dry very slowly (doubling the thicknessincreases the drying time by 3-4 times).5. Since the basic cause of the �V� crack is the differencein radial and tangential shrinkage of wood, choosing aspecies with a low ratio of tangential (perpendicular tothe rays)-to-radial (parallel to the rays) shrinkage willminimize the tendency to form a �V� crack. Mesquite is adomestic wood that offers these properties. There are alsosome foreign woods with similar radial and tangentialshrinkage properties.

Bending of Wood

Wood bending is more of an art than a science.This ancient art is still of economic importance todaybecause it is easier, faster, more efficient and requires lessexpensive equipment than machining. The bending ofwood is critical in the manufacture of some furnitureitems (figure 3), various tools and novelty items. Bentwood is usually stronger and stiffer than machined partsof equal dimension and requires less surface preparationfor finishing.

The two common methods to plasticize woodbefore it can be bent are by using steam and with liquidammonia. Species that often bend well using either thesteam or ammonia treatment are the oaks, yellow-poplar,black cherry and sweetbay. For native Louisiana woodspecies, hackberry is probably one of the easier to bendby either method. Sweetgum, black gum, yellow-poplarand red maple often are difficult to bend. The bending

Figure 3. (Below) This is a

classic chair produced with

typical wood bending. Allpieces except the front legs are

f rom bent wood. (Left) A

restraining strap, center,follows the convex side of this

two-plane bend. An

adjustable end-pressure stop iscontrolled by the operator�s

hand at lower left.

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behavior of any wood will depend on the species, treat-ment (steam or ammonia) and setting procedures. Forany species to yield good bent wood structures, it mustbe knot and decay free, contain no cross or twisted grainand not be excessively low in density. Flat sawn lumberusually bends well. There is a great deal of disagreementconcerning what moisture content is best for bendingwood, but most moisture contents reported by research-ers are less than 25%. Steam-bent wood success dependson steam time. It is somewhat weakened and may changeits curvature with time, depending on exposure condi-tions and amount of restraint during use.

Steam bending is often done at atmosphericpressure (212 degrees Fahrenheit) or at gauge pressuresbelow 5 psi. Most Louisiana woods need only 45 minutesto be sufficiently heated. The softening effect of ammo-nia is similar to that of steam, except that ammonia hasbetter dissolving properties. The use of ammonia allowsthe hobbyist to make a variety of artistic, decorative andarchitectural products.

Bending can be done with and without (free) endpressure (figure 4). Free bending is satisfactory for mildbends. Bent wood will tend to return to its original formunless restrained until set. The setting of a bend is the

Figure 4. A hydraulic press is

used to produce extreme bends

in chair arms and some otherfurniture parts.

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Figure 6. The process of bending wood is

also used to produce parts of a boat. Theconcave strap restrained by the rods

maintains end pressure and shape. The

wood braces reduce twisting. (Photooriginally from USDA Forest Service,

Forest Products Laboratory, Madison,

WI).

Figure 5. This bent ash wood hassplits as a result of inadequate endpressure .

opposite of the plasticization of wood in that the woodis now cooled. A furniture maker will set the bendtogether while drying to the expected moisture contentin service. The straps on the convex side and end stopsshould be restrained during setting. The opening ordistance between the ends of a bent piece should also becontrolled. A split can occur if the end pressure is toohigh (figure 5).

Wood bending is used to produce several recre-ational items. Among these are boat parts (figure 6) andtennis racquets and snow skies (figure 7).

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Source for all pictures: Peter Koch. 1985. Utilization ofhardwoods growing on southern pine sites. USDA AgricultureHandbook 605. Vol II - Processing. U.S. Gov. Printing Office.Washington, DC.

Acknowledgment: The author is grateful to the assistancereceived from Dr. Chung Hse, Principal Wood Scientist, USDAForest Service, Southern Research Station, Pineville, La.; Dr. PeterKoch, President, Wood Science Laboratory, Inc., Corvallis, Mont.;Dr. Elvin Choong, Professor of Forestry, Louisiana State UniversityAgricultural Center, School of Forestry, Wildlife, and Fisheries,Baton Rouge, La.; and Dr. Poo Chow, Professor of Wood Science,University of Illinois, Department Natural Resources and Environ-mental Sciences, Urbana, Ill.

Figure 7. Wo o dbending is used to

produce re c reational

items such as thesetennis racquets (left)

and snow skis

(bottom).

Visit our website: http://www.agctr.lsu.edu/w w w a c

Dr. Todd F. Shupe, Assistant Specialist (Forest Products)

Louisiana State University Agricultural Center,William B. Richardson, ChancellorLouisiana Cooperative Extension Service,Jack L. Bagent, Vice Chancellor and Director

Pub. 2643 (1.5M) 5/97Issued in furtherance of Cooperative Extension work, Acts of Congress of

May 8 and June 30, 1914, in cooperation with the United States Department ofAgriculture. The Louisiana Cooperative Extension Service provides equal opportu-nities in programs and employment.