howP E G

h e l p st h eh o b b y i s t

whow o r k sw i t hw o o d

by H.L. MITCHELLU.S. DEPARTMENT OF AGRICULTURE

FOREST SERVICE

FOREST PRODUCTS LABORATORY

1972

TABLE OF CONTENTS

What is PEG?

How PEG Stabilizes Wood 2

How Moisture Content Relates to Shrinkage of Wood 3

Mixing PEG Solutions 6

Vats for Treating 6

Treating Schedules for Wood Disks 7

Drying Treated Disks 7

Protecting Valuable Thick Stock During Drying 8

Processing Lumber for Maximum Dimensional Stability 8

Gluing PEG-Treated Wood 9

Finishing Treated and Dried Disks 9

Stabilized Disks Exposed to Weather 11

Producing Specific ProductsDecorator ClocksBowls and Other TurningsGreen Wood Carvings and StatuaryRifle Stocks with Maximum Dimensional Stability

Protecting Imported Carvings 17

Preserving Archaeological Specimens 19

Bibliography 20

SummaryPolyethylene glycol-1000 (PEG) is introduced to

the hobbyist as an agent to dimensionally stabilizewood. Directions are given for mixing PEG solutions,preparing treating vats, and drying and gluing treatedwood. Directions are also given for producing decora-tor clocks, bowls and other turnings, green woodcarvings, statuary, and rifle stocks with maximumdimensional stability. “How to’s” for protecting im-ported wood carvings and for preserving archaelo-

Page

1

1111111313

gical specimens are also included.

HOW PEG HELPS THE HOBBYIST

WHO WORKS WITH WOOD

By

H. L. MITCHELL 1

Forest Products Laboratory,2 Forest ServiceU.S. Department of Agriculture

You, the hobbyist, particularly appreciate wood,one of our abundant renewable natural resources.

You are well aware of the almost limitlessadvantages of wood: Ready availability, beauty,warmth, variety of color. You appreciate thefavorable strength-to-weight ratio of wood, thegood insulation properties, how easily wood canbe worked, how easily it can be sanded andfinished. You know, too, that wood can be treatedto resist fire and attack by insects and decayorganisms.

But wood is an exceedingly complex and variablematerial--not without certain deficiencies. Youwell know that wood shrinks and swells withchanges in atmospheric moisture. This deficiencyplus related problems of checking, splitting, warp,and broken glue joints are all too familiar to you.But, now there is help for this.

Treatment with polyethylene glycol-1000, com-monly known as PEG, greatly improves thedimensional stability of wood. Many of the prob-lems you have encountered while working on woodcan now be minimized or eliminated.

What is PEG?

PEG is a white, waxlike chemical that resemblesparaffin. A solid at room temperature, PEG meltsat 104° F., has an average molecular weight of1000, dissolves readily in warm water, is non-toxic, noncorrosive, odorless, colorless, and hasa very high fire point (580° F.). Although chemi-cally related to common antifreeze (ethyleneglycol, a monomer), PEG is a polymer (manymonomer units linked to form larger molecules)

more closely related to various other polyethyleneglycol polymers with substantially higher or lowermolecular weights and different properties. PEGhas its own unique properties not possessed bythe others. Accordingly, none of the related chemi-cals can be successfully substituted for PEG inprocessing wood or wood products. Some hobbyistshave learned this the hard way!

IRetired Chief of the former Division of Wood Quality Research.

2Maintained at Madison, W i s . , i n c o o p e r a t i o n w i t h t h e U n i v e r s i t y o f W i s c o n s i n .

How PEG Stabilizes Wood

What exactly is meant by “dimensionalstability?”

It is the extent to which a given material re-sists changes in dimension with variations inenvironmental factors. Steel, high in dimensionalstability, expands only slightly with increases intemperature, and does not expand with variationsin relative humidity. Wood, however, althoughlittle affected by changes in temperature, changesconsiderably in volume (shrinks and swells) withvariations in moisture content. Thus, comparedwith a material like steel, wood is inherently lowin dimensional stability.

However, when green or partially dry wood issoaked for an appropriate period in a 30 to 50percent (by weight) solution of PEG, the wood doesnot shrink appreciably when dried. Equally impor-tant, wood thus treated and then dried swells verylittle when exposed again to high humidities.PEG enters the fine structure of the wood by dif-fusion, thus application of pressure is of littlevalue. The large molecules of the chemical dis-place the natural moisture in the microscopic,lattice-like structure of the wood-fiber walls.

Heavily treated (bulked) wood retains its turgid(green) dimension indefinitely: thus the wood ispermanently restrained from shrinking, swelling,or warping regardless of atmospheric humidity.For maximum dimensional stability, PEG mustbe diffused into the wood in amounts of 25 to 30percent of the dry weight of the wood. This treat-ment reduces maximum potential shrinkage andswelling about 80 percent.

Rather sophisticated laboratory equipment isrequired to determine the amount of PEG takenup by wood. Anyone seriously interested in thissubject can write to the U.S. Forest ProductsLaboratory, P.O. Box 5130, Madison, Wis. 53705,for information on the methods and equipment

required.PEG may also be used as a drying agent. Rela-

tively light treatments will effectively preventdrying degrade in green wood bowls, art carvings,turnings, and other craft items. The objective issimply to get enough PEG into the outer shell ofthe preshaped project to prevent splitting, check-ing, and warp during drying.

Treating wood with PEG does not prevent theabsorption of atmospheric moisture. PEG is evenmore hygroscopic than wood itself. However,PEG-treated wood will not swell appreciably asit absorbs moisture, whereas untreated woodwill swell. Treated wood, not yet finished, whenexposed to high humidities for long periods willbecome moist to the touch, and in extreme condi-tions will become oily if some PEG goes into solu-tion on the wood surface. However, the appearanceof treated wood properly finished with coatingsthat effectively lock the chemical into the wood isno different than untreated wood.

PEG treatment has no effect on color or odorof wood and only a slight effect on mechanicalstrength and hardness. The wax-impregnatedfibers of treated wood have less tendency to fuzzand splinter than do those of untreated wood, andare much easier to machine and work with cuttingtools. Treated wood can be efficiently sanded,stained, and glued, when the right techniques andmaterials are used.

Many common finishes do not adhere well tothe rather waxy surface of PEG-stabilized wood.These include shellac, lacquer, linseed oil, andcertain varnishes. Two types of polyurethaneresin varnish and a Danish-oil type finish doadhere well to PEG-stabilized wood, and theyeffectively lock the stabilizing chemical into thewood.

How Moisture Content Relates

to Shrinkage of Wood

Wood in its natural state in the trunk of a livingtree contains from 30 to 300 percent of water(dependent largely on growth conditions andspecies) based on the weight of the ovendry wood.This water is (1) contained as free water in thecell cavities and the intercellular spaces of thewood and (2) held as absorbed water in the capil-laries of the walls of wood elements, such asfibers and ray cells. The absorbed water relatesto shrinkage. When all of the free water is re-moved and all of the absorbed water remains,wood has reached the fiber saturation point,approximately 30 percent moisture content for allspecies. Shrinkage occurs only when the moisturecontent of wood is reduced to some level belowthe 30 percent fiber saturation point. “Partiallydry” wood refers here to wood that has not beendried to a moisture content of 30 percent or less.

The moisture content of wood, normally deter-mined with a rather expensive electric moisturemeter, can be measured in a home workshopas follows:

1. Cut three to five approximately l-inch squareblocks from the piece of wood in question:

2. Remove all loose wood fibers and sawdustfrom the blocks;

3. With an ammunition reloading balance, apostage scale, or some equally accurate weigh-ing device, obtain and record the green weightof each block;

4. Dry the blocks in an oven with temperatureadjusted to about 220° F. until periodic weightchecks indicate that a stable weight condition hasbeen reached;

5. Then weigh the blocks rapidly directly fromthe oven and record the weights; and

6. Determine the moisture content according tothe following formula:

Moisture content (percent) =

Original (green) weight - ovendry weightOvendry weight

x 100

Any piece of wood will give off or take on mois-ture from the surrounding atmosphere until theamount of moisture in the wood balances that inthe atmosphere. The moisture content of the woodat the point of balance is called the equilibriummoisture content, and is expressed as a percent-age of the ovendry weight of the wood as deter-mined here. For example, green fresh-sawedlumber exposed continuously to an environmentof 90 percent relative humidity (R.H.) and 80° F.temperature will eventually reach a moisturecontent (M.C.) of 20 percent. At 65 percent R.H.and 80° F., it will reach 12 percent M.C.; and at30 percent R.H. and 80° F., will be reduced to6 percent M.C.

Wood, like many other materials, shrinks as itloses moisture and swells as it absorbs moisturefrom the atmosphere. When wood dries (withreduced R.H.) from its green to ovendry condi-tion, it shrinks in volume by about 6 to 20 percent,depending on the species. When the hobbyist,through ignorance or neglect, uses wood that isinsufficiently dry for the intended purpose, volumeshrinkage of these magnitudes often results inpartial or complete ruin of a painstakingly con-structed project. Glue joints pop, tabletops warpand split, bowls and art carvings check andcrack.

To further complicate the problem, woodshrinks (and swells) differently in different direc-tions or planes. For example, tangential shrink-age is greatest. That is, “flat sawed” boardssawed from the log parallel to the annual growthrings, approximately at right angle to a line(radial) from the pith center to the circumference,shrink much more (twice as much) as “quarter-sawed” boards sawed in the radial plane. That iswhy quartersawed lumber always sells for morethan flat-sawed lumber. It shrinks much less inwidth, and is less apt to warp. Wood shrinks andswells least of all in the longitudinal (along thegrain) direction. Longitudinal shrinkage andswelling is normally not a serious problem

3

The differential shrinkage of wood during thedrying process, mainly the twofold differencebetween tangential and radial shrinkage, sets upenormous internal stresses that cause wood towarp (cup, twist, crook, and bow) during drying.Accordingly, during drying there is some warpingand checking of lumber whether dried under con-trolled temperature and humidity conditions in amodern commercial dry kiln or air-dried “on thefarm” with no control of atmospheric temperatureand humidity. The main differences are that kiln-drying is faster, much more expensive, tends tominimize degrade due to warp, checking, andsplitting, and is capable of reducing the moisturecontent of lumber to any desired level,

How dry (what M.C.) should lumber be beforeyou use it for a particular project? Fortunately,there is a simple answer.

If you are making furniture or other items thatin use will be exposed for at least part of the yearto the arid environment of a heated home, office,or store in winter, all wood should be dried to auniform 6 percent M.C. That is the moisture con-tent to which all wood will be reduced on continu-ous exposure to the 30 percent R.H. and 73+ ° F.temperature of heated or air-conditioned build-ings.

If you use wood of a higher moisture content,remember that for each 1 percent reduction inM.C. down to the equilibrium moisture content(6 pct.) for this atmospheric environment, thewood will shrink one-thirtieth of its total poten-tial volume shrinkage. Shrinkage of this magni-tude will result in internal stresses that can causeserious warp, checks, cracks, broken glue joints,and all the other problems you are familiar with.

If you use rough l-inch-thick “farm” or “smallsawmill” lumber that has been air-dried outdoorsfor 6 months or so, and has about 18 to 20 percentM.C. in the Gulf States or 14 to 16 percent M.C. inmost other parts of the country, the lumber shouldfirst be conditioned in a heated room for 4 to 6weeks to reduce the M.C. to the desired 6 percent.Pile the lumber in layers separated by l-inchstickers to permit good air circulation around theboards. After drying, flatten one side of each boardon a jointer to remove any warp, then surface(dress) to desired thickness, preferably in aplaner, and use with assurance that it will per-form satisfactorily.

Supposedly kiln-dried lumber purchased from alumberyard or other supplier should similarly beconditioned. This is important!

PEG is not recommended for the routine proc-essing of l-inch lumber destined for use in furni-ture, cabinets, and other indoor uses. It is notnormally needed. Simply follow the suggestionsfor drying and conditioning to the proper M.C.,and no serious problems should result.

If, however, a project requires lumber 1-1/2inches or more in thickness, special problemsmust be considered. Thick lumber for indoor useis very difficult, time-consuming, and expensiveto dry to the required uniform 6 percent M.C.For example even in the best of modern dry kilns,from 60 to 90 days are needed to dry 2-1/2-inchwalnut planks to a uniform 6 percent M.C. Thecost of this stock is accordingly high. However,the cost of using only partially dry thick stock forbowls, art carvings, gunstocks, and the like is aptto be even higher.

Do not depend on some magic finish or “sealer”to prevent bowls, carvings, and like items madefrom partially dry thick stock from degrade whilelosing moisture to equilibrium (6 pct. M.C.) withthe arid environment of a heated room. No finishor coating material known will prevent the move-ment of moisture into and out of wood with varia-tions in atmospheric humidity, Even the best, suchas polyurethane and epoxy varnishes, only slow upthe process. The end result is just as inevitable.The widely promoted wood ‘sealers,” althoughthey may serve a useful function as a base forfinish coats, have even less effect on moisturemovement. Linseed oil, tung oil, and most other“natural” finishes are little better than goosegrease or crankcase oil for effectiveness asmoisture barriers, Although these finishes arefine, beautiful, and widely used, do not expect themto do what they are not intended to do.

Kiln-dried lumber does not always assuresuccess. Most lumberyards and other suppliersstore their lumber under cover, but it is exposedto ambient temperature and humidity. The stockyou buy has probably absorbed moisture from theatmosphere up to 8 to 12 percent M.C., and islittle better for your purpose than is air-driedfarm lumber. Items made from this lumber arejust as apt to warp, check, and split during theinevitable drying process. Normally, only inmodern furniture factories is kiln-dried lumberstored in air-conditioned rooms and kept at thedesired 6 percent M.C. until fabricated into thefinal product.

Here again, however, there is a simple, prac-tical solution for the hobbyist. With PEG treat-

4

ment, it is possible to process flawless bowls,art carvings, lamp bases, candlesticks, gunstocks,and many other craft items from either green orpartially dry thick stock. Another advantage is thatraw material can be obtained at little or no cost.Odd chunks of green scrap wood, such as loggingand sawmilling residues, are normally availablefor the asking and can be used. Bolts from the

trunks or limbs of the dead or unwanted shadetrees that are cut down almost daily in everylarge city can also be used.

With PEG treatment, it is also possible to proc-ess flawless, crack-free tabletops and other craftitems from green log and limb cross sections(fig. 1).

F i g u r e I . - - S t a b i I i z e d d i s k s . T o p , P E G - s t a b i l i z e d t a b l e t o p , 3 8 - i n c h d i a m e t e r , c u t f r o mbu t t o f 784 -yea r -o ld redwood ; bo t t om, k i n g - s i z e P E G - s t a b i l i z e d c r o s s s e c t i o n t h r o u g hl a r g e b o x e l d e r b u r l . M I26 056

M l26 475

5

Mixing PEG Solutions

The two solutions commonly used are 30 and50 percent PEG by weight. Exactly 4.46 poundsof polyethylene glycol-1000 dissolved in 5 quartsof water yield 7 quarts of 30 percent solutionwith a specific gravity of 1.05 at 60° F. Tenpounds of polyethylene glycol dissolved in an equalweight of water (about 4.8 qt.) yield about 7.4quarts of 50 percent solution with a specificgravity of 1.093 at 60° F. A precision hydrometeraccurate for the desired range of specific gravi-

Vats for Treating

Glass, earthenware, plastic, or similar typesof containers are adequate for treating a limitednumber of small disks. A plastic trash containeris an excellent treating vat. Metals, except certaintypes of stainless steel, should not be used becausethey react with extractives from the wood and formcompounds that discolor the wood. PEG, however,is colorless.

A serviceable, one-time treating vat can bemade from a fiberboard box lined with a heavypolyethylene bag or sheet. Cross sections of logsup to 60 inches in diameter have been success-fully treated in circular vats roughly fashionedfrom scrap lumber or plywood and lined with asheet of heavy plastic.

After the disk is placed in the vat, it should besupported by a few narrow strips of wood to per-mit free circulation of the solution beneath thedisk; it should be weighted down with a rock toassure complete immersion. Stones or bricks canbe nested in irregular or empty spaces around alarge disk to raise the solution level to reduce theamount of solution needed for complete immersion.

Growth of molds, bacteria, and fungi during longtreatment can be prevented by adding borax orsodium pentachlorophenolate to the PEG solution,to a 1 or 2 percent concentration.

ties will help maintain the proper concentrationof the solution during treatment.

PEG does not volatilize at the temperaturesnormally used in treating vats, but water doesevaporate during long treating cycles, even incovered (but not airtight) vats. Accordingly, itis usually necessary to replace water lost byevaporation before chemical must be added tomake up for that diffused into the wood duringtreatment.

PEG solutions can be reused indefinitely by add-ing chemical (or water) to maintain at the desiredconcentration. When not in use, they should bestored in glass containers.

Hobbyists and small commercial users of PEGusually construct 50- to 100-gallon capacity treat-ing vats of 1/2-inch exterior grade Douglas-firplywood glued together with resorcinol or someequally water-resistant adhesive. This type oftank is frequently lined with fiberglass, the kindused to cover wooden boats.

For best results, treating tanks should beequipped with electric heating elements (withbuilt-in thermostat) of the type used in waterheaters. It is desirable to cover the sides, bot-tom, and especially the cover, with fiberglass in-sulation (4 in.) to conserve heat. In a productionoperation, the PEG solution is normally circulatedperiodically with a pump system activated by acycle timer. Brass or copper pipe fittings are notobjectionable if they are a 1 ways completelycovered by the solution.

Commercial operators who treat with PEG fre-quently employ stainless steel vats similar tothose used by the dairy and food-processing in-dustries.

6

Treating Schedules. for Wood Disks

Disks from fresh-cut wood should be wrappedin plastic or immersed in water to keep in greencondition until they can be treated with PEG. Crosssections from dead tree trunks or sound portionsof old logs, regardless of when cut, are essentially“green” for this purpose. However, they should besoaked in water for 2 or 3 days before treating;this assures good penetration of the stabilizingchemical into the wood.

The soaking time to obtain sufficient penetrationof PEG into disks, or cross sections, to protectthem against splitting and checking depends on theconcentration and the temperature of the treatingsolution, the size and thickness of the disks, andthe species of wood. Diffusion of the chemical intowood can be greatly accelerated by increasing thetemperature and the concentration of the solution.

The treating schedules in the following tabula-tion are based on experience with walnut:

Solution concentrationand temperature Suggested period of soak for walnut disks--

Up to 9 inches in More than 9 inchesdiameter and 1 to in diameter and1-1/2 inches thick 2 to 3 inches thick

(Days) (Days)

30 percent, 70° F. 20 60

50 percent, 70° F. 15 45

30 percent, 140° F. 7 30

50 percent, 140° F. 3 14

Treating time will generally vary with the den-sity of the wood. Walnut, a medium-density wood,has an average specific gravity of about 0.55 on agreen volume, dry weight basis. For cross sec-tions of lower density woods such as white pine,spruce, redwood, and the “soft” hardwoods (softmaple, cottonwood, willow, and others), soak forone-half to two-thirds the time suggested for wal-nut. For the higher density woods like hard maple,yellow birch, beech, the oaks, and apple, double ortriple the treating period. Elevated temperaturesare normally required for fully effective treatmentof very dense woods, such as manzanita, mesquite,desert ironwood, and burls of most species.

Some experimentation is usually required towork out the best treating schedule for disks of agiven size, thickness, and species. It should bepointed out that the wood in the core of certaintree trunks is so defective that even with PEGtreatment flawless disks cannot be processedtherefrom, The pith center of all trees tends tobe punky, have incipient fractures due to wind ac-tion, sometimes is partially decayed, and alwaysis weaker than the more normal wood formedlater in the life of the tree. Some trees have muchpoorer pith centers than do others, and wood ofthis type should not be used for hobby projects.

Drying Treated Disks

For drying, pile the treated disks and separatewith dry stickers. under cover in a well-ventilatedroom or building, preferably a room heated inwinter. Time needed for air-drying will vary withthickness of disks, temperature, and relativehumidity. Treated cross sections up to 4 inchesthick and 40 inches in diameter will dry suffi-ciently for finishing if placed 6 to 8 weeks in aheated room in winter. Thinner, smaller diskswill dry much faster in a similar environment.Stored outdoors in the summer, even under cover,drying takes somewhat longer than in a heatedroom, However, since treated disks shrink very

little during drying, it is not essential that theybe completely or uniformly dry before work onthem is started. It is only necessary that thesurface 1/4 inch be sufficiently dry to sand welland accept the finish.

Bark normally loosens and falls away fromuntreated log cross sections as they dry becausethe wood shrinks away from the encircling bandof bark, Since treatment with PEG largely pre-vents this shrinkage, bark firmly attached priorto treatment will remain so indefinitely regard-less of how dry the stabilized disks become.

7

Protecting Valuable Thick Stock During Drying

Thick planks of certain species and grades ofwood are almost impossible to dry free of de-grade (checks, splits) either by air-drying follow-ed by a long period of conditioning in a heatedroom or by kiln-drying in a controlled environ-ment. For example, highly figured walnut gun-stock blanks with a large proportion of burl, butt,stump, or crotch figure are very difficult to drydefect free. The wild grain patterns--knot swirls,interlocked, distorted, and other abnormal grain--so rare and beautiful in a finished stock are veryprone to splitting, checking, and warp.

Because of the high values (up to $100 for asingle blank) and the great risk of loss involved,treatment with PEG is fully justified. In a recentstudy, 40 highly figured green walnut gunstockblanks, all 2-1/2 inches thick, were randomlydivided into two groups, Twenty were kiln-driedunder a relatively mild schedule developed forwalnut stock blanks. About one-third of theseblanks dried to 6 percent moisture content haddeep checks and splits, and would have beengraded as culls. This confirmed general industryexperience with this class of material.

The other twenty were soaked in water for a

few days, then treated for 10 days in a 50 percentwater solution of PEG maintained at 140° F. Theywere dried rapidly to 6 percent moisture contentunder drastic conditions. The moisture wasliterally baked out of them at temperatures up to200° F. with no humidity control. All were inperfect condition--free of checks, splits, honey-comb, and warp. Probably equally good resultscould have been obtained by treating at ambienttemperature for a longer period, 6 to 8 weeks.

This simple, relatively inexpensive and effec-tive preventative treatment is, of course, recom-mended only for rare, highly figured defect-pronewalnut blanks of the type described here. Conven-tional drying methods are adequate for generallystraight-grained material.

The objective here was simply to get enoughPEG into the outer shell to provide protectionduring drying. PEG penetration was not great, andmost if not all of the treated wood will be cutaway in shaping the finished stocks from theseblanks. The stocks will be no more stable thanthose from untreated wood, but they will be freeof seasoning checks and splits.

Processing Lumber for

Maximum Dimensional Stability

We do not recommend routine PEG treatmentfor lumber destined for indoor use such as forfurniture. Treatment is too expensive, and is notnormally necessary. However, PEG treatment isessential for lumber used in special products forwhich a high level of dimensional stability isneeded to meet exacting performance require-ments. Examples include laminated stocks forprecision rifles, straight edges, patterns, essen-tial structural parts for large cameras, andmounting blocks for large metal engravings.

To attain maximum possible dimensional sta-bility in lumber it is necessary to diffuse PEGinto the wood to minimum retentions of 25 to 30

percent of the dry weight of the wood. This levelof treatment can only be achieved at high temper-atures. When 1-1/4-inch-thick lumber is treatedsufficiently long at high temperature to attain thedesired 25 to 30 percent PEG in the center 1/8inch of the boards, the surface 1/8 inch will havetaken up approximately 40 percent PEG. Thus,the wood near the surface of the boards is some-what over-treated to attain the minimum requiredlevel of treatment in the center of the boards. Theexistence of this gradient, which becomes morepronounced as thickness (and treating time) isincreased, makes it impractical to treat, formaximum stability, lumber more than 1-1/4

8

inches in thickness.If greater thicknesses of stable lumber are

required, as in certain patterns and target gun-stocks, they are most efficiently achieved bylaminating (gluing together flatwise) boards pre-viously stabilized in thickness of from 3/4 to1-1/4 inches.

Rough-sawed green walnut boards of nominal1-inch thickness can be treated for maximum di-mensional stability by soaking for 30 days in a50 percent PEG solution maintained at 140° F.Three-quarter-inch-thick boards of walnut canbe similarly stabilized by soaking in such a solu-tion for 25 days; and 1-1/4-inch-thick boards,for 45 days.

The same treating schedules can be used withequal success for green lumber of red oak, syca-more, yellow birch, myrtlewood, soft maple,true mahogany, Hawaiian mango, Hawaiian koa,monkeypod, mesquite, and sapwood of hard maple.Probably many other hardwoods in the samegeneral density range can be similarly treatedwith equal success,

There are, however, certain exceptions. For

example, the heartwood of hard maple is impos-sible to treat with PEG by any known practicalmethod. Cherry lumber invariably honeycombsin the treating vat when treated according to theschedule developed for walnut. However, 1-inch-thick cherry boards can be successfully stabilizedby soaking for 45 days in a 50 percent PEG solu-tion maintained at 110° F.

In the treating vat, layers of unsurfaced greenlumber should be separated by stickers to permitfree movement of the solution around each board.Circulating the PEG solution periodically with apump system activated by a cycle timer is alsoadvocated.

Heavily PEG-treated lumber is so stable that itcan be air-dried or kiln-dried in a drasticschedule with little or no loss due to drying de-grade. There will be no surface checking andwarp, and end splitting will be minimal regard-less of how rapidly the remaining moisture isremoved. Heavily treated green wood rifle stockshave been successfully dried in 8 hours in anelectric oven (220° F.).

Gluing PEG-Treated Wood

Although many common glues, including thepopular white glue (a polyvinyl), do not work wellon PEG-treated wood, several widely availablehigh-quality glues do work well. Among them aretwo-component waterproof resorcinol and epoxyglues of various brands. The hobbyist’s oldstandby, the urea-resin type, also works well, butis not quite as resistant to water.

For best results it is important to use a good

PEG solvent on the wood surface to cut the waxand expose the fibers just before the glue isapplied. For critical joints that require maximumstructural strength, as in laminated stock blanks,scrub the surface with a toluol-soaked cloth.When dry, wash again with wood alcohol justbefore you apply the glue. For less critical joints,the toluol step can be omitted.

Finishing Treated and Dried Disks

When disks are sufficiently dry for working,smooth both surfaces on a sharp jointer or with ahand plane; for small cross sections, smooth bysanding with coarse grit. Probably the best tech-nique to smooth very large disks is with a router.Mount the router above the disk on a plank frameconstructed to permit horizontal movement intwo directions at a constant height. After smooth-

ing, sand the faces of the disk with coarse grit.If the grit loads up with waxy sawdust, clean thesandpaper with a wire brush, preferably in astream of warm water to dissolve the accumulatedPEG. After coarse sanding, sponge the surfacewith warm water and finish sanding with 3/0 andthen with 6/0 wet-or-dry sandpaper.

Then bleach the disk to bring out the natural

9

color of the wood, to accentuate the contrast be-tween heartwood and sapwood, and to remove theaccumulation of iron compounds and natural pig-ments that often forms near the surface of treat-ed wood, especially during high-temperaturesoaking, For the bleach, dissolve 2 heapingtablespoons of oxalic acid crystals in a halfglass of hot water, and apply liberally with abrush. Wear rubber gloves, and keep the solu-tion out of eyes. After an hour or so, neutralizethe excess acid by sponging with cloth dampenedwith dilute solution of household ammonia. Letdry for 1 day, resand with fine grit, then finishaccording to either of the following systems:

1. A built-up finish with varnish. Apply, at 1-day intervals, four or five coats of a newlydeveloped moisture-cure-type of polyurethaneresin varnish. Sand well between each coat with220 or finer sandpaper. This type of varnish willset up rapidly on a wet surface or when exposedto atmospheric moisture, is extremely hard anddurable, and most importantly, is quite effectivein sealing PEG in the wood during prolongedperiods of high humidity. However, when properlyformulated to meet these use requirements, thisspecial type of varnish is so thick and sticky ittends to leave brush marks and other surfaceimperfections. For best results the moisture-cure-type base coating should be followed withone or two finish coats of any good grade of aconventional (chemically hardened) polyurethanevarnish that is easy to apply and is widely usedon furniture and interior trim, (The first finishcoat over the moisture-c&e base may take aslong as 2 days to set up. However, subsequentcoats will set up in 12 hours or less.) Then polishwith pumice in oil lubricant, using care not tocut through the final finish coat of the conven-tional polyurethane varnish, and buff with apaste wax.

For products that are less difficult to finishthan treated log disks, the base coats of moisture-

cure-type polyurethane are not normally needed.Examples include gunstocks, bowls, carvings, andsimilar products with a minimum of exposed endgrain that are not apt to be exposed in use toextremely high humidities for prolonged periods.For these products, use conventional polyurethanevarnishes. They are easy to apply, adhere wellto treated wood, and are fairly hard, durable, andbeautiful.

2. An oil finish. Of the many oil-type “natural”finishes tested, only a Danish-oil penetratingfinish performed s a t i s f a c t o r i l y over PEG-stabilized wood, especially end grain. This finishis easy to apply. If tabletops or other items madeof PEG-stabilized log disks are apt to be exposedin use to long periods of high humidities, this isprobably the best finish to use. It may also beused with equally good results for less criticalapplications, That is, for gunstocks, bowls, carv-ings, and other PEG-treated products.

Special instructions are needed for tabletops,decorator clocks, and other items to be finishedwith bark attached. Do not use any type of varnish- -on PEG-treated bark, Bark is so porous thatduring treatment it absorbs much greater quanti-ties of PEG than does wood. Therefore, duringprolonged periods of high humidity, the hygro-scopic PEG in the bark sops up so much atmos-pheric moisture that the bark becomes sufficientlywater soaked to ruin any built-up varnish finish.Use only Danish-oil on bark. It penetrates intothe porous structure, does not form a thick con-tinuous surface film, and will therefore withstandhigh humidities. If a varnish finish is preferredon a tabletop with bark attached, then first giveonly the bark portion two coats of Danish-oil.Then apply the various varnish coats as directedto the entire flat surface of the top; this includesthe sanded edge of the oil-finished thin ribbon ofbark around the circumference. Use care not torun varnish over the edge onto the rough surfaceof the oil-finished bark.

10

Stabilized Disks Exposed to Weather

PEG-stabilized disks are sometimes used forpaving outdoor patios and for rustic steps thatcan be in contact with moist soil. Thus thesedisks must be protected against attack by insects(termites, borers) and decay organisms. Also,since PEG is soluble in water, it will eventuallyleach out of unprotected treated disks exposed torepeated rains. The protection needed againstinsects, decay, and leaching is probably bestprovided by applying a good water-repellent pre-servative stain that contains both a wax to shedwater and an active insecticide-fungicide such aspentachlorophenol. Good water-repellent preser-vative stains are available at most paint stores

or directions for preparing and mixing them canbe obtained from the U.S. Forest Products Labo-ratory. The more wax and pigment you use in theformulation, the better will be performance underadverse conditions.

Apply several coats of the water-repellentpreservative to the previously stabilized disksbefore installing in an outdoor exposure, and addone coat annually thereafter. For outdoor use youshould also consider using a species of wood withhigh natural resistance to insects and decay, suchas the heartwood of redwood, cedar, white oak, orcypress. Before PEG treating, cut away the outerband of the less resistant sapwood.

Producing Specific ProductsDecorator Clocks

The clock frame and face are usually madefrom a blank sawed at a 45° angle from a largelimb or from the trunk of a small tree. Eachshould be about 2-1/2 inches thick and be PEG-stabilized as has been described for green logand limb cross sections.

Following treatment, each section should bedried, sanded, and finished exactly as for disks:this should include the special attention to thebark that has been suggested. Then select thebest face; mark the point where the hands shouldbe located, and drill a small pilot hole throughthe blank. Turn the blank over, place in drillpress, and bore a hole in the back large enoughin diameter to accommodate the mechanism ofthe round electric clock, and just deep enough sothat the shank of the clock works will protrudesufficiently through the front face to permitanchoring with a brass hex nut and proper mount-ing of three hands.

Make a paper template so that the hour markerscan be properly positioned. The markers can bemade of sapwood of maple, holly, or any otherlight wood. They are 1/8 inch square and ofdifferent lengths. The markers can be attached tothe clock face with contact cement (fig. 2).

Bowls and Other Turnings

You can start your bowl with green or partiallydry wood. Simply rough turn the bowl from greenstock to a wall thickness about 3/8 to 1/2 inch iffinished thickness dimension is to be 1/8 to 1/4inch. The base can be 3/4 inch or more in thick-ness. Next, transfer the precarved green woodbowls to the PEG treating vat.

If blocks of wood are used that are partiallydry, they should first be soaked in water for afew weeks. As a minimum, if partially dry woodis used, soak the rough-turned bowls in waterfor several days prior to PEG treatment. Thisassures adequate diffusion of the large moleculesof the chemical into the wood.

Period of soak again depends on the species ofwood, the thickness and shape of the bowl, and theconcentration and temperature of the PEG solu-tion. For rough-turned green wood bowls, such asdescribed, and of walnut, poplar, ash, red oak,koa, mango, apple, elm, sapwood of hard maple,myrtlewood, white pine, and southern yellow pine,a 3-week soak in a 30 percent PEG solution at73° F. proved adequate. Equally good results canbe obtained in a much shorter time, 3 to 7 days,by increasing the PEG concentration of the solu-tion up to 50 percent and the temperature to 140° F.

11

F i g u r e 2 . - - D e c o r a t o r c l o c k s r e s u l t f r o m P E G - s t a b i I i z e d , b i a s c u t s t h r o u g h l a r g e l i m b so f w a l n u t l e f t i n w o o d s b y c o m m e r c i a l l o g g e r .

M l25 952

Bowls from burls and very dense heavy woods,inclusive of many tropical hardwoods, are muchmore difficult to treat than are the species men-tioned. Bowls of some species can be adequatelytreated only at elevated temperatures. For bestresults the hobbyist should develop treatmentschedules (solution concentration, temperature,period of soak) specifically suitable for hisproduct, wood species, and end use.

After treating, the rough-turned bowls shouldbe air-dried, preferably in a heated room, for3 to 4 weeks. If treatment is adequate, they willnot develop checks, cracks, or splits during the

drying process. The only exception will be areasof large hard knots, into which little PEG pene-trates; in these areas some fine checks normallydevelop when thoroughly dry. If objectionable,avoid large knots in selecting raw material.Thanks to PEG stabilization differential shrink-age is minimized. Your bowl should be roundrather than slightly elliptical, typical of so manywooden bowls.

When sufficiently dry for finishing, replace thebowl on the lathe and turn to final desired dimen-sions. In the process you will remove the waxyaccumulation of PEG that formed on the treated

12

surfaces; this facilitates sanding and finishing.Sand thoroughly, preferably on the lathe, bleach,and finish with polyurethane varnish, Danish-oil,or the systems to be described.

Probably the safest finish for the inside of abowl to hold food is olive oil, linseed oil, or someequally safe vegetable oil. only the outside sur-face of the bowl should be finished with one of themore durable, attractive finishes that have beendescribed (fig. 3). PEG is widely used in cos-metics, ointments, lotions, suppositories, and asbinders for pharmaceutical tablets, and has beendeclared safe for these uses.

The PEG process for bowls can also be usedfor table lamp bases and stems, floor lamps,candlesticks, serving trays, and a great varietyof other turnings, also made of green scrap wood,including limbs of trees,

Green Wood Carvings and Statuary

The procedure for carvings and statuary isessentially the same as that for bowls. Rough-shape carving or statue from green wood veryclose to final dimensions. When not working onthe carving, immerse it in water or wrap inplastic film to prevent drying. If the base portionis rather massive, hollow it from the bottom ordrill holes up into it. This tends to relieve naturalstresses in the wood, minimizes the tendency tosplit, and more importantly permits penetrationof PEG into a high hazard area from both outsideand inside; thus better treatment is ensured. Thentreat in PEG solution.

Since most carvings and statues will be some-what thicker than bowls, treat about twice as longas bowls, After treating, air-dry, add final details,sand, bleach, and finish with one of the two sys-tems recommended for stabilized wood.

If a carving is so large it would be impracticalor too expensive to treat by soaking, melt somePEG and brush it on the carving or statue. Thenwrap the carving in plastic film to keep thehumidity high; this facilitates diffusion of themolten chemical into the wood. Repeat this oper-ation about four times at l-week intervals. Re-move the plastic, air-dry, and proceed as sug-gested. This technique, not as sure-fire assoaking, frequently produces good results, andwill minimize checking and splitting (fig. 4).

Rifle Stocks with MaximumDimensional Stability

Two methods can be used to make rifle stocksthat will not shrink, swell, or warp at any extremeof atmospheric humidity, from the aridity of theMojave Desert to the stifling humidity of anAmazon jungle. Chemically stabilized stocks caneven withstand complete immersion in water forextended periods with no appreciable change indimension.

Stable stocks for target rifles, because of theirlarge size, especially their width, are best madeby laminating heavily treated and dried l-inchboards into blanks of the required size. Normallythe blanks are three-, five-, or seven-ply, depend-ing on the thickness needed. The target stocks arethen shaped from the dry, stabilized laminatedblanks. The much smaller stocks for sporterrifles may be made from laminated blanks, usuallythree-ply, but they may also be made from solidwood.

Fresh-sawed green blanks of the required sizeare first shaped and completely inletted on amultiple-spindle duplicator to within 1/8 inch ofthe final finished dimension (fig. 5). They arethen treated for 10 to 14 days, depending on thethickness of critical areas, in a 50 percent PEGsolution maintained at 140° F. This schedule isdesigned to attain the required 25 to 30 percentPEG retention in the thin critical areas aroundthe action and barrel groove, where a high levelof dimensional stability is essential for top per-formance of precision rifles. Additional treatmentwould result in deeper penetration of the chemicalinto the thick part of the butt, where a high levelof stability is unimportant to performance. Thiswould also result in an undesirable increase inthe weight of sporter stocks.

Since treated wood does not shrink appreciablyin drying, there is less wood substance (mass) ina given volume of dry treated wood than in anequal volume of untreated wood seasoned by con-ventional methods. A properly stabilized solid-wood sporter stock is little or no heavier than acomparable untreated stock.

The treating schedule given here is suitable forprecarved stocks made from fresh-cut, greenblanks of the following species: Walnut, mango,koa, yellow birch, sycamore, myrtlewood, sap-wood of hard maple (including bird’s eye), true

13

F igu re 3A and B . - -Sawmi l l and l ogg ing was te (3A) can provide the raw material for thebowls shown in 3B. The bowls were rough-shaped f rom green scrap wood, PEG-treated,a i r - d r i e d , t u r n e d t o f i n a l d i m e n s i o n , s a n d e d , g i v e n f i n i s h , a n d p o l i s h e d .

M l 2 2 8 l 0

14

M l22 824

F igu re 4 . - -Rough -shaped g reen che r r y wood t ha t has been PEG- t rea ted and a i r - d r i ed( l e f t ) e v o l v e s i n t o f i n i s h e d c a r v i n g t h a t h a s b e e n s a n d e d , f i n i s h e d , a n d p o l i s h e d .

M I I9 797

15

F i g u r e 5 . - - A m a s t e r s t o c k m a k e r s h a p e s a n d i n l e t s s p o r t e r r i f l e s t o c k s f r o m g r e e n ,f r e s h - c u t w a l n u t b l a n k s . T h e s t o c k s w i l l b e P E G - t r e a t e d a n d k i l n - d r i e d t o 6 p e r c e n tmo i s t u re con ten t .

M I I8 0 0 5

mahogany, and mesquite. It will probably workequally well for most other hardwoods of the den-sity range suitable for rifle stocks. However, itwill not work for cherry. Precarved cherry stocksshould be soaked in PEG for about 45 days at alower temperature, 110° F., for equally goodresults.

After stabilization, the precarved stocks canbe dried equally successfully either rapidly in akiln under drastic conditions without fear ofdegrade or air-dried followed by conditioning to6 percent M.C. in a heated room.

The stabilized stocks should be sanded, bleach-ed, and finished as described for the other treated

wood products. These stocks can also be glassbedded. Simply scrub the wood with alcohol tocut the wax just before the bedding compound isapplied.

It should be emphasized that this treatmentworks only on green or partially dry wood. Not onlong-dry stocks! When wood is dried below thefiber saturation point (30 pct. M.C.) certainirreversible chemical and physical changes takeplace in the cell walls that prevent effectivestabilization. You can get PEG into dry wood bywater soaking previous to treatment, but the effectis not the same.

16

Protecting Imported Carvings

Almost all wooden carvings from tropicalc o u n t r i e s--southeast Asia, equatorial Africa,tropical Latin America, and other areas of highhumidity--are carved from partially dried wood.Consequently, the majority of these carvings willcheck and split badly when brought into theUnited States and exposed to our relatively lowrelative humidities. Many carvings are ruined

after only 1 or 2 weeks in the arid environment ofour heated and air-conditioned stores, offices,and homes (fig. 6).

At the first sign of checking or splitting, thecarvings should be immersed in water until theycan be treated. If incipient cracks swell shut asthe wood regains its turgid “green” condition,then it is certain that PEG treatment will be

F i g u r e 6 . - - M a n y o f t h e c a r v i n g s i m p o r t e d f r o m t r o p i c a l c o u n t r i e s d e v e l o p t h e s e r i o u sc h e c k s a n d s p l i t s s h o w n a b o v e w h i l e t h e y d r y o u t i n t h e a r i d e n v i r o n m e n t o f o u rh e a t e d a n d a i r - c o n d i t i o n e d s t o r e s , o f f i c e s , a n d h o m e s .

M l 2 l 8 8 l

17

completely successful. The cracks will remainclosed, and no others will develop when the carv-ings are later dried to the 6 percent M.C. reachedin a heated room.

If, however, the cracks do not completely closeas the wood swells when immersed in water,these cracks will remain open in the treated andsubsequently dried carving. All that can then beaccomplished with PEG treatment is the preven-tion of further damage. Preventing further check-ing and splitting, to the point of ruin, can be anadvantage. Use the schedules recommended forgreen wood bowls and carvings for treating thecarvings with PEG after they have been thoroughlysoaked in water. It is better to overtreat thanundertreat. Many extremely dense hard tropicalhardwoods are difficult to treat at ambient tem-peratures. For adequate penetration of the chem-

ical into these woods, it may be necessary toincrease the temperature of the solution to 100°F. or more. An electric water-heating element,preferably with a thermostat, is safe and efficient.

After treating, air-dry carvings for about 3 to6 weeks (depending on size and shape), preferablyin a heated room. Water-soaking and treatingwill, fortunately, remove most of the shoe polishor cheap varnish with which these carvings wereoriginally finished, but remove the remainder bysanding. Then bleach, sand, and refinish with thefinish suggested for bowls and green wood carv-ings, For imported carvings we strongly recom-mend the Danish-oil finish.

If carvings can be treated when purchasedwhile the wood is still in the essentially greencondition, they can be fully protected againstfuture checking and splitting (fig. 7).

F i g u r e 7 . - -The ca r v i ngs shown f r om Sou theas t As i a we re PEG- t rea ted on a r r i va l i n t h i scoun t r y ; t hus t hey we re p ro tec ted aga ins t d r y i ng deg rade .

M l 2 l 8 9 5

18

Preserving Archaeological Specimens

Archaeologists and other scientists who dig inold bogs and who probe the depths of the oceansfrequently find wood artifacts they want to pre-serve. Usually these artifacts are badly deterio-rated by marine insects or various bacteria anddecay organisms that dissolve much of the carbo-hydrate portion of the wood and leave mainly theresistant lignin. When these partially decomposedwooden artifacts are exposed to drying they breakdown into fragments or become dust.

If not too badly disintegrated, however, theycan be preserved by treating with PEG-1000.Most of, these artifacts can be successfully treated

by soaking 3 to 4 weeks in a 50 percent PEGsolution at ambient temperature. The PEG readilydiffuses into the partially deteriorated, water-soaked fine structure of the wood, supports it,and keeps it intact during the drying process.For badly deteriorated artifacts it may be nec-essary to start with a less concentrated PEGsolution and to use a polyethylene glycol polymerof higher molecular weight, such as PEG-1540 toeven PEG-4000. For extreme deterioration it maybe best to start with a liquid, like PEG-400, andthen gradually increase to the higher molecularweights until the desired result is attained.

Fungicides used improperly can be injurious toman, animals, and plants. Therefore, for safe andeffective usage it is essential to follow the direc-tions and heed all precautions on the labels.

Store fungicides in original containers--out ofreach of children and pets--and away from food-stuff.

The use of trade, firm, or corporation namesin this publication is for information and con-venience of the reader. It does not constitute anofficial endorsement or approval of any productor service by the United States Department ofAgriculture to the exclusion of others that maybe suitable.

Some of the distributors from whom small Crane Creek Companyamounts of polyethylene glycol can be procured P.O. Box 5553include: Madison, Wisconsin 53705

Robert M. Albrecht8635 Yolanda AvenueNorthridge, California 91324

Wilkens-Anderson Company4525 West Division StreetChicago, Illinois 60651

19

BIBLIOGRAPHY

Anonymous1972. How to make it with PEG. The Crane

Creek Company, P.O. Box 5553, Mad-ison, Wis. 53705.

, and Iversen, E. S.1961. Seasoning green-wood carvings with

polyethylene g l y c o l -1000. ForestProd. J. 11(1): 6, 7.

Bruce, Peter , and Fobes, Eugene W.1963. Magic potion for rifle stocks. Guns and 1962. Protect imported carvings with PEG.

Hunting 6(6): 34, 35. Forest Prod. J. 11(10): 476, 477.

Dawson, Robert E., Usher, Edward G., Jr., andMitchell, Harold L.1962. Stabilized wood gunstocks in Marine

Corps markmanship competition.Forest Prod. Lab. Rep. No. 2245,Forest Serv., U.S.D.A., Madison,Wis.

1963. PEG-treated walnut limbwood makeshandsome decorator clock. ForestProd. J. 13(9): 416.

1964. Burlwood--Royalty’s wood? AmericanForests 70( 12): 22-25.

Englerth, George H., and Mitchell, Harold L.1963. New horizons in bowl turning. Forest

Prod. J. 13(2): 48, 49.

, and Mitchell, Harold L.1967. Bowls from scrap wood and PEG. Work-

bench 23(4): 56, 57.

1966. Profitable use of low-grade timber. InBlack Walnut Culture, North CentralForest Exp. Sta., Forest Serv.,U.S.D.A., pp. 72-76. Papers pre-sented at Walnut Workshop, Carbon-dale, Ill., Aug. 2 and 3, 1966.

Fuglsby, Glen O.1964. Dimensional stabilization of wood.

School Shop 24(1): 23-25, 56.

Hoadly, R. Bruce1972. PEG-1000. Workbench 28(2): 48, 49.

Magnuson, Karen Lee1972. How to work with green wood. Popular

Science 200(4): 146-148.

1968. PEG is the sweetheart of the woodc r a f t s m a n . U.S.D.A. Yearbook ofAgriculture: S c i e n c e for BetterLiving, pp. 147-149.

Mitchell, H. L., and Wahlgren, H. E.1959. New chemical treatment curbs shrink

and swell of walnut gunstocks. ForestProd. J. 9(12): 437-441.

Rakusan, Jerome1964. Are we exhausting our walnut supply?

The Shooting Industry 9(6): 26-28, 41,42; and 9(7): 16-19, 40.

n.d. The trading post. The Shooting Industry13(9): 22, 23.

Stamm, A. J.1959. Effect of polyethylene glycol on the di-

mensional stability of wood. ForestProd. J. 9(10): 375-381.

20

U.S. GOVERNMENT PRINTING OFFICE: 1977–750-844