HEAT STABILIZED WOOD 

(Staybwood) 

No. 1621 

Revised April 1955 
- - - - 

HEAT-STABILIZED WOOD (STAYBWOOD)

1, 2 

By 

ALFRED J. STAMM, Chemist 

HORACE K. BURR, Chemist 

and 

ALBERT A. KLINE, Technologist 

3 Forest Products Laboratory, Forest Service 

U. S. Department of Agriculture 

Introduction 

Heating wood under drying conditions at higher temperatures or for 

longer periods of time than are normally encountered in kiln drying 

has been shown to reduce appreciably the hygroscopicity and subsequent swelling and shrinking4 (3, 4, 6, 7, 9). 

Experiments in which the specimens were heated in bombs containing different gases indicate that the nature of the gas has only a minor 

An up-to-date version of an article published in Industrial and 

Engineering Chemistry, June 1946. 

Staybwood is uncompressed wood that has been dimensionally stabilized by heat alone. 

scopicity of the wood is reduced. 

It differs from staypak (7) 

The wood is 

3 

Maintained at Madison, Wis., in cooperation with 

in that the hygronot compressed. 

the University of 

Wisconsin. 

4 

Numbers in parentheses indicate literature cited and refer to the 

listing at the end of the paper. 

Report No. 1621 Agriculture - Madison 

1 

2 
effect upon the resulting hygroscopicity, an oxidizing atmosphere being 

only slightly more effective than a reducing atmosphere. 

Previous data also show that the effect of heat upon the hygroscopicity 

of wood is permanent (6, 9). After an initial humidificationdehumidification cycle or soaking-drying cycle subsequent to heating, 

no gain in hygroscopicity occurs within the time or number of cycles 

tested. The phenomenon thus cannot be explained on the basis of a 

hysteresis effect. 

Heating in Molten Metal 

It seemed desirable to extend heating measurements over a much 

broader range of heating temperatures 

nature of the phenomenon better and 

of such a process for commercially 

wood. To minimize oxidation and its 

possible rapid heat transfer so as to 

length of time required to attain the 

and times to analyze the 

to determine the practicability 

stabilizing the dimensions of 

effect on strength and to make 

avoid making corrections for the 

desired temperature in high 

temperature-short time combinations, a method of heating wood beneath the surface of molten metal was adopted (6). A lead-tincadmium alloy (approximately 50 percent tin, 30 percent lead, and 

20 percent cadmium) with a melting point of about 150° C. (302° F.) 

was used. This alloy had but little tendency to stick to the specimens 

when they were removed from the bath except when used at temperatures only slightly above its melting point. Any small amount of adhering metal was readily scraped off with a steel spatula. In later 

work, Wood's metal was tried in order to determine if it could be 

used at lower temperatures. This metal tended to stick to the wood 

sections and hence its use was abandoned, 

The experiments with western white pine were made by heating the 

wood in an iron pan, under a steel weight with an insulated handle 

for holding the specimens immersed. The pan was heated with two 

Buns en burners. 

Report No. 1621 -2-
The bath, with the weight immersed, was heated to about 2° C. (3.6° F.) 

above the desired temperature for temperatures below 270° C. (518° F.). 

The weight was rapidly lifted, the specimens were placed on the moltenmetal surface, and the weight placed on them to immerse them in the 

molten metal. The wood specimens cooled the bath by about 2° C. (3.6° F.) 

thus minimizing the necessary manual adjustment of the burners. When 

the heating temperature was above 270° C. (518° F.) the exothermic reaction was sufficiently great so that one of the burners could be removed, 

slightly before or at the time the specimens were put into the bath, and 

not replaced for a period of 2 to 5 minutes. This agrees with the findings 

of Kobe and Goin (2) who found 273° C. (523° F.) to be the temperature at 

which exothermic decomposition of wood becomes appreciable. 

The temperature control by this manual method was accurate to only 2° 

to 5° C. (3.6° to 9.0° F.). It was not suitable for long-period heating. 

For this reason the heating of the Sitka spruce veneer specimens and the 

larger eastern white pine specimens was conducted in an electrically 

heated bomb with an automatic temperature control. The bomb lid was 

designed to hold the specimens immersed. The molten metal could not 

be used for heating at temperatures below 175°C. (347° F.). Because 

of this, the veneer specimens were heated at 125° and 150° C. (257° and 

302° F.) in sand with dry nitrogen slowly circulating through the sand to 

prevent oxidation. Veneer specimens were also heated in an electrically 

heated oven at 125°, 150°, and 175° C. (257°, 302°, and 347° F. ) to 

determine the extent to which oxidation affects the results. 

Hygroscopicity and Dimensional Stability 

The heated specimens, when removed from the heating medium, were 

cooled in a desiccator, scraped free of any adhering metal, weighed, and 

measured. They were then subjected to relative-humidity cycles in which 

they were successively brought to equilibrium with 90 percent and 30 percent relative humidity in rooms held at 26. 7° C. (80° F. ), together with 

matched unheated controls that were oven-dried to determine the ovendry weight at the conclusion of the measurements. 

The western white pine cross section specimens were all cut from a 

single flat-sawn board 1 inch thick and 4-3/8 inches wide into pieces 

one-half inch in the fiber direction. Every seventh specimen was used 

as a control. The flat-sawn western white pine specimens were also cut 

from a single flat-sawn board 13/32 inch thick into strips 1-1/4 inches 

Report No. 1621 -3-
wide in the tangential direction. The strips were cut into 6-inch lengths. 

All the specimens heated at a given temperature, together with two controls taken from the two ends, were cut from a single stick, thus assuring 

optimum matching, The rotary-cut 1/16-inch Sitka spruce veneer specimens were 5-7/8 inches long and 1 inch wide. They were similarly matched 

except that specimens Nos. 1, 6, 11, and 16, all cut from a single 1-inchwide strip, were used as controls. The eastern white pine specimens were 

cut into 6 -inch lengths from a single stick 2 inches wide by 15/16 inch 

thick. The two end specimens were used as controls. 

The specimens were put through three complete relative-humidity cycles 

of 90 percent to 30 percent to 90 percent, with the exception of the eastern 

white pine, allowing sufficient time under each condition (about 2 weeks) 

to come to moisture equilibrium. The percentage reduction in hygroscopicity and the percentage increase in the dimensional stability (antishrink efficiency) was calculated for each half cycle of 90 to 30 percent 

and 30 to 90 percent relative humidity. The percentage reduction in 

hygroscopicity is equal to 100 times the difference between the weight 

change of the control and the relative weight change of the heated specimen divided by the weight change of the control. The percentage increase 

in the dimensional stability is equal to 100 times the difference between 

the dimension change of the control and the relative dimension change of 

the heated specimen divided by the dimension change of the control. The 

percentage reduction in hygroscopicity and the antishrink efficiency are 

practically equal under the experimental conditions of this research. In 

view of this equality and inasmuch as the accuracy of weighing is greater 

than that of measuring, the values recorded in this paper are actually the 

reduction in hygroscopicity as found by weighing except in the case of the 

eastern white pine. In this case dimension changes between the watersoaked and the oven-dry condition were measured. 

Figure 1 gives the average antishrink efficiencies for the complete second 

and third cycles plotted against the heating temperature for several different times of heating obtained on the western white pine cross sections. 

Figures 2 and 3 give similar data for the flat-sawn western white pine and 

the rotary-cut Sitka spruce veneer, respectively. Figure 4 gives the antishrink efficiency values from a water-soaked to an oven-dry condition for 

the specimens of eastern white pine. These measurements were made on 

1/8-inch-thick cross sections cut from the ends of the larger specimens. 

An approximately linear relationship exists between the antishrink efficiency 

and the temperature for each time of heating, the lines becoming slightly 

steeper with increasing time. 

Wood when heated at temperatures and for periods sufficient to give an 

appreciable antishrink efficiency is materially darkened. The lightReport No. 1621 -4-
colored softwoods used in these experiments darkened to about the color 

of walnut. 

The minimum heating times used in these experiments, 15 minutes for 

the 1/2-inch and 13/32-inch-thick western white pine and 1 minute for 

the 1/16 -inch-thick Sitka spruce veneer, have been theoretically shown 

to be such that the center of the wood rises in temperature at least 95 

percent of the temperature rise at the surface in a fraction of the total 

time. Practically uniform antishrink efficiencies should hence result 

throughout the thickness of the wood, even in these short heating times. 

Figure 5 shows the data of figures 1 to 4 plotted as the logarithm of the 

heating time against the heating temperature necessary to obtain three 

different antishrink efficiencies. The slope of the lines is such that the 

reaction rate practically doubles for each rise in temperature of 10° C. 

(18° F.). 

Figure 6 shows the relationship between the antishrink efficiency and 

the loss in weight caused by heating, for the data of figure 3. The loss 

in weight per unit antishrink efficiency obtained is greater when the wood 

is heated in air than in the molten metal. Evidently a greater degree of 

oxidation occurs in air; this contributes less to the antishrink efficiency 

than the other degradative reactions. 

Figure 7 gives the relationship between the toughness and the antishrink 

efficiency of the flat-sawn western white pine specimens in equilibrium 

with 30 percent relative humidity, The measurements were made on 

the Forest Products Laboratory toughness machine (1), using a 4-inch 

span. The toughness values are for the actual cross-section of the 

specimens corrected, for the slight variation in thickness and width, 

to an area of 0. 53 square inch by considering the toughness directly 

proportional to the cross-sectional area. As was expected, the toughness of the wood was appreciably decreased by the heat treatment. The 

reduction in toughness of wood on heating is not attributable to the reduced moisture content at test which results from the reduced hygroscopicity of the wood. The variation in toughness of wood with changes 

in moisture content is small compared to the loss in toughness obtained 

on heating. The loss in toughness must thus be due to an embrittlement 

of the fiber itself. 

Figure 8 shows the relationship between the face hardness and the antishrink efficiency of the eastern white pine specimens as determined by 

the Forest Products Laboratory method in which the load required to 

embed a 0. 444-inch diameter ball one-half its diameter is measured. 

Report No. 1621 -5-
The specimens were preconditioned at 65 percent relative humidity. The 

hardness is reduced significantly at the higher degrees of heating, but 

not so much as the toughness. 

Figure 9 shows the relationship between the antishrink efficiency of the 

spruce veneer and the modulus of rupture in equilibrium with 30 percent 

relative humidity. The measurements were made on a cantilever beam 

stiffness tester. The veneer heated in the molten metal showed a much 

smaller decrease in modulus with increase in antishrink efficiency than 

did the veneer heated in air. 

The modulus of elasticity is also decreased by the heating process, but 

to an appreciably lesser degree than the modulus of rupture. Heating 

in air again causes a greater loss in strength than when heating is done 

beneath the surface of molten metal. It is thus of considerable importance 

to heat wood in the absence of air in any commercial operation aimed at 

stabilizing wood for uses in which moderate strength is important. 

A few preliminary pure-culture decay tests were made on the flat-sawn 

western pine specimens, Both the heated specimens and controls were 

cut into 2-inch lengths, oven-dried, weighed, and placed in jars containing a good growth of the wood-destroying fungus, Trametes serialis. 

The jars were held under favorable growth conditions for 2 months. The 

fungus grew over the surface of all the specimens. This growth was 

carefully removed and the specimens were again oven-dried and weighed. 

The Controls sustained an average weight loss of 28. 4 percent. Heated 

specimens with antishrink efficiencies of 30 to 33 percent gave an average weight loss of 12.5 percent. Specimens with antishrink efficiencies 

of 33 to 38 percent gave weight losses of 0 to 4.5 percent. All the specimens with antishrink efficiencies of 40 percent or more gave no weight 

loss. 

Decay resistance produced by heating the wood is similar to that resulting from resin treatment in that the resistance results largely from the 

inertness of the material rather than to the presence of a toxic ingredient. 

The decay resistance in both is presumably due to either or both the reduced hygroscopicity whereby sufficient water is prevented from entering 

the cell-wall structure to support decay, or to a chemical change occurring within the wood that made it no longer susceptible to fungus attack (7). 

Abrasion resistance tests have been made on specimens heated so as to 

attain an antishrink efficiency of 30 to 40 percent. The abrasion resistance was found to be only one-third of that of the unheated controls. The 

Report No. 1621 -6-
seriousness of this reduction can be best appreciated from the fact that 

fibers can be pulled from the surface of a specimen. This indicates 

that the fiber bond is greatly reduced by the heat treatment. 

Conclusions 

An appreciable degree of dimensional stabilization and decay resistance 

can be imparted to wood by dry heat, This can be accomplished over a 

broad range of temperatures and times, the time of heating decreasing 

rapidly with an increase in temperature, 

This heat treatment causes a serious loss in strength properties, especially toughness and abrasion resistance. When the wood is heated 

beneath the surface of a molten metal the strength loss for any antishrink efficiency is less than when heated in air. The strength loss is 

still too great to make this a commercially applicable method for obtaining dimensional stabilization of wood, except perhaps for some 

abnormal use where strength is of relatively little importance. 

Report No. 1621 -7-
Literature Cited 

(1) Forest Products Laboratory Report No. 1308*, November 1941. 

(2) KOBE, K. A. , and GOIN, T. L. 1939. Ind. Eng. Chem. 31: 1171. 

(3) KOEHLER, A., and PILLOW, M. Y. December 19, 1925. So. 

Lbrman., p. 219. 

(4) PILLOW, M. Y. October 1929. Wood Working Industries, p. 8. 

(5) SEBORG, R. M., MILLETT, M. A., and STAMM, A. J. March 

1944. Forest Products Laboratory Report No. 1580*. 

(6) STAMM, ,A. J. September 1942. U. S. Patent No. 2, 296, 316. 

(7) and HANSEN, L. A. 1937. Ind. Eng. Chem. 29: 831. 

(8) STAUDINGER, H. 1936. Trans. Faraday Soc. 32: 323. 

(9) TIEMANN, H. D. 1915. Lumber World Review 28(7): 10. 

(10) Tinius Olsen Testing Machine Co. , Bull. No. 11. 

*Available from the U. S. Forest Products Laboratory, Madison 5, 

Wisconsin. 

Report No. 1621 -8- 1.-15 
Figure 1.--Antishrink efficiencies obtained by heating Western white pine cross 

sections one-half inch thick in the fiber direction under molten metal at 

Z M 68534 F different temperatures for different periods of time. 
Figure 2.--Antishrinkefficiencies obtained by heating flat-sawn Western white pine 

specimens thirteen-sixteenths inch thick under molten metal at different temperaZ M 68535 F tures for different periods of time. 

Figure 4.--Antishrink efficiencies obtained by heating Eastern white 

pine specimens 6 by 2 by 15/16 inches under molten metal at 

different temperatures for 2 hours. 

Z M 68537 F 
Figure 5.--Heating time and temperatures necessary to obtain antishrink efficiencies 

of 10, 25, and 40 percent on Western white pine cross sections, flat-sawn Western 

white pine, rotary-cut Sitka spruce veneer, and Eastern white pine boards. 

Z M 68538 F 

Figure 7.--Forest Products Laboratory toughness versus the antishrink 

efficiency obtained by heating flat-sawn Western white pine specimens 

as indicated in figure 2. Z M 68540 F 
Figure 8.--Hardness (ASTM standard test method) versus the antishrink 

efficiency obtained by heating Eastern white pine specimens as 

indicated in figure 4. Z M 68541 F 
Figure 9.--Modulus of rupture in bending versus the antishrink 

efficiency obtained by heating rotary-cut Sitka spruce veneer 

specimens as indicated in figure 3. Z M 68542 F