The Effects of Heat-Treatment on Tensile
Elastic Modulus and Creep in Hickory
Wood: Applications to Bow-Making
Nicholas Wenner
23 May 2014
ME348
Independent Research Project
2
Table of Contents:
Motivation and Objectives ………………………………. 3
Background ………………………………. 4
Equipment and Experimental Procedure ………………………………. 5
Results ………………………………. 9
Discussion ………………………………. 11
Conclusions ………………………………. 14
Appendix I: Stress Calculations …………………………………. 16
Appendix II: St. Louis’ Experiments ………………………………. 17
Appendix III: Selections from Esteves and Pereira (2009) ………………………………. 19
3
Motivation and Objectives:
This experiment was conceived and carried out with an eye towards designing an archery bow
from natural materials. Over several meetings with a local bowyer, an interest in the effects of
heat-treatment on bow wood developed based on anecdotal evidence suggesting that certain
forms of heat-treatment can improve the elastic qualities of wooden bows, giving both increased
draw strength in a given bow and decreased levels of string follow (the tendency of a bow to
deform and lose strength over time and use).
In The Traditional Bowyer’s Bible, Vol. 4 (2008), bowyer Marc St. Louis discusses his informal
experiments with the heat-treatment of bows, finding potential for both increasing the draw
weight of a given bow and decreasing its string follow (see Appendix II for details).
St. Louis discusses the following benefits of heat-treatment:






“A bowyer can make heat-treated bows of a much narrower profile than an untreated
bow.”
“Dry heat can also be used to temper static recurves on a bow after they have been steam
bent, and this can reduce the mass of the limb tips.”
“Heat-treating and reducing the mass of the tips is advantageous for those wishing to use
lighter arrows than normal, as light tips reduce handshock. This can also be beneficial to
those wanting to make bows to compete in flight archery.”
“In effect, bows made from the better second-string wood species such as HBB or elm
can exceed the performance of the higher density wood species.”
“The process can be used to make very high performance bows, improve the performance
of simple flatbows, or even to revitalize older bows. The tempering process reduces mass,
thereby increasing performance. The reshaping of the wood and adding reflex increases
string tension, which also improves performance.”
It also “seems to increase a bow’s resistance to moisture incursion, which is always a
concern with whitewood bows.”
St. Louis also mentions that heat-treatment can be taken too far, at which point any benefits are
overcome by an increase in the brittleness of the wood. St. Louis gives some guidance to the
appropriate level of heat-treatment: “A good rule of thumb when tempering: brown is good,
black is bad.”
The general aim of this experiment is to build on the evidence St. Louis provides by more
rigorously quantifying the effects of heat-treatment on wood and providing more specific
guidelines for the proper time and temperature needed.
Wood is a highly diverse and complicated material. The effects of a given heat-treatment will
depend strongly on the type of wood, temperature, time, humidity, and many other factors. Due
to limitations of resources and the relatively small scope of the assigned project, this experiment
aims only to provide a small-sample exploration of the effects of one particular heat-treatment on
one particular wood species. The heat-treatment was chosen based on reports given by St. Louis
4
(2008) and informed by a review of the scientific literature on wood heat-treatment provided by
Esteves and Pereira (2009). Hickory wood (Carya illinoensis) was chosen based on availability
and on reports from both a local bowyer and from St. Louis on its “excellent” response to heattreatment.
Background:
Esteves and Pereira (2009) offer an excellent review of the effects of heat-treatment on wood,
showing that wood in heat-treatment undergoes complex chemical and anatomical changes with
effects on factors including mass, equilibrium moisture content, dimensional stability, rot
resistance, mechanical properties, color, wettability, thermal conductivity, and factors involved
in the finishing and gluing processes.
Of the effects Esteves and Pereira discuss, the most applicable to bow making are:



Mass loss
Decreased equilibrium moisture content, resulting (among other things) in increased
dimensional stability
Mechanical properties
o Increased brittleness
o Time and temperature-dependent effects on modulus of elasticity
 “The modulus of elasticity seems to increase for softer treatments and
decrease for more severe treatments.”
See Appendix III for more information on each of these effects.
5
Equipment and Experimental Procedure:
Equipment:
 Two hickory wood specimens of clear grain and approx. dimensions of 1.5” x .5” x 6.25”
 Table saw, band saw, disc sander, and wood planer for sample preparation
 Thermolyne small benchtop muffle furnace FB1415M (120V)
 Dial calipers (resolution .001”)
 Digital scale (resolution .1 gram)
 Two single element, pre-wired strain gages and installation materials
 P 3500 strain indicator
 Gear motor driven, deflection controlled loading system
 Four-point bending loading fixture
 Voltmeter for monitoring applied load
Sample Preparation:
Because only two strain gauges were available for the purposes of this experiment, special effort
was taken to maximize homogeneity between the experimental and control samples. Both
samples were cut from the same piece of hickory wood in adjacent growth rings. The samples
were cut to dimensions as equal as possible. Dimensions were chosen to approximate those of a
bow limb and to accommodate the size of the loading fixture.
Original Dimensions: Experimental (1.455” x .456” x 6.274”), Control (1.460” x .463”
x 6.277”)
Care was taken to maintain as similar grain orientation and growth ring size as possible:
6
The samples were prepared with the grain oriented in the direction that would be found in a bow:
Mass and dimensional data was taken in both specimens immediately prior to heat-treatment and
immediately prior to loading. Moisture content was not measured due to lack of appropriate
equipment.
Heat-treatment:
The experimental specimen was heated in a benchtop muffle furnace at 200 degrees C for 91
minutes. This relatively mild treatment was chosen based on a conclusion by Esteves and Pereira
(2009) that the modulus of elasticity “seems to increase for softer treatments and decrease for
more severe treatments.” (See Appendix III for more details.)
7
The specimens were left side-by-side for about 1 week to allow the heat-treated specimen to
come back to equilibrium moisture content.
Loading and Data Collection:
Strain gauges were attached with cyanoacrylate adhesive to the underside of each specimen as
close to the centers as possible and oriented as closely as possible with the longitudinal axis.
Gauges were attached to the sides of the specimens that would experience tension under bending.
The samples were placed in the four-point bending fixture in the loading system.
8
Data was taken in two rounds, the first for determination of elastic modulus and the second for
testing the effects of creep. In the first round, each sample was loaded in increments of about 30
pounds, pausing at each increment to record loading and strain values with a photograph. The
intent was to load each specimen to 1% strain, an approximation of the strain experienced by a
bow at full draw. The loading machine was unable to apply sufficient load, however, reaching a
maximum in the control of about .47% strain and about .37% in the experimental specimen. Each
specimen was unloaded immediately and residual strain measured.
In the second round, each specimen was loaded continuously and at the same rate up to a
maximum strain of .35% and left there for 60 minutes, at which point the specimen was unloaded
and residual strain measured.
A specimen under approximately .35% strain.
9
Results:
Modulus of Elasticity:
Modulus of elasticity increased by 26% with heat-treatment. The stress-strain curve
demonstrated highly linear behavior:
Creep:
No significant creep was observed in either specimen. Both specimens behaved elastically both
when unloaded immediately after reaching a maximum load (Phase 1) and after being held for 1
hour at 3500 microstrain (Phase 2).
Creep (με)
10
Mass Loss:
Due to equipment malfunction, mass data taken prior to heat-treatment was erroneous.
Comparing the densities of the control and the heat-treated specimens, however, provided an
indirect measure of mass loss, yielding:
Percent mass loss: 1.7%
Dimensional Shrinkage:
Dimensions of both specimens were taken immediately prior to treatment of the experimental
specimen and immediately prior to collecting stress-strain data on the loading machine. To
separate dimensional changes due to heat-treatment from those due to changing environment,
shrinkages observed in the control were subtracted from those observed in the experimental
specimen, yielding a “corrected” percent shrinkage.
Corrected percent shrinkage due to heat-treatment
11
Discussion:
Modulus of Elasticity
In “The Wood Handbook” (2010), Kretschmann gives the following values:
Assuming 12% moisture content in the wood used in this experiment, the elastic modulus of the
control specimen was found to be 58% greater that reported by Kretschmann. While this could
be due to natural variation between wood specimens, Kretschmann also reports a coefficient of
variation for elastic modulus of only 22%, leaving a 36% increase unexplained.
According to The Wood Database (http://www.wood-database.com), the strength characteristics
of Carya illinoensis are influenced by the spacing of the growth rings, with wood from fastergrowing trees (wider spacing) tending to be harder, heavier, and stronger than that from slowergrowing trees (closer spacing). The relatively high elastic moduli of the specimens in this
experiment could be explained, then, by the relatively high density of the specimens, with the
specific gravity of the control being .88 (33% greater than the value reported by Kretschmann).
While the magnitude of the elastic moduli reported is perhaps unexpected, there is regardless a
clear increase of elastic modulus (26%) between the control and the experimental specimens,
verifying and quantifying the effects reported by St. Louis (2008).
The surface stresses reached in the experiment (12721 psi in the control specimen and 12795 psi
in the experimental) are approaching the modulus of rupture reported by Kretschmann (13700
psi).
Creep:
That no significant creep was observed in either specimen could be due to relatively low strains
and relatively short loading times. String follow in bows results from repeated shooting from full
draw (at approximately 1% strain). Strains in this experiment were limited to .35% due to
12
limitations of the available equipment. Creep in bows may also be a result of dynamic bending
effects, which were not explored in this experiment.
Mass Loss:
The observed mass loss is on the order of that expected by data given in Esteves and Pereira
(2009). (See Appendix III, Figure 1.)
Studying the effects of the observed mass loss (1.7%) on bow performance would be an
interesting avenue for future study.
Dimensional Shrinkage:
The Wood Database (http://www.wood-database.com) gives the following values for
dimensional shrinkage with moisture content (green to oven-dry): Radial: 4.9%, Tangential:
8.9%, Volumetric: 13.6%, T/R Ratio: 1.8. The longitudinal shrinkage is negligible.
That only negligible amounts of radial shrinkage occurred in the heat-treated specimen and that
the T/R ratio was not maintained at approximately 1.8 suggest that a mechanism alternative to
moisture loss contributed to the relatively large tangential shrinkage observed in the specimen
(~1.9%).
The corrected percent shrinkage values reported in this experiment are likely to be
underestimates of the real values given that heat-treated wood has been shown to have higher
dimensional stability than untreated wood (Esteves and Pereira, 2009).
That significant tangential shrinkage occurred with heat treatment fits with St. Louis’
observations that heat-treating the belly of a bow causes it to become slightly concave as well as
smaller in width.
Sources of Error
General
 While great effort was taken to ensure homogeneity between the two samples, inevitable
differences between them must account for some of the reported changes.
 The relation provided by the instructor to convert measured voltage to applied load was
of unknown accuracy.
 Strain gauge application was not perfectly aligned with the longitudinal axis of the
material. The contribution to error due to this alignment, however, is small for small
angles: If the angle were off by 1 degree (and it was surely less than that), the error
incurred would be only about 0.02%.
 The strain gauge was not perfectly placed in the center of the beam. This is mitigated by
the fact that the internal moment is expected to be constant throughout the middle section
of a beam in four-point bending.
13
Temperature compensation for changes in resistivity and for differences in the
coefficients of thermal expansion between the gage and the substrate were not taken into
account. Temperatures could have varied on the order of 1-4 degrees C.
 Transverse sensitivity of the strain gage was not taken into account.
 It was not possible for the top and bottom loading fixtures to be perfectly aligned,
resulting in four-point bending that was asymmetrical to some extent.
Modulus of Elasticity
 Deformation of the sample at the loading points clearly occurred, potentially affecting
measured strains.
 In calculating stress, elastic strain under pure bending with uniaxial bending stress was
assumed.
Creep
 Loading profile over time between the two samples could have varied stochastically
between the two samples as a function of deformation at the loading points.
Mass Loss
 Equipment malfunction rendered pre-heat-treatment mass values unusable and mass loss
had to be calculated between the experimental and the control specimens via density.
Dimensional Stability
 While the average of several measurements were taken for each dimension and each
dimension was measured in roughly the same location pre-heat-treatment and preloading, inevitable variation in the specimen’s shapes could have accounted for some of
measured dimensional changes.

14
Conclusions:
The general aims of this experiment were to build on the evidence provided by St. Louis (2008)
about the effects of heat-treatment on bow wood by 1) more rigorously quantifying those effects
and 2) providing more specific guidelines for the proper time and temperature needed.
For a particular heat-treatment in a particular wood species, the first goal was achieved: For a
heat-treatment of 200 C for 1.5 hours, an increase in elastic modulus of 26%, no significant
change in creep, mass loss of 1.7%, and significant tangential shrinkage (1.9%) were observed.
The second goal was approached, showing that the particular heat-treatment explored had
positive effects on the elastic modulus of hickory wood, but not exploring the effects on modulus
of rupture and the effects of different heat-treatments.
While necessarily lacking in scope, this study served well as a next step in the attempt to
quantify the effects observed by craftsmen in their trade and – perhaps – to provide those
craftsmen with useful theoretical frameworks and predictions.
Special thanks to bowyers Richard Baugh and Jim Langell for guidance, inspiration, and
sharing of resources, time, and materials.
Recommendations for future research:

Further inquiry
o Test more wood types at a greater range of heat-treat temperatures and times.
 Identify the times at a given temperature at which elastic modulus begins
to decrease.
o Test creep over a greater range of maximum strains and loading times.
o Test in dynamic loading environments to more accurately reflect the loading and
unloading seen by a typical bow.
o Provide frameworks for estimating temperature based on metrics available to the
bowyer (e.g. temperature of thermometer held at surface, color of wood reached).
o Take data on how modulus of rupture is affected by heat-treatment. (Requires
thinner samples or loading machine with higher range to reach sufficient strains.)
o Study the effects of the observed mass loss (1.7%) on bow performance.
o Study the effects of the observed increase in modulus of elasticity (26%) on bow
performance.

Improvements on methodology
o Multiple samples and multiple tests with each sample.
15
o Compare samples from the same growth rings but different (but adjacent)
longitudinal locations (as opposed to the same longitudinal locations but different
(but adjacent) growth rings) to maximize homogeneity between the experimental
and the control specimens.
o Take mass data before as well as after heat-treatment using a scale with precision
of at least .1 gram.
o Take data on moisture content.
o Use thinner samples (or a loading machine with a higher range) to achieve strains
typical of a bow at full draw (on the order of 1% strain).
16
Appendix I: Stress Calculations
The modulus of elasticity, E was calculated from the slope of the stress-strain curve, assuming
elastic strain under pure bending with uniaxial bending stress.
σ = 3My / (2bh3)
Where the applied moment, M is given as:
M = F/2*l
Where l is the distance between the inner and outer loading points.
In this experiment, the lower loading fixture spanned 5.5 inches and the upper loading fixture
spanned 3.5 inches, such that l was 1 inch.
17
Appendix II: St. Louis’ Experiments
St. Louis (2008) explores the use of heat-treatment to introduce recurve into bows by drawing
them backward and applying heat to the tensioned belly. Recurve bows have tips that curve away
from the archer when the bow is strung, storing more energy than a simple straight limbed bow
and permitting a shorter bow for a given performance.
Using a heat gun, St. Louis heats the wood until it turns “brown.” Measuring the heat at the
surface of the wood with a thermometer in one occasion, he measured about 400 degrees F (204
C).
Elm static recurve bow
 About fifteen minutes at four inches from a hot plate with the bow reflexed.
 Belly turned a “dark brown.” Gave off a pleasant aroma.
 Back was hot enough that he could only put his hand on it for a second or two and water
brushed on the back started to steam immediately.
 Belly shrank from the heat, leaving the back wider than the belly.
 In the end the bow was reflexed several inches and the color turned from pale white to
dark brown.
 Left for several days to come back to equilibrium moisture content.
 Noticed the belly had become slightly concave as well as shrunk in width.
 From the side, slight discoloration could be seen up to about halfway through the limb.
 Shavings from the belly seemed “crisper.”
 Seemed to be a definite increase in draw weight. Drawing well past brace height had
“hardly any difference on reflex.”
 During tillering, “the limbs just did not want to take any appreciable amount of set.”
 Tillered to 50# at 28” draw, where it kept “at least 50 percent of the reflex” he had heated
into it.
 It was “very fast”: nearly 180 fps with a 10 grain per pound arrow.
 It was “stable and shot like a bow at least 10# heavier in draw-weight.”
 It “gave nothing away in performance” compared to the high-performance sinew-backed
reflexed recurves he had made in the past and it was much easier to make and much more
stable with humidity.
 Years later, after having shot “many arrows” with the bow, it still holds a “substantial
reflexed profile” when unbraced.
Eastern Hop Hornbeam static recurve bow
 Same procedure as previous. Again noticed shrinking of the belly. Image shows about
1/8” belly over a limb about 2” wide.
 Heat-treated again until the bow started to turn black.


18
Tillered same as before. Also retained “more than 50%” of the several inches of reflex he
introduced.
Performance was “spectacular”, but the bow began to fail not long after finishing it.
Large fractures formed on the belly (compression) side.
Eastern Hop Hornbeam pyramid flat bow
 “Though being relatively short for the draw length, it still retained nearly 2” of reflex.”
Eastern Hop Hornbeam flat bow with shallow static recurves and a bit a set with about 1”
string follow
 The limbs returned to nearly their original straight profile and again became somewhat
concave.
 The draw weight had “increased substantially.”
 The limbs did not take “any set at all” after being pulled back “a few times.”
 At 28” draw, the draw weight increased from the original 52 lbs. to 60 lbs.
 After tillering back to the original draw weight, the bow shot “an additional 10 yards, or
an additional 6-8 fps” for a 500 grain arrow. The bow was averaging 156 fps with highs
of 160 fps.
White ash native style D bow
 Held by hand about 12” above hardwood coals for about 30 minutes per limb.
 It “lost a bit of reflex during the process” but still had about 2” before tillering was
started.
 After repeated pulls to full draw, the bow held 1” of the initial reflex.
19
Appendix III: Selections from Esteves and
Pereira (2009)
The following selections are taken from:
Esteves and Pereira (2009). “Heat treatment of wood,” BioResources 4(1), 370-404.
Mass Loss:

“Mass loss of wood is one of the most important features in heat treatment and is
commonly referred to as an indication of quality. Several authors studied mass loss with
heat treatment and concluded that it depends on wood species, heating medium,
temperature, and treatment time (Fig. 1).”
Equilibrium Moisture Content:


“The main effect of the heat treatment is the decrease in equilibrium moisture content.
The reduction was already reported in 1920 by Tiemann, who showed that the drying at
high temperatures decreased the equilibrium moisture of wood and consequently its
swelling and shrinking. This is the basis for all heat treatment processes. As in mass loss,
improvement of equilibrium moisture content depends on wood species, temperature,
time, and type of treatment.”
“The minimum temperature necessary to perform a heat treatment is 100oC according to


20
some authors. Kollmann and Shneider (1963) carried out tests with beech wood, oak, and
pine at temperatures 70oC- 200oC and 6 -24 hours, and concluded that the absorption of
water decreased at temperatures higher than 100oC, decreasing with the increase in
treatment time. The same was confirmed by Nikolov and Encev (1967) and D ́Jakonov
and Konepleva (1967). However, other authors disagree and believe it depends on the
wood species. For example, Kollmann and Fengel (1965) reported that wood degradation
begins at 100oC for pine but only at 130-150oC for oak.”
“The reasons for the decrease of the equilibrium moisture are as follows: There is less
water absorbed by the cell walls as a result of chemical change with a decrease of
hydroxyl groups; there is enhanced inaccessibility of cellulose hydroxyl groups to water
molecules due to the increase of cellulose crystallinity; and cross-linking occurs in
lignin.”
“The lower equilibrium moisture content might affect positively the strength properties of
heat-treated wood, but this effect is superseded by the degradation of the chemical
compounds.”
Mechanical Properties
“The downside of the treatment is the degradation of mechanical properties. The effect on MOE
is small, whereas static and dynamic bending strength and tensile strength decrease. Brittleness
of wood increases with the deterioration of fracture properties due to the loss of amorphous
polysaccharides. The degradation of hemicelluloses has been identified as the major factor for
the loss of mechanical strength, but also the crystallization of amorphous cellulose might play an
important role. Polycondensation reactions of lignin, resulting in cross-linking, have been
mentioned as having a positive impact mainly on longitudinal direction.”
Modulus of Elasticity




“The modulus of elasticity seems to increase for softer treatments and decrease for more
severe treatments. Results reported by Esteves et al. (2007b) with steam heat-treated pine
wood (Pinus pinaster) showed a small increase until about 4% mass loss, followed by a
decrease for higher mass losses. With the same treatment conditions, heating time, and
temperature, the reduction of MOE was higher for the treatment in air, and relationship
prevailed also when comparing at constant mass loss.”
“Results reported by Esteves et al. (2007b) with steam heat-treated pine wood (Pinus
pinaster) showed a small increase until about 4% mass loss, followed by a decrease for
higher mass losses. With the same treatment conditions, heating time, and temperature,
the reduction of MOE was higher for the treatment in air, and relationship prevailed also
when comparing at constant mass loss.”
“In static bending tests (Fig 8) with Pinus radiata wood treated at 120oC, 150oC, and
180oC during 6 to 96 hours, Kim et al. (1998) showed that there was a close relationship
between the decrease of bending properties (MOR, MOE and WML) and the process
conditions (time and temperature). The work for the maximum load (WML) suffered an
accentuated decrease, while the modulus of elasticity was affected less.”
“Shi et al (2007) studied the mechanical behaviour of Quebec wood species heat-treated
using the Thermowood process and concluded that the modulus of rupture decreased




21
between 0% and 49% for heat-treated spruce, pine, fir, and aspen, while for birch the
modulus increased slightly (6%) after the heat treatment. Heat- treated spruce and pine
modulus of elasticity decreased between 4% and 28%; however for fir, aspen, and birch
the modulus generally increased. Mburu et al. (2008) with heat- treated Grevillea robusta
wood found reductions on MOR and MOE reaching about 65% and 28%, respectively.”
“Rusche (1973a, b) made heat treatments with and without oxygen using pine and beech
and concluded that the modulus of elasticity decreased significantly for mass losses from
8 to 10%. Similar results were reported by Vital et al. (1983), with Eucalyptus saligna
treated at 105-155oC for 10-160 hours. Mitchell (1988) studied heat-treated Pinus taeda
at 150oC and 1, 2, 4, 8, and 16 h with equilibrium moisture of 0%, 12%, and green, in
oxygen, nitrogen and air, and found that the MOE decreased irregularly with the time of
treatment, decreasing more for green wood. For the heat treatment in air, MOE decrease
was 14 times higher in green than in dry wood. In nitrogen there was no decrease of
MOE, while with air the decrease was smaller than with oxygen. Santos (2000) reported
for heat-treated Eucalyptus globulus a steep increase of the modulus of elasticity from
15,974 MPa to 27,646 MPa, although the time and the temperature of the treatment were
not mentioned. Different results were reported by Esteves et al. (2007b) for the same
wood treated at temperatures between 180 and 210oC. They found a slight increase at the
beginning of the treatment, followed by a decrease. Sailer et al. (2000) did not find
differences in the modulus of elasticity for oil and air heat-treated wood at 180oC,
200oC, and 220oC, and only for impact bending that decreased 51% in the case of oil and
37% in air. Kamdem et al. (2002) used spruce and beech treated by the French method
(Rectified wood) between 200oC and 260oC, and obtained a decrease of 11% and 20%
for MOE, 8% and 40% for MOR, respectively for spruce and beech. Goroyias and Hale
(2002) studied the heat treatment of Pinus sylvestris chips for OSB production between
200oC and 260oC for 20 minutes. MOR did not decrease significantly for 200oC and
210oC, decreased slightly for 220oC, 230oC, and 240oC, and decreased significantly for
250oC and 260oC. The variation in MOE was similar with a decrease for temperatures
higher than 240oC.”
“Kubojima et al. (1998) made some vibrational studies with Picea sitchensis, and
observed that the Young modulus in longitudinal and radial directions increased in the
first two hours of treatment and remained constant afterwards for wood treated at 120oC
and 160oC. At 200oC, the Young modulus increased in an initial phase, decreasing after
that. The shear modulus in longitudinal and radial directions increased in an initial phase,
becoming constant for 120 and 160oC, while for 200oC increased initially and decreased
afterwards. The Young modulus increased with the increase of cellulose crystallinity and
with the decrease of wood moisture. The effect of crystallinity prevails in the beginning
of the treatment but with the continuation of the treatment the heat degradation is
dominant, leading to the decrease of the Young modulus.”
“Kubojima et al. (2000a) reported that the effects of heat treatment were similar in green
and dry wood. The same authors (Kubojima et al. 2000b) also reported that the Young
modulus and the bending strength increased in the beginning of the treatment, and
decreased afterwards, more for the treatments in air than in nitrogen.”
“Korkut et al. (2008a) studied heat-treated Scots pine wood and concluded that
compression strength parallel to grain, bending strength, modulus of elasticity in bending,
janka-hardness, impact bending strength, and tension strength perpendicular to the grain

22
decreased. Similar results were presented by Korkut et al. (2008b) for Red-bud maple
(Acer trautvetteri).”
“According to Boonstra et al. (2007b) heat-treated wood can be used in construction if
the stresses that occur in construction are taken into account. These authors treated
Norway spruce construction wood and obtained a 6% reduction in bending strength and a
17% increase on MOE. They also mentioned that only a combination of several defects,
such as large knots, enclosed pith, and an abnormal slope of grain, decreases the bending
strength and MOE of treated posts … “