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SHRINKAGE OF THREE TROPICAL HARDWOODS BELOW AND ABOVE
THE FIBER SATURATION POINT
Roger E. Hernández†
Professor
and
Michele Pontin
Graduate Student
Centre de Recherche sur le Bois
Département des Sciences du Bois et de la Forêt
Université Laval
Québec, Canada, G1K 7P4
(Received November 2005)
ABSTRACT
Two experimental techniques were used to perform moisture sorption tests at 25°C on samples of three
tropical hardwood species: tornillo (Cedrelinga cateniformis Ducke), pumaquiro (Aspidosperma macro-
carpon Mart.), and huayruro (Ormosia coccinea Jackson) woods. The first technique used saturated salt
solutions at a relative humidity from 0% to 90%, and the second one used the pressure membrane method
at above 96% relative humidity. These sorption tests were combined with shrinkage measurements. The
fiber saturation point (FSP), estimated by interpolation to zero volumetric shrinkage, was 28%, 22.5%,
and 21.5% for tornillo, pumaquiro, and huayruro, respectively. Results confirmed that at equilibrium
moisture content, radial, tangential, and volumetric shrinkage occur above the actual FSP. This behavior
can be explained by the effect of hysteresis at saturation on wood properties. This hysteresis indicates that
loss of bound water takes place in the presence of liquid or capillary water, which contradicts the
traditional concept of FSP. The initial equilibrium moisture content at which bound water starts to leave
cell walls varied largely among the species: 52%, 36%, and 77% for tornillo, pumaquiro, and huayruro,
respectively. The liquid water remaining in wood could be principally located in the least permeable flow
paths of these wood species.
Keywords: Equilibrium moisture content, fiber saturation point, shrinkage, Cedrelinga cateniformis,
Aspidosperma macrocarpon,Ormosia coccinea.
INTRODUCTION AND BACKGROUND
The fiber saturation point (FSP) is a very im-
portant property of wood as it governs the
changes in its properties (Stamm 1964; Siau
1984; Skaar 1988). The FSP was initially de-
fined by Tiemann (1906) as the moisture content
(MC) at which the cell walls are saturated with
bound water with no free water in the cell cavi-
ties. It is assumed that the FSP is the MC below
which the physical and mechanical properties of
wood begin to change as a function of MC
(USDA 1974; Siau 1984). Therefore, the FSP is
used in models to adjust the mechanical proper-
ties of wood as a function of its MC (Bodig and
Jayne 1982), as well as in wood shrinkage and
density adjustment models (Siau 1984; Skaar
1988).
However, several studies show that this as-
sumption is not realistic. Stevens (1963) indi-
cated that shrinkage in beech wood begins tak-
ing place above the FSP. The MC gradient effect
advanced by his study for explaining this behav-
ior appears invalid because shrinkage values
were obtained at equilibrium moisture content
(EMC). Goulet and Hernández (1991) reported a
† Member of SWST
Wood and Fiber Science, 38(3), 2006, pp. 474 – 483
© 2006 by the Society of Wood Science and Technology
large hysteresis effect on the EMC and on the
perpendicular-to-the-grain tangential tension
strength of sugar maple wood at high relative
humidities (RH). The difference for the tangen-
tial tension strength between adsorption and de-
sorption states was 20% at 26% EMC. This ef-
fect was attributed to the hysteresis at saturation
phenomenon, which affected the wood moisture
sorption above 63% RH (Hernández 1983). This
hysteresis implies that during desorption the loss
of bound water begins before the removal of all
liquid water from the wood.
Even though desorption curves had 26% EMC
as an upper limit, Goulet and Hernández (1991)
suggested that the effect of EMC on sugar maple
wood properties could be extended beyond the
FSP, estimated to be 31% MC. To this end, two
studies focusing on high humidities (above 90%
RH) were performed on sugar maple (Hernández
and Bizonˇ 1994) and yellow birch woods
(Almeida and Hernández 2006a). The results
confirmed that at the EMC, changes in shrinkage
and in transverse strength occur above the FSP
for both species. The initial EMC, at which
bound water starts to leave the wood, was 42.5%
for sugar maple and about 41% for yellow birch.
This occurs even in the presence of liquid water
within the wood structure. The results of
Hernández and Bizonˇ (1994) were taken into
account by Siau (1995) when describing the fi-
ber saturation point. However, more research on
wood species with different structures is needed
in order to corroborate these conclusions.
The purpose of this investigation was to study
the effect of EMC on shrinkage properties of
three tropical hardwoods below and above the
cell-wall saturation. Two moisture sorption tech-
niques, combined with shrinkage measurements,
were applied to large specimens at 25°C.
MATERIAL AND METHODS
The experiments were carried out on three
tropical hardwood species: tornillo (Cedrelinga
cateniformis Ducke), pumaquiro (Aspidosperma
macrocarpon Mart.), and huayruro (Ormosia
coccinea Jackson). The specimens for moisture
sorption tests had a cross-section of 20 mm (R)
by 20 mm (L) and a height of 60 mm (T). The
choice of dimensions was limited by the match-
ing techniques used, by the length of the sorp-
tion experiments and to reduce the effect of the
growth ring curvature.
One hundred twenty-two green defect-free
flatsawn pieces were carefully selected and
stored in a conditioning room maintained at
20°C and 60% RH. After reaching equilibrium
moisture content (13% for tornillo and 11% for
pumaquiro and huayruro), the pieces were cut to
obtain boards of 20 mm (R) by 60 mm (T) and
500 mm (L). The twenty best boards were se-
lected for each species on the basis of their
growth ring orientation, growth ring uniformity
as well as reduced wood density variation
among boards (by eliminating the heavier and
lighter boards). Each board was then cross-cut to
yield 20-mm-thick specimens. Thirteen adjacent
specimens were chosen from each board to in-
vestigate thirteen moisture conditions. This lon-
gitudinal matching yielded thirteen comparable
groups of twenty specimens each. An additional
group of matched specimens was used to study
the anatomical structure of these species
(Almeida and Hernández 2006b).
The average basic wood density (oven-dry
mass to green volume) was 490 kg/m
3
for tor-
nillo (coefficient of variation (CV) of 3.5%);
585 kg/m
3
for pumaquiro (CV of 2.8%), and 640
kg/m
3
for huayruro (CV of 3.5%).
Experiments
The experiments consisted of moisture sorp-
tion tests associated with shrinkage measure-
ments. Dimensions of specimens were taken as
soon as the EMC was reached. The sorption con-
ditions studied are summarized in Table 1. Prior
to the desorption tests, specimens were saturated
in three steps until their full moisture content
was reached. This was done in order to avoid
internal defects caused by a rapid moisture ad-
sorption (Naderi and Hernández 1997). The full
saturated masses were then taken to the nearest
0.001 g using a digital balance, and dimensions
in all principal directions to the nearest 0.001
mm were measured with a digital micrometer.
Hernández and Pontin—CHANGES IN SHRINKAGE OF TROPICAL HARDWOODS 475
The group to be conditioned in adsorption over
distilled water was kept at 20°C and 60% RH
prior to the test.
The sorption experiment was performed ac-
cording to Almeida and Hernández (2006a).
Briefly, this sorption required two experimental
techniques. The first technique involved satu-
rated salt solutions, and the second involved
conditioned specimens using a pressure mem-
brane procedure. The first technique was carried
out at between 0% and 90% RH, as well as over
distilled water, using sorption vats equilibrated
at 25°C. For the saturated salt solutions, the time
of conditioning varied between 105 days (tor-
nillo in desorption at 33% RH) and 249 days
(tornillo in desorption at 90% RH). For each
point of sorption, control specimens were
weighed weekly, without being removed from
the desiccator. It was assumed that the equilib-
rium moisture content (EMC) was reached when
the loss in MC was under 0.007% per day.
The pressure membrane procedure was used
to determine five additional points of desorption
between 96.431% and 99.927% RH (Table 1). A
series of studies has shown the suitability of the
technique used for this humidity range (Stone
and Scallan 1967; Griffin 1977; Fortin 1979;
Hernández and Bizonˇ1994; Almeida and
Hernández 2006a). The procedure introduces the
concept of water potential (or WP), which is
derived from classical thermodynamics and is
defined as the difference between the specific
Gibbs free energies of water in the state under
study and in a standard reference state (Siau
1995). A detailed description of the apparatus
used for the pressure membrane method is given
by Cloutier and Fortin (1991). For each point of
longitudinal desorption, twenty fully saturated
specimens were placed into the pressure extrac-
tor on a saturated cellulose acetate membrane.
Pressure was then gradually applied until the
required level was reached. Flow of water was
collected in a burette. EMC was considered as
reached when outflow became negligible (no
outflow during seven successive days). These
experiments required between eight and seventy
days of desorption, depending on the consid-
ered and the wood species tested.
As soon as each sorption test was completed,
the sample mass was measured to the nearest
0.001 g. Dimensions in all principal directions
were taken to the nearest 0.001 mm with a mi-
crometer. Differences in dimensions of speci-
mens after full saturation and as soon as the
TABLE 1. Characteristics of the moisture sorption conditions.
State of sorption
Chemical or saturated
salt solution
Nominal relative
humidity (%)
Water potential
(Jkg
−1
)
Radius of curvature of
the air-water meniscus
1
(m)
Full saturation under distilled water
Saturation H
2
O 100 0 ⬁
Equilibrium under a pressure membrane at 25°C
Desorption — 99.927 −100 1.44
Desorption — 99.782 −300 0.480
Desorption — 99.492 −700 0.206
Desorption — 98.557 −2 000 0.072
Desorption — 96.431 −5 000 0.029
Equilibration over saturated salt solutions at 25°C
Adsorption H
2
O≈100 — —
Desorption ZnSO
4
90 −14 495 —
Desorption KCl 86 −20 750 —
Desorption NaCl 76 −37 756 —
Desorption NaBr 58 −74 941 —
Desorption MgCl
2
33 −152 526 —
Desorption P
2
O
5
0 −950 346 —
1
r=−2␥cos
; where: ␥is the surface tension of water (0.072 N m
−1
at 25°C);is the contact angle between the liquid and the surface of the capillary (0°). Under
about 92% RH, equation is not applicable.
WOOD AND FIBER SCIENCE, JULY 2006, V. 38(3)476
equilibrium was reached were used to estimate
partial percent shrinkage in the tangential (
TH
)
and radial (
RH
) directions of the wood. Volu-
metric shrinkage was estimated to be the sum-
mation of these two directional shrinkages
(
TH
+
RH
−
TH
⭈
RH
). The mass of the
specimens at equilibrium and their mass mea-
sured after oven-drying were used to calculate
the EMC, expressed as a percentage of oven-dry
mass.
RESULTS AND DISCUSSION
Wood hygroscopicity
The relationship between water potential and
EMC for the three tropical hardwoods is given in
Fig. 1. Previous results obtained for sugar maple
wood (Hernández and Bizonˇ1994) are also
added for comparative purposes. This figure dis-
plays only the desorption curve obtained by us-
ing either the pressure membrane or the satu-
rated salt solution methods. The point obtained
at full saturation is also shown. An excellent
continuity is apparent between the results ob-
tained by the two sorption methods. This con-
firms the suitability of the pressure membrane
method for determining EMC in wood under
high relative humidity conditions, which is con-
sistent with several earlier reports (Stone and
Scallan 1967; Cloutier and Fortin 1991; Hernán-
dez and Bizonˇ1994; Defo et al. 1999; Almeida
and Hernández 2006a).
The hysteresis at saturation has been de-
scribed by Goulet and Hernández (1991) as the
difference between the equilibrium obtained in
desorption when starting from the FSP and that
reached in desorption when starting from wood
having liquid water. Some researchers have in-
dicated that high EMCs in desorption are ob-
tained using never-dried specimens (Higgins
1957; Spalt 1958). Skaar (1988) ascribed such
behavior to an initial irreversible loss in hygro-
scopicity after the initial drying of green or wa-
ter-soaked wood. However, many studies have
shown that this effect is apparent during subse-
quent desorptions (Fortin 1979; Hart 1984; Gou-
let and Hernández 1991; Cloutier and Fortin
1991; Hernández and Bizonˇ1994; Almeida and
Hernández 2006a). Experimental data for sec-
ond desorption above 76% RH show an equilib-
rium higher than the one expected when starting
from the FSP of each species (about 28% MC
for tornillo, 22.5% for pumaquiro, and 21.5% for
huayruro). This confirms that the hysteresis at
saturation is not limited to the first drying, but
rather to any desorption made in the presence of
liquid water.
The importance of this hysteresis will vary
according to the initial MC of the wood. In this
study, desorption was carried out beginning
from the fully saturated state, and the curve ob-
tained corresponds to the maximum EMC ex-
pected for each RH condition. The term bound-
ary desorption curve is therefore used to de-
scribe this feature. Thus, any desorption curve
obtained from a lower initial MC would be lo-
cated below this boundary desorption curve
(Defo et al. 1999).
The region between 96% and 100% RH (WP
higher than −10
4
Jkg
−1
) is greatly expanded
FIG. 1. Equilibrium moisture content as a function of the
water potential for tornillo, pumaquiro, and huayruro hard-
woods at 25°C and for sugar maple at 21°C (maple from
Hernández and Bizonˇ1994). Gray symbols represent full
saturation under distilled water; black symbols are the val-
ues obtained by the pressure membrane method and white
symbols are the values obtained under the saturated salt
solution method (standard errors do not exceed the symbol
size shown).
Hernández and Pontin—CHANGES IN SHRINKAGE OF TROPICAL HARDWOODS 477
when using the water potential concept to rep-
resent sorption isotherms. This region is very
important when studying the wood-water inter-
actions given that it is mainly controlled by the
capillary forces and consequently by the micro-
structure of wood species. Since wood is a po-
rous material, an important effect to be consid-
ered in the interpretation of Fig. 1 is the “ink-
bottle effect.”The capillary system of wood
consists of cavities interconnected by narrow
channels. The variation in dimensions between
the different types of cavities connected in series
suggests that desorption tends to be governed by
a lower water potential, which is determined by
the narrower sections of the pores. In contrast,
adsorption tends to be governed by a higher wa-
ter potential that depends on the larger sections
of the pores. Thus, the desorption isotherm will
depend on the size of channels connecting the
lumina, whereas the adsorption isotherm will de-
pend on the size of these lumina (Fortin 1979).
The boundary desorption curves presented in
Fig. 1 show that a great variation exists among
the three species of wood. This variation is more
pronounced as WP increases. Pumaquiro wood
exhibited a more distinct drainage behavior than
the other hardwoods, especially at high values of
WP. Figure 1 shows that the loss of EMC be-
tween full saturation and −100 Jkg
−1
WP was
48% for pumaquiro, 11% for tornillo, and 7%
for huayruro. In terms of mass units within a
specimen, this corresponds to a loss of 7.43 g,
1.60 g, and 1.16 g liquid water for pumaquiro,
tornillo, and huayruro, respectively. This water
would have occupied a volume within the wood
specimen of about 31% for pumaquiro, 7% for
tornillo, and 5% for huayruro (mean volume of
the specimens at 12% EMC was 24 cm
3
). Ac-
cording to previous studies, this water should
have been removed from the larger capillaries,
especially the vessel lumina (Hernández and Bi-
zonˇ1994; Almeida and Hernández 2006a).
Quantitative anatomical measurements per-
formed from matched samples showed that the
proportion of vessel lumina within the wood vol-
ume was 28% for pumaquiro, 8% for tornillo,
and 5% for huayruro (Almeida and Hernández
2006b). Table 1 shows that at −100 Jkg
−1
WP,
capillaries with radius larger than 1.44 m are
already empty. The tangential radius of vessel
elements was 45 m for pumaquiro, 128 m for
tornillo, and 86 m for huayruro (Almeida and
Hernández 2006b). Therefore, it is apparent that
the majority of vessel elements were already
empty at this stage of desorption. However, the
presence of some vessels that terminate within
the samples (Petty 1978), the cut cells at the
transverse faces, as well as deposits of extrac-
tives in the vessels can also affect the flow in
wood. For instance, it is known that huayruro
wood possesses some deposits of gum within its
vessels (JUNAC 1981).
The boundary desorption curve of pumaquiro
shows a constant decreasing EMC rate between
−100 Jkg
−1
and −2000 Jkg
−1
WP, with a plateau
occurring between −2000 Jkg
−1
and −5000
Jkg
−1
WP (Fig. 1). This plateau indicates that
pore openings controlling the retention and flow
of water are scarce within this WP range. Below
this WP region, the water remaining in wood
would be localized in capillaries having a radius
equal to or smaller than 0.029 m (Table 1). As
discussed later, this would correspond to the
transition between the drainage of the fiber cavi-
ties and that of the ray parenchyma as noted by
Hart (1984).
The boundary desorption curves presented in
Fig. 1 change abruptly at about −300 Jkg
−1
WP
for tornillo and at −700 Jkg
−1
WP for huayruro.
Below these WP values, curves show a quite
constant EMC decreasing rate up to −5000 Jkg
−1
WP, without presence of a plateau as occurring
for pumaquiro wood (Fig. 1). A more uniform
distribution of pore openings in wood may ex-
plain the absence of a plateau for these species.
Another possibility is that the intervals of WP
used were too large for detecting it. For soft-
wood species, Fortin (1979) and Tremblay et al.
(1996) also observed a drainage curve without
an intermediate plateau. Such results confirm
that at high humidities, the EMC-WP relation-
ship is strongly dependent on species. For tor-
nillo and huayruro, it should be more difficult to
determine the location of liquid water within the
wood elements at these WP levels. For instance,
these species have an important proportion of
WOOD AND FIBER SCIENCE, JULY 2006, V. 38(3)478
axial parenchyma (12% for tornillo and 34% for
huayruro; Almeida and Hernández 2006b). The
drainage of the liquid water could even occur
simultaneously among different elements of the
wood. In this case, the range of dimensions of
the openings connecting the different wood ele-
ments could be overlapped.
Figure 1 also shows that a high proportion of
drainage occurs at lower values of WP for tor-
nillo and huayruro (curves shifted to the left) as
compared to drainage for pumaquiro and sugar
maple woods. The pore openings connecting the
lumina at these levels of WP are smaller for the
later species than for pumaquiro and sugar
maple. The difference among the boundary de-
sorption curves could also indicate the facility of
a given wood species to reach full impregnation.
In fact, tornillo and huayruro were least perme-
able and hence more difficult to impregnate up
to full saturation. These species exhibited a very
low loss of EMC at higher ranges of relative
humidity (Fig. 1). In contrast, pumaquiro and
sugar maple are more permeable woods and
were saturated up to full moisture very easily.
Therefore, these species showed a higher loss of
EMC even at the beginning of the desorption
(Fig. 1).
The boundary desorption curves of the three
species nearly join below 58% RH (Fig. 1). A
similar result was observed between yellow
birch and sugar maple woods (Almeida and
Hernández 2006a). This was expected given
that desorption of liquid water at this level of
RH is almost achieved. For sugar maple,
Hernández (1983) reported that loss of liquid
water was accomplished at about 63% RH.
Thus, the bound water desorption was quite
similar for the three hardwoods studied (values
at 33% and 58% RH).
For the three hardwood species, it was not
possible to determine the FSP by the adsorption
over distilled water method (Table 1). Equilib-
rium was not reached because condensation of
water occurred simultaneously with bound water
adsorption (Hernández 2006). For this reason,
the FSP was determined by the volumetric
shrinkage intersection point method. In this
method, the FSP is defined as the MC at which
the extended straight linear portion of the shrink-
age-MC curve intersects the line of zero shrink-
age (Stamm 1964; Skaar 1988; Siau 1995). For
this estimation, only volumetric shrinkage val-
ues obtained between 33% and 76% RH were
used. This was done because of the non-linearity
of the shrinkage-MC curve at low moisture con-
tents (Stamm 1964) and the effect of the hyster-
esis at saturation on shrinkage at high moisture
contents (Hernández and Bizonˇ1994). The esti-
mated FSP were 28%, 22.5% and 21.5%, for
tornillo, pumaquiro and huayruro, respectively.
The FSP values estimated by extrapolation to
zero volumetric shrinkage (actual FSP) are used
in the discussion that follows.
Wood shrinkage—EMC relationships
The relationships between the EMC and the
radial, tangential, and volumetric shrinkages for
the three hardwoods are shown in Fig. 2. Free-
hand curves were drawn taking into account the
sorption state, in such a way that the actual FSP
is not linked to the others. The standard errors of
the shrinkage values do not exceed the symbol
size shown. As expected, the three tropical hard-
woods showed a low degree of shrinkage when
compared to temperate woods having a similar
density. This behavior can be explained by the
presence of extractives in the woods studied
(Choong and Achmadi 1991).
For the three hardwoods, Fig. 2 shows that
radial, tangential, and volumetric shrinkages
started before the FSP was reached. In order to
determine the EMC at which shrinkage starts to
take place, the differences between the dimen-
sions at full saturation and those at each EMC
studied were calculated. This was made for the
tangential and radial dimensions of specimens.
Since the dimensions at full saturation and at
each EMC studied were taken on the same speci-
men, a paired t-test (one-tail) was performed
(SAS Institute 2002–2003). This test deter-
mined if changes in dimensions between these
two MCs are statistically larger than zero at the
0.01 probability level. The results of the paired
t-tests indicating the EMC at which shrinkage
Hernández and Pontin—CHANGES IN SHRINKAGE OF TROPICAL HARDWOODS 479
begins to take place in both principal directions
of wood are shown in Table 2.
The EMC at which shrinkage was larger than
zero varied among wood species (Table 2). This
was expected since water drainage at high mois-
ture contents is highly dependent on wood struc-
ture. Table 2 shows that in tornillo wood tan-
gential and radial shrinkages started at 51.5%
EMC, which corresponds to −2000 Jkg
−1
WP.
Since the actual FSP for tornillo was estimated
to be 28%, shrinkage started before the FSP was
reached. It is clear that, even at equilibrium, loss
of bound water within the cell walls provokes
shrinkage of wood before all liquid water has
evaporated. This implies that about 23.5% MC
in liquid form is still retained in the wood when
shrinkage of tornillo starts taking place at −2000
Jkg
−1
WP (51.5%–28.0% EMC). In terms of
mass units, this corresponds to 3.06 g of liquid
water that would have occupied a volume of
about 13% within a wood specimen (mean vol-
ume of the specimens at 12% EMC was 24 cm
3
).
This remaining liquid water could be entrapped
in the ray elements, given that these wood ele-
ments are considered as the least permeable flow
path in hardwoods (Siau 1995). Wheeler (1982)
noted that the parenchyma-parenchyma pit
membranes are thicker than both the intervessel
pit membranes and the fiber-fiber pit mem-
branes, and consequently are less efficient path-
ways for liquid flow. This entrapped water could
in fact fill the 14% volume of ray tissue mea-
sured on matched specimens of tornillo wood by
Almeida and Hernández (2006b). The entrap-
ment of liquid water in ray tissue reported by
Hart (1984) for hickory and oak adds further
support to this hypothesis. Menon et al. (1987)
studied the water location during drying of
Douglas-fir and western red cedar woods using
proton magnetic resonance techniques. Under
unequilibrated conditions, they noted that liquid
water remained in the ray and tracheid compart-
ments when bound water begins to leave the cell
walls. The liquid water was completely lost
when the MC reached values as low as 9%.
For the other species, shrinkage was statisti-
cally different from zero at 36.3% EMC for pu-
maquiro and at 77.3% EMC for huayruro (Table
2). These EMCs are also larger than the actual
FSP of these species (22.5% EMC for puma-
quiro and 21.5% for huayruro). About 13.8%
MC in liquid form is still retained in pumaquiro
wood when shrinkage starts taking place at
−2000 Jkg
−1
WP (36.3%–22.5% EMC). For
huayruro, 55.8% MC in liquid form is still re-
tained in the wood when shrinkage starts taking
FIG. 2. Shrinkage of tornillo, pumaquiro, and huayruro
hardwoods as a function of the EMC at 25°C. The symbol
•represents the FSP estimated by the volumetric shrinkage
intersection method (standard errors do not exceed the sym-
bol size shown).
WOOD AND FIBER SCIENCE, JULY 2006, V. 38(3)480
place at −700 Jkg
−1
WP (77.3%–21.5% EMC).
In terms of mass units, this corresponds to
2.07 g for pumaquiro and to 9.11 g for huayruro
of liquid water that would have occupied a vol-
ume of about 9% (pumaquiro) and 38% (huay-
ruro) within a specimen (mean volume of the
specimens at 12% EMC was 24 cm
3
). The vol-
ume of ray tissue in pumaquiro wood was 9%
(Almeida and Hernández 2006b). As was the
case for tornillo wood, the liquid water in pu-
maquiro wood appears to be trapped principally
in the ray tissue.
The case of huayruro was different: even
though having a high density, this species ex-
hibits a high proportion of aliform and confluent
parenchyma (34%) and ray parenchyma (20%;
Almeida and Hernández 2006b). Shrinkage of
this wood started earlier (77.3% EMC) at a high
WP (−700 Jkg
−1
). The capillary tension forces at
this level of WP could be too low to cause a loss
of bound water in wood. However, these could
be enough to provoke some localized collapse
on the thin cell walls of axial parenchyma (Hart
1984). Localized collapse in parenchyma cells
above FSP has been reported previously (Hart
1984; Demanet et Morlier 2000; Wu et al. 2005).
Furthermore, the action of the capillary tension
could be combined with hydrostatic tension
forces occurring at the interface between the
confluent parenchyma and fibers of huayruro
(Bariska 1992; Demanet and Morlier 2000). In
fact, this species exhibited fibers with very thick
cell walls and very small lumina. This should
contribute to increase the gradient of drying
stresses between these two wood elements (as
noted between earlywood and latewood in tem-
perate woods). However, judging by the total
shrinkage measured for this wood (11.3%), the
magnitude of this localized collapse can be con-
sidered as negligible. Thus, the occurrence of a
localized collapse from cavity water loss for
some heterogeneous hardwoods can not be dis-
carded. However, the hypothesis of a cell-wall
shrinkage from bound water loss taking place in
presence of liquid water appears as more plau-
sible for homogeneous hardwoods. It is hence
postulated that these two mechanisms occurred
simultaneously in huayruro wood. Additional
work is, therefore, needed in order to better un-
derstand the mechanisms releasing the begin-
ning of shrinkage at equilibrium.
General discussion
The results show that, for the three tropical
woods, shrinkage started before the FSP was
reached as a result of the loss of bound water in
the presence of liquid water (Fig. 2). Such be-
havior in shrinkage has been also reported for
tangential compression strength in previous
studies. The changes in physical properties
started at about 42.5% EMC for sugar maple
(Hernández and Bizonˇ1994) and at 41% EMC
for yellow birch (Almeida and Hernández
2006a). In the present study, wood species pre-
senting more variable anatomical structures
TABLE 2. Statistical analysis of the difference in dimensions of specimens after full moisture saturation and at a given EMC
for the three wood species.
Wood
species
EMC
1
(%)
Tangential direction
2
Radial direction
3
T
FS
(mm)
T
EMC
(mm)
Diff T
(mm) t Value
R
FS
(mm)
R
EMC
(mm)
Diff R
(mm) t Value
Tornillo 52 62.407 62.344 0.063 4.56** 20.709 20.691 0.019 2.01 ns
(0.014)
4
(0.009)
Pumaquiro 36 62.502 62.463 0.039 7.80** 20.603 20.552 0.051 6.34**
(0.005) (0.008)
Huayruro 77 62.341 62.320 0.021 6.04** 20.667 20.636 0.032 4.01**
(0.004) (0.008)
1
Equilibrium moisture content where Diff T (difference between T
FS
and T
EMC
dimensions) and Diff R (difference between R
FS
and R
EMC
dimensions) were
statistically higher than zero (**).
2
Average of 20 measurements: T
FS
⳱tangential dimension at full saturation, T
EMC
⳱tangential dimension at EMC.
3
Average of 20 measurements: R
FS
⳱radial dimension at full saturation, R
EMC
⳱radial dimension at EMC.
4
Values between parentheses represent the standard error.
Hernández and Pontin—CHANGES IN SHRINKAGE OF TROPICAL HARDWOODS 481
showed that changes in wood properties began at
different EMC values: at nearly 52% EMC for
tornillo, 36% EMC for pumaquiro, and 77%
EMC for huayruro. These values do not corre-
spond to any abrupt transition from bound water
to liquid water as currently stated. It is hence-
forth established that during boundary desorp-
tion a region exists where the loss of bound wa-
ter takes place in the presence of liquid water.
The FSP or cell-wall saturation can not be di-
rectly determined from desorption experiments.
The range of EMC of this region depends on the
size distribution of wood capillaries, which will
vary among wood species. The occurrence of
localized collapse above FSP for highly hetero-
geneous woods must also be considered in this
kind of studies.
CONCLUSIONS AND RECOMMENDATIONS
Moisture adsorption and desorption experi-
ments were performed in specimens of tornillo,
pumaquiro, and huayruro woods at 25°C. Spe-
cial attention was paid to the fiber saturation
zone. Once equilibrium was reached, shrinkage
measurements were undertaken. The results of
these tests lead to the following main conclu-
sions:
1. At equilibrium, the radial, tangential shrink-
age, and consequently the volumetric shrink-
age begin well above the actual fiber satura-
tion point for the three species studied. The
EMC at which shrinkage begins to take place
varied among species.
2. In the desorption phase, loss of bound water
begins at nearly 52% EMC for tornillo, 36%
EMC for pumaquiro, and 77% EMC for
huayruro in the presence of liquid water. The
volume of liquid water that remains in wood
at the beginning of shrinkage was estimated
to be 24% EMC for tornillo, 14% EMC for
pumaquiro, and 56% EMC for huayruro. This
volume of entrapped water depended on the
micro-structure of each wood species.
3. The cell-wall saturation or FSP can be deter-
mined sufficiently by interpolation to zero
shrinkage from the linear portion of the volu-
metric shrinkage-EMC relationships.
ACKNOWLEDGMENTS
The authors are grateful to Professor Yves
Fortin and Ph.D. Student Giana Almeida for
valuable suggestions and help. This research
was supported by the Natural Sciences and En-
gineering Research Council of Canada and by
the International Tropical Timber Organization.
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Hernández and Pontin—CHANGES IN SHRINKAGE OF TROPICAL HARDWOODS 483
