Wood Science and Technology
Vol. 2 (1968) p. 279-291
FOREST PRODUCTS LABORATORY (Madison, W i s . , 53705)
F o r e s t S e r v i c e , U.S. DEPARTMENT O F AGRICULTURE
Approve d T e c h n i c a l A r t i c l e
Factors Affecting Permeability and Pit Aspiration
in Coniferous Sapwood
By G. L. C OMSTOCK , Madison, Wis., and W. A. CÔTÉ JR., Syracuse, N. Y.
Summary
The influence of drying methods on the permeability of red pine and eastern hemlock
sapwood was investigated. Permeability was found to be reduced by normal drying procedures
to only a small percentage of the green permeability. The reduction was more severe a t higher
drying temperatures; less severe but still very large a t - 18° C. Pit aspiration was shown to be
responsible for the reduction. Replacing the sap with surfactant solutions and organic liquids
and evaporating them revealed that pit aspiration occurred with surfactant solutions having
surface tension values of less than 20 dynes/cm and did not occur with organic liquids having
surface tension values as high as 44 dynes/cm. It is suggested that a critical factor in pit
aspiration is the adhesion of the torus to the pit border, and the failure of the organic liquids
to cause pit aspiration is due to their inability to promote adhesion between the torus and pit
border.
Zusammenfassung
Der Einfluß verschiedener Trocknungsverfahren auf die Durchlässigkeit des Splintholzes
bei Kiefer und Eastern Hemlock wurde untersucht. Es zeigte sich, daß bei Anwendung
üblicher Trocknungsverfahren die Durchlässigkeit gegenüber jener des frischen Holzes sehr
deutlich vermindert war. Höhere Trocknungstemperaturen setzten die Durchlässigkeit
weit stärker herab; bei - 18° C war die Verminderung etwas weniger stark, jedoch noch immer
beträchtlich. Als Ursache für die Durchlassigkeitsverminderung wurde der Tüpfelverschluß
gefunden. Beim Austausch des Zellsaftes gegen oberflächenaktive Lösungen bzw. gegen
organische Flüssigkeiten und bei nachfolgender Verdampfung derselben ergab sich, daß die
oberflächenaktiven Lösungen mit Oberflächenspannungswerten von höchstens 20 dyn/cm
einen Tüpfelverschluß erzeugten, die organischen Lösungen mit Oberflächenspannungswerten
bis zu 44 dyn/cm jedoch nicht. Es wird postuliert, daß beim Tüpfelverschluß das Haften des
Torus am Tüpfelrand das Kriterium bildet; die Unfähigkeit bestimmter organischer Lösungen,
einen Tüpfelverschluß zu bilden, ist darin zu sehen, daß sie die Adhäsion zwischen dem
Torus und dem Tüpfelrand nicht zu fördern vermögen.
Introduction
The permeability of wood is a measure of the ease with which fluids flow
through it. Longitudinal permeability of coniferous woods is controlled almost
exclusively by the bordered pits. In green sapwood these pits are quite permeable
and permit easy passage of fluids and small suspended particles. During drying,
however, these pits frequently become aspirated, resulting in a marked reduction
in permeability. The research reported here is aimed a t determining quantitatively
the extent t o which permeability is reduced by drying as influenced by drying
conditions and the properties of the liquid evaporated from the wood. The influ­
ence of surface tension of the evaporating liquid is given particular attention,
because it has previously been regarded as the most important factor in pit
aspiration.
Pit Aspiration
Most important conifers, in particular those belonging to the Pinaceae family,
possess a bordered pit structure with pit membrane characterized by a centralized
280
G. L. COMSTOCK and W. A. CÔTÉ, JR.
thickened disk, the torus, and a supporting membrane, or margo, consisting of
strands of cellulose microfibrils [LIESE, 1965]. I n the green condition, most of the
sapwood pit membranes are centrally located and quite permeable. Drying the
wood generally causes the torus to be displaced and come into contact with one of
the pit borders. This phenomenon is referred to as pit aspiration and the forces
which cause it to occur were discussed in detail by HART and T HOMAS in 1967.
I n 1933, PHILLIPS made what appears to be the first comprehensive study of pit
aspiration. He found that drying sapwood caused a gradual increase in the number
of aspirated pits with loss of moisture down to the vicinity of the fiber saturation
point. A t this point, virtually all the springwood pits became aspirated; whereas
about one-third of the summerwood pits remained unaspirated. He ascribed
the greater tendency of summerwood pits to resist aspiration to the greater
rigidity of the summerwood pit membrane. LIESE and B AUCH in 1967 reported
observing the same phenomena. BRAMHALL’S 1967 permeability studies indicate
that there is little change in summerwood permeability upon drying, but springwood permeability can vary as much as 30 times, depending upon the drying
method.
G RIFFIN in 1919 and 1924, PHILLIPS in 1933, ERICKSON and C R A W F O R D
in 1959, and LIESE and B AU C H in 1967 observed t h a t pit aspiration does not
occur in coniferous sapwood when the water is replaced by alcohol prior to drying.
The prevention of aspiration is invariably ascribed to the lower surface tension of
alcohol compared to water. E RI CK S ON and C R AW F O RD observed that when sap­
wood was air dried the permeability was reduced by drying to only 1 . . . 3 percent
of its original value in the green condition; whereas drying by solvent exchange
with alcohol, acetone, or alcohol-benzene followed by evaporation of the solvent
prevented pit aspiration and maintained the permeability at its original level.
LIESE and BAUCH dried several conifers by solvent exchange, using alcohol-water
and acetone-water mixtures. They observed t h a t for solvent concentrations in
excess of 75 percent ethanol or 80 percent acetone, pit aspiration was incomplete.
These concentrations correspond to a surface tension of about 26 dynes/cm in the
pure liquids, and LIESE and B AUCH concluded that a surface tension of 26 dynes/
cm is required to cause pit aspiration in springwood.
The concept of the authors regarding pit aspiration is that three factors are
involved, any one or combination of which may control aspiration. These are:
(1) Surface tension forces tending to pull the torus into contact with the pit
border.
( 2 ) Rigidity or stiffness of the pit membrane which results in a force opposing
the surface tension forces exerted by the evaporating liquid.
(3)Adhesion of the torus to the pit border when they are brought into contact.
The first factor, surface tension forces of the evaporating liquid, is primarily
dependent on the surface tension of the liquid and the sizes and shapes of the pit
aperture, chamber, torus, and pores in the margo. I n 1967, HART and T HOMAS
discussed in detail the forces which may develop due to the presence of air-liquid
menisci in a pit. They concluded that the forces associated with a meniscus be­
tween the torus and pit border increase with loss of liquid as the torus is drawn
closer to the pit border.
Coniferous Sapwood Permeability and Pit Aspiration
281
The relative rigidity of the pit membrane compared to the surface tension
forces will determine whether the torus is displaced sufficiently from the central
position to come into contact with the pit, border. The stiffer and more rigid the
membrane, the greater will be the force required to aspirate the pit. Summerwood
pits tend to aspirate less than springwood pits, evidently because summerwood
pits are usually smaller and have a thicker, more rigid membrane. Another factor
which may affect, the rigidity of the pit membranes, when drying from non­
aqueous liquids, is the degree of swelling of the liquid. It is well known that most
strength properties of wood, including the modulus of elasticity, increase with loss
of moisture below the fiber saturation point). Strength of wood swollen with
liquids other than water would be expected to change in similar fashion with the
degree of swelling of the wood. For example, ERICKSON and REES observed in
1940 that the crushing strength of wood decreased with increasing degree of
swelling caused by the organic liquid in which the wood was soaked. It is antici­
pated, therefore, that the stiffness of the pit membrane may be increased by re­
placing water with liquids of lower swelling power.
The third factor involves adhesive forces between the torus and the pit border.
Assuming that the surface tension forces are sufficient to overcome the stiffness of
the pit membrane and thus bring the torus into contact with the pit border, the
torus will stay in this position only if there are adhesive forces between it and the
pit border. This particular aspect has apparently been ignored in the past and is
worthy of consideration. Whether an intermediate substance, such as a resin, is
involved as an adhesive or whether there is direct bonding of the surface of the
torus to the pit border, is unknown. The type of forces involved is likewise un­
known.
Permeability Measurements
The purpose of these experiments was to determine the extent to which the
permeability of coniferous sapwood is changed by various drying methods and to
determine whether the changes observed are related to aspiration of the bordered
pits. Permeability measurements prior to drying were made with water. Measure­
ments after drying were made with nitrogen gas. That comparisons of the measure­
ments obtained by these methods are valid has been demonstrated by COMSTOCK
in 1967 and 1968. He showed that the permeability values obtained with gases
and liquids are essentially identical if viscosity is included in the permeability
equation and a correction is made for slip flow of the gas. That gas permeability
increases with loss of moisture in the hygroscopic range because shrinkage of the
wood causes the intertracheid pores to become larger was also shown. This effect
was found to be very small in solvent-dried wood and somewhat larger in air-dried
wood.
Water permeability was determined using the technique described earlier
[COMSTOCK 1965]. Permeability values were calculated using DARCY’S law.
(1)
where k = permeability (Darcys)
Q = flow rate (cm3/sec)
L = specimen length (cm)
23
Wood Science a n d Technology, Vol. 2
A
= viscosity (centipoise)
= flow area (cm2)
P = pressure drop (atm)
282
G. L. COMSTOCK and W. A. CÔTÉ JR.
The superficial gas permeability, kg, was calculated according to DARCY’S law for
gases, which takes into account the compressibility of the gas.
where Pis the absolute pressure at which Q is measure and P is the mean absolute
pressure in the specimen.
Slip flow was then corrected for by use of the following expression [COMSTOCK
1967]:
(3)
where = mean free path of the gas
= radius of the intertracheid pores
The procedure used for gas permeability measurements was to measure k,
at mean pressures of one and three atmospheres and to extrapolate the plot of k,
vs. 1/P . . . 1/P equal to zero. The value of k obtained in this way is the slipcorrected permeability. The apparatus used for gas permeability measurements is
similar to that described by COMSTOCK in 1968.
Influence of Temperature and Rate of Drying on Permeability
This experiment was conducted to determine the extent to which the reduction
in permeability on drying depends on the drying conditions employed. Five
drying temperatures were used: -18°,20°, 60°, 100°, and 140° C. At the three
intermediate temperatures, two rates of drying were used. Only slow drying was
carried out at -18°C and only rapid drying at 140° C.
Drying at -18°C was accomplished by placing the specimens in a desiccator
over phosphorus pentoxide and placing the desiccator in a room controlled at
-18° C. Drying at 140° C was accomplished by putting the specimens between
two platens heated to 140° C and bringing the platens into contact with the wood.
All other drying was done in a humidity- and temperature-controlled cabinet.
Rapid drying was achieved by placing the specimens directly into the air stream
in the cabinet. Slow drying was achieved by placing the specimens in a desiccator
over P2O5 and placing the desiccator in the cabinet at the desired temperature.
Since no circulation was provided in the desiccator, drying was much slower than
in the open air stream.
Specimens were taken from three eastern hemlock trees, and two specimens
from each tree were dried at each set of conditions. The specimens used in this
experiment were approximately 1.3 cm in diameter by 2 cm along the grain.
Specimens from a given tree were all taken from the same growth rings to minimize
the influence of within-tree variation on the results.
Water permeability of the green sapwood of several specimens was determined
prior to drying so that the reduction due to drying could be estimated. The average
permeability before drying was found to be 2.78 Darcys.
The average permeability values after drying are plotted graphically as a
function of drying temperature in Fig. 1. Specimens dried at -18°C had perme-
Coniferous Sapwood Permeability and Pit Aspiration
283
ability values considerably higher than those dried at the higher temperatures,
but they were still only about 5 percent as permeable as they were before drying.
Above 27° C there is a grad­
ual trend toward decreasing
permeability with increasing
drying temperature. Rate of
drying had no effect on per­
meability. This is contrary to
the findings of ERICKSON
and CRAWFORD in 1959, which
showed that slow drying at
room temperature resulted in
higher permeability, whereas
drying at 100° C had the same
effect as drying rapidly at room
temperature.
The results of this experi­
ment indicate that pit aspira­
tion is most severe in specimens
dried at high temperatures,
but it appears to be reasonably
complete in specimens dried at
-18°C, since the permeability
of specimens dried at this tem­
perature is only about onetwentieth as large as it was be­
1. Slip-corrected nitrogen permeability of dry specimens
fore drying. It was expected Fig.
as a function of drying temperature and rate of drying.
Permeability before drying averaged 2.78 Darcys.
that pit aspiration would not
occur in specimens dried at
- 18° C, because freezing of the free water in the wood should eliminate the surface
tension forces which cause aspiration. In 1967, THOMAS observed that vacuum freeze
drying did prevent aspiration from occurring in pine, but the drying temperature
was probably lower. It is speculated that sufficient liquid phase was present at
-18°C to exert the attractive force to cause pit aspiration.
The more severe aspiration at higher temperatures is apparently due to the
greater plasticity of the wood at this temperature rather than surface tension,
since surface tension decreases with increasing temperature.
Influence of Surface Tension and Other Properties on Pit Aspiration
To isolate some of the factors involved in pit aspiration, the sap in a series
of sapwood permeability specimens was exchanged for liquids having different
surface tension and swelling properties. Aqueous surfactant solutions and organic
liquids were used to vary these properties.
Two experiments were conducted using this exchange procedure. The first
was done using eastern hemlock samples from the three trees used in the drying
condition experiment. Two specimens from each tree were assigned randomly
to each of the exchange treatments. The treatments are given in Table 1. The
23*
284
G. L. COMSTOCK and W. A. CÔTÉ, JR.
Table 1. Exchange Treatments given Samples Prior to Drying
code letter
Treatment
0.1 % fluorocarbon surfactant (cationic)
0.1 % fluorocarbon surfactant (nonionic)
0.1 % organo-silicone surfactant (nonionic)
0.1 % organo-silicone surfactant (cationic)
Ethanol
Ethanol
Methanol
Methanol
water1
acetone
acetone
pentane1
toluene1
Controls - air-dried immediately
Arrow
indicates succession of treatments on the same specimens.
surfactants used are powerful surface tension depressors, which lower the surface
tension well below the value of 26 dynes/cm suggested by LIESE and B AU CH in
1967 as a lower limit which causes aspiration. The other treatments were aimed
a t determining the effect on permeability of drying from nonaqueous liquids of
varying surface tension.
Table 2. Surface Tension of Several Exchange Solutions and the
Permeability of Eastern Hemlock Sapwood after Drying from these
Solutions1
Treatment
None (water permeability)
R 2 - air-dried
A - FC-134
B - FC-170
C - L-77
C - L-79
K 1 - Water
K 2 - Ethanol
N - Toluene
L - Pentane
Surface tension
Final3
Initial2
dynes
per cm
Permeability 4
Darcys
-
-
2.78
18.7
21.8
22.8
24.3
72.0
24.0
29.6
18.4
19.4
23.2
23.5
28.2
-
.053
.071
.114
.098
.094
.088
3.01
4.44
5.10
Measurements of permeability were made with nitrogen gas
after drying wood over P2O5. Permeability values are corrected
for gas slip.
Value obtained from tensiometer measurements on liquids
before specimens were introduced.
Value obtained from tensiometer measurements on liquids
after specimens were removed.
Average of six or more specimens.
The results of this experiment are shown in Table 2. The surface tension
values listed were determined on the solutions in which the specimens were im­
mersed using a ring tensiometer. Air drying is seen to reduce permeability to
about one-fiftieth of the permeability before drying. The effect of drying after
285
Coniferous Sapwood Permeabllity and Pit Aspiration
exchanging the sap for surfactant solutions is seen to be nearly like air drying
directly from the green condition, even though the surface tension of these solu­
tions was as low as 19.4 dynes/cm. On the other hand, drying from any of the
organic liquids maintained the high permeability of the green sapwood and may
have increased it somewhat in the case of the pentane and toluene treatments.
Treatment K1 in which the procedure was to replace sap with ethanol and then
replace the ethanol with water resulted in a permeability reduction on drying
similar to the air-dried controls.
A second experiment was conducted using a large number of liquids, mostly
organic, with a wide range of surface tension and swelling properties to see if the
important properties controlling pit aspiration could be isolated. In this experi­
ment sapwood of red pine and eastern hemlock was used. All specimens were
closely matched and two from each species were randomly assigned to each
treatment. The first step in all treatments was complete exchange of the water
for acetone, a dehydration step. The acetone was then exchanged for the liquids
shown in Table 3. Old solution was discarded and new added three times for each
exchange step to ensure essentially complete exchange. After exchange was
complete, the liquids were allowed to evaporate from the wood by simply air drying.
Table 3. Permeability of Red Pine and Eastern Hemlock Sapwood before Drying
and after Evaporation of Liquids of Varying Surface Tension and Swelling
Properties
Surface
tension
Treatment
dynes
per cm
None (water permeability)
Air dried
Pentane
Ethylether
Ethanol
Methanol
Acetone
1, 1, 1 Trichloroethane
Chloroform
Toluene
Benzene
Cellosolve
P-chlorotoluene
Dioxane
Furfural
Water
Swelling relative
to water
Eastern
Red pine1 hemlock1
%
%
of water
of water
Permeability
Red pine1
Eastern
hemlock1
Darcys
Darcys
-
-
-
5.30
5.30
-
-
-
17.8
18.4
23.3
23.5
24.8
26.5
28.4
28.8
29.0
29.6
32.1
33.8
44.4
62.5
3
3
81
94
63
6
6
5
4
92
3
10
68
100
2
4
90
97
77
11
10
8
8
98
4
7
56
100
.24
6.47
6.12
6.54
6.88
5.99
6.28
5.83
6.02
6.26
6.85
5.88
6.14
6.17
.76
.016
5.48
6.68
6.94
5.69
6.40
6.38
6.40
6.68
6.04
6.92
6.86
6.85
6.67
.024
Each value is the average for two specimens.
Surface tension values were determined on the pure liquids and on the liquids
after the specimens were removed. There was little change except for water,
which had a reduced surface tension after being exposed to the wood specimens.
The surface tension values determined on the liquids after the specimens were
removed are shown in Table 3.
286
G. L. C OMSTOCK and W. A. CÔTÉ, JR.
The swelling by the different liquids was determined experimentally using
specimens about 2 cm square by 5 mm along the grain. These were ovendried,
measured, and immersed in the desired organic liquid and the change in dimension
after 6 clays of immersion was taken as the swelling. Radial and tangential swelling
were measured on two specimens of each species for each liquid. The average of
the radial and tangential values is expressed as the percentage of the swelling in
water in Table 3. Some swelling occurred in organic liquids which were cited by
STAMM in 1964 as nonswelling, such as benzene. This could have been due to trace
amounts of water in the liquids or adsorption of atmospheric moisture during the
measurements. The values in general, however, agree well with STAMM’S values,
with the exception of dioxane, for which STAMM reports a value of 62 percent as
opposed to 10 and 7 percent found here.
Results of permeability measurements described earlier are also shown in
Table 3. It is interesting that in no case involving solvent drying is there evidence
of pit aspiration even though surface tension values as high as 44 dynes/cm were
observed. The only cases where permeability was reduced were those of drying
directly from the green condition and drying after going from sap to acetone and
back to water. The latter drying procedure gave somewhat higher permeability
than air drying, but still reduced permeability well below the values for the green
wood. Red pine was reduced to about 15 percent of its original permeability and
eastern hemlock was reduced to less than 1 percent of its original permeability.
Permeability values after drying from any of the organic liquids were similar and
slightly higher than the values measured with water before drying. The slightly
higher values after solvent drying may be due to removal of extractives or perhaps
deaspiration of some pits by the solvents. There does not appear to be any trend
in permeability with either surface tension or swelling of the liquids, so it seems
unlikely that the higher permeability is produced by evaporation of the solvent
from the wood. The increase produced by solvent drying is small compared to the
decrease caused by evaporation of water from the wood and mill not be considered
further here.
Electron Microscopic Examination of Pits in Hemlock
Specimens of eastern hemlock dried from several of the different liquids were
examined to see whether changes in the permeability were consistent with visual
changes in the bordered pits. Replicas were prepared from split radial surfaces using
the direct carbon replica technique described by CÔTÉ KORAN, and DAY in 1964.
The results of these investigations agreed with those from the permeability
studies. Pits in air-dried specimens were almost invariably tightly aspirated as
shown in Fig, 2. Tight aspiration is inferred when the imprint of the pit aperture
on the torus is clearly visible, indicating that the torus is sealed tightly against
the pit border.
Examination of samples dried from surfactant solution C revealed that the pits
are aspirated, but not tightly, since the aperture imprint was missing. Fig. 3 is
typical of the pits found in specimens examined. The less severe aspiration is
manifested also in many supporting strands in the margo being elevated above the
back border. The less severe aspiration is evidently responsible for the slightly
higher permeability compared to the air-dried controls.
Coniferous Sapwood Permeability and Pit Aspiration
287
Fig 2. Electron micrograph of a tightly aspirated pit in air-dried eastern hemlock sapwood.
Magn.: 5 800:1.
Fig. 3. Electron micrograph of a n eastern hemlock s a p w o o d pit dried after the water had been replaced
by surfactant solution C. Magn.: 5 600: 1.
Fig 4. Electron micrograph of an unaspirated eastern hemlock sapwood pit dried by acetone exchange.
Back border was torn away during replication, revealing the fibrillar structure of the margo.
Magn.: 4 200:1
Fig. 5. Electron micriograph of two unaspirated pits in eastern hemlock sapwood dried by ethanol
exchange. Torus extensions which are charateristic of hemlock are clearly visible. The dark band
around the torus is the outline of the replica of part of the pit border behind the torus.
Magn.: 3 150:1.
Coniferous Sapwood Permeability and Pit Aspiration
289
Fig. 6. Electron micrograph of an unaspirated pit in eastern hemlock sapwood dried by solvent exchange
using benzene as the evaporating liquid. The light circular area on the torus is the area of the pit
aperture behind the torus. The dark band occurred because the entire pit border behind the torus was
replicated and remained intact. Magn.: 4 700:1.
Specimens dried after solvent-exchanging water for ethanol, acetone, benzene,
and ethyl ether were examined and the invariable result was that the majority
of the pits were unaspirated, which corresponds to the permeability findings. Typi­
cal electron micrographs are shown in Figs. 4, 5, and 6. Notice first that the tex­
ture of the margo is clearly visible in these unaspirated pits and the margo is seen
to have a very high porosity. This corresponds to the 1966/67 findings of THOMAS
and NICHOLAS on solvent-dried southern pines. Although different pits often
exhibited greatly different margo densities, no consistent differences were observed
between samples dried from the different organic liquids. This also agrees with
the permeability results.
Direct carbon replicas of unaspirated pits exhihit some peculiar traits not found
in replicas of aspirated pits. For example, the dark band which appears around
the edge of the torus in Figs. 5 and 6 is due to the deposition of a layer of carbon
and some metal on the pit border behind the torus. Where the replica of this
border occurs behind the torus there is a darker appearance because two layers
of carbon must be penetrated by the electron beam, one being the replica of the
torus, the other the replica of the back border. In Fig. 6 the entire back border is
intact and the lighter area in the center of the torus corresponds to the pit aperture.
Frequently, in replicating unaspirated pits, the back border is torn away
during the process of replication because of its loose attachment to the annulus.
When this occurs, a replica similar to that shown in Fig. 4 results, showing the
290
G. L. COMSTOCK and W. A. CÔTÉ, JR.
torus, margo, and annulus with the back border missing. The margo texture is of
course clearly visible in replicas of this type and the extreme porosity of the margo
is evident.
Discussion of Results
It is clear from the electron microscopic studies that changes in permeability
during drying are directly related to the condition of the bordered pits. When
aspiration occurs, permeability is low; if aspiration is prevented, permeability is
maintained at a high level.
It is obvious from the results shown in Tables 2 and 3 that we cannot speak
glibly about the role of surface tension in pit aspiration as though it were the only
important' factor. The evidence clearly indicates that pit aspiration can occur
using aqueous surfactant solutions with surface tension values of less than
20 dynes/cm as the evaporating liquid. On the other hand, pit aspiration did not
occur when drying from organic liquids with surface tension values as high as
44 dynes/cm.
It seems that either the influence of these liquids on the rigidity of the pit
membrane or on the adhesion of the torus to the pit border may be the determining
factor. The experiments involving organic liquid displacement followed by water
displacement showed that the effect of the organic liquid was not that of any
permanent fixation of the pit membrane. LIESE and BAUCH also found this in
1967.
The influence of the various liquids on the rigidity of the pit membrane should
be in nearly direct proportion to the swelling of the wood by the liquid. If this
factor were of major importance it should have manifested itself in a reduced
permeability with liquids like cellosolve and furfural which have both a high
swelling and high surface tension. The fact that permeability remained high after
drying from those liquids suggests that swelling was not the controlling factor.
By a process of elimination, it thus appears that a key factor involved in the
mechanism of pit aspiration is the adhesion of the torus to the pit border. Even
though the surface tension is presumably sufficiently large to bring the torus into
contact with the pit border, adhesive forces between the torus and pit border
apparently do not develop when the organic liquids are evaporated from the wood.
This is analogous to the greatly reduced strength in paper when formed from
nonaqueous liquids as reported by BROUGHTON and WANG in 1955. The role of
water in the adhesion process appears to be an overriding factor which provides
the molecular forces necessary for adhesion to occur. The exact nature of the
adhesive forces is not known, but it is clear that substances soluble in organic
solvents are not acting as an adhesive, since pit aspiration occurs in solventextracted sapwood when water is evaporated. It appears, therefore, that there is
direct bonding of the surface of the torus to the pit border, presumably by some
type of Van der Waals-forces. From the chemical nature of wood and the impor­
tance of water, it would seem that these adhesive forces may be hydrogen bonding
forces, but this is speculative at this stage.
The 1967 results of LIESE and BAUCH with varying concentrations of water
in acetone and in ethanol can be explained on the basis of varying adhesion of the
torus to the pit border, depending on the concentration of water present at the
Coniferous Sapwood Permeability and Pit Aspiration
291
final water stage of pit aspiration. The exact amount of water required is uncer­
tain, because the concentration at the time of pit aspiration can be quite different,
than the initial concentration, due to different evaporation rates of the water and
the organic solvent.
References
BRAMHALL, G. : Longitudinal Permeability Within Douglas-fir (Pseudotsuga menziesii (Mirb.)
Franco) Growth Increments. MS Thesis, University of British Columbia (1967).
BROUGHTON, G., and J. P. WANG: The Mechanical Properties of Paper. Part III. TAPPI
Vol. 38 (1955) No. 7, p. 412/415.
COMSTOCK, G. L. : Longitudinal Permeability of Green Eastern Hemlock. For. Prod. J. Vol. 15
(1965) No. 10, p. 441/449.
-: Longitudinal Permeability of Wood to Gases and Nonswelling Liquids. For. Prod. J.
Vol. 17 (1967) No. 10, p. 41/46.
-: Physical and Structural Aspects of the Longitudinal Permeability of Wood. Ph. D. Thesis,
N.Y.S.U. College of Forestry, Syracuse (1968).
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(Received May 10, 1968)
W. A. CÔTÉ JR., Professor of Wood Technology, New York State University College of Forestry at Syracuse, N. Y., USA. GILBERT L. COMSTOCK, Associate Wood Scientist, U. S. Forest Service, Forest Products Laboratory, Madison, Wis., USA. PURCHASED BY THE
U. S. DEPARTMENT OF
FOREST PRODUCTS
AGRICULTURE, FOR,
LABORATORY, OFFICIAL USE.