|Table of Contents|
How it works
The 5-C's of chemical seasoning plus antishrink effect
General statement about the chemical seasoning of Douglas fir
Back Cover 1
Back Cover 2
A 1PI I CS 11411 CEIHIFICAL
SIASCNIN Of UOCIJLAS IIHP
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST PRODUCTS LABORATORY
In Cooperation with the University of Wisconsin
A PRIIJER ON T0 CHEMICAL SEASONING OF DOUGIAS FIR,;
STATUS NOVEMBER 1938
Forest Products Laboratory Research Supplemented by a Field
Study Made in Cooperation With the West Coast Lumbermen's
W. Karl Loughborough
The growth of a tree or an idea cannot be taken for granted.
Lacking a suitable environment neither will grow enough to be useful to
society. If by accident or otherwise a dormant tree is given enough light,
its rate of growth is apt to take a spurt; a dormant idea under the stimu-
lus of new concepts and information may take on new life. Then the Forest
Products Laboratory discovered that wood, after chemical treatment, could
be made to dry from the inside out an olcl dormant idea was released from
inhibiting influences. Old drying processes which involved the use of
chemicals were immediately dressed in new clothes. Past industrial ex-
perience with chemicals as an aid to seasoning could no longer be used to
evaluate the new process which offers possibilities of solving the age-old
problem of seasoning wood without checking.
To solve this-industrial problem, the West Coast Lumbermen s
Association cooperated with the Forest Products Laboratory in a. prelimi-
nary study of the chemical seasoning of Douglas fir.
This article may be considered the first in a series of progress
reports to be issued by the West Coast Lumbermen's Association on the
chemical seasoning of their products. The primary purpose of this article
is to set forth the principles of chemical seasoning in as much detail as
consistent with our present understanding of the subject.
Acknowledgment is made of spontaneous inspiring cooperation of the West
Coast Lumbermen's Association, of its officers, members of its Grading
and Trade Promotion Committees, and to the Weyerhaeuser Timber Company
at Longview, Wash., who were hosts to the field study.
How It Works
In the following paragraphs the movement of water in wood, as it
dries, will be explained so that it will be easier to understand the es-
sential differences between ordinary methods of drying and chemical
Moisture moves from the center toward the surface of a piece of
wood when the water in the central portion has a higher vapor pressure
(analogous to steam pressure) than the water in the outer portions of the
piece. In short, moisture moves from a zone of high vapor pressure to a
zone of low vapor pressure just as steam would rush from a boiler having
a high steam pressure into a boiler having a low steam pressure.
In drying wood by the common methods of air drying or kiln dry-
ing the required difference in vapor pressure is obtained by drying the
surface fibers first thus setting up a moisture gradient.
In the process of attaining a comparatively low moisture content
the vapor pressure of the surface fibers is diminished to a value which
almost corresponds to the temperature and humidity of the surrounding air.
In drying by conventional means the surface fibers uist always be drier
than any other part of the stick. Because of the upset in moisture, re-
fractory items of wood are apt to surface check in drying. The steeper
the moisture gradient and the greater the tendency of the wood to shrink
with a loss in moisture the greater the checking hazard becomes.
Figure 1 shows the moisture content of natural wood in equi-
librium with various temperatures and relative humidities and correspond-
ing vapor pressures. With the aid of this family of curves and suitable
moisture distribution data one can construct a curve that shows the pro-
gressive diminution of vapor pressure from the center to the surface of a
piece of wood while drying. Repeating for sake of emphasis, when wood is
air dried or kiln dried the difference in vapor pressure, the driving
force of moisture movement, is produced by a moisture content gradient.
This is not true of chemically treated wood.
In Chemical Seasoning Vapor Pressure in Wood
Depends on Absorbed Chemical
The relationships shown in figure 1 do not hold when the mois-
ture in the cell walls (hygroscopic moisture) acts as a solvent for a
chemical. It is a well known fact that at a given temperature the nor-
mal vapor pressure of water is lowered when chemicals are dissolved in
it. The degree to which the vapor pressure is reduced depends on the
characteristics and the amount of the dissolved chemical. If then the
hygroscopic moisture is saturated with a given chemical the wood will not
tend to lose moisture unless the vapor pressure of the air is lower than
thevapor pressure in equilibrium with the saturated solution.
:Relatie va-pr pressure of~
Satiated aqueous :tha air over the solution
solution of -- t@ OF
--- -- --- -- --- -- --- -- --------------------~~c--
valcium~ chloride.... 0-32
Magesium chllride..: 32
Calcium nitate....: -59
Amonium nitrite....: .68 ;
Sodium. nitrate...... .76
Sodium chloride.....: .779
Uirea .. ... .. ........: .90
Ammnium sulfates....: .91
ata from an unublished reort "~he Effectl:'
of Solutions of Variougs Chemical.-- and
M~ixtres of Chemic~ls on Rlative lli.-Lmidity
Equilibrium jIoisture Cotent of Wood a&d
on Shrinkage" b,,- E C. Peck.
lip1cn thin wood that has been soakeO- in a chemical solution~~~:~
successivel:T brought to an equilibrium YcsreIht vt aiu eaieh
mriditles it will attain ealiilibrium mosture vaues tat ae higbnr t)l
correpodiing values for normal -.7ood. Fi,, r2 shows tho equilibriuma
moisture contnt of natural wood and thin specimens of vcood treated ii~j
saturatca solution of sdium echlori !P.: ;
in~ese clires contain tho fg*,.rt basic pri~nciples of ceia
seasor-in ,, wn~iich may be dscried a th ,aer rtenion rflunae
They showou that when chemicall-j-traed roogd is usqunl ridi
given relative humlidity the: zones vfhich ctaly onan hechmia
have, a higer moisture content tha the adjaen ntete zn. h
Moisture will condense on tetd7o hna ie
,ure the vapor pressure of the :air ecesth ao resr -
containred chemical soalutin B a o lusrton ale1s
relative vapor )rssurn of the air atb0F a hc wocn
enug ch( micl--l to saturate its byroscpic moisture, starts to
lose moistue. For exampe, aftor soa~ii a thin piece of greer
ar saturted solutionr of sodiur chillorideo for a day~ Or So at room;11
ture it canot (try i a temperature Of 69f0 F. an a rlative hle)
79 pErcentG because the vapr pressure of the soluttin cntained~
wood is in equilbrium rith the vapor pressure of the air.
R-1 27 F:
moisureretention property of the chemical. tends to maintain the surface
ft a hi r tha nora lmoisture content and accordingly tends to
the dryiig stresses. here the relative humiditY during a subse-
qun ryine, process is in equtilibriu~m with the relative vapor pressure of
treating solution the surface fibers of the wood will not dry or tend
ik but the untreated portions of the wood dry at a normal rate.
Thu b,,rdinary methods orQod dries from the outsiide in, while chemicall-
treated wood dries from the inside out.
sure Gradient in hemically-Treated Wood
Yrow a moisture gradient standpoint the movement of water in
chemically-treated wood seems to be an "uphill" process. This is not the
case, however, as the controlling vapor pressure gradient is quite normal
due toa chemical gradient extending from the surface into the vood.
The moisture distribution from the center of the specimen to the
zone in which the concentration of chemical is becoming appreciable is
quite normal. Projection of a moisture distribution curve to the surface
gives a moisture content value at the surface that would have beon obtained
by dring untreated wood. in the same temperature and relative humidity.
See figure 3, which illustrates the moisture and salt distribution in a
treated 6 by 12 D 1ouglas fir timber after a 5-day drying period *n 160O, F.
and 72 percent relative humidity. In this dryingw condaiton normal wood
rilattain an equilibrium moisture content of about 10 percent. The fact
tht the inner part of the chemically-treated rood has a moisture gradient
identical to that which develops in untreated wood in the same interval of
time indicates that the driving forces in the two cases must be equal.
Hence, provided the drying conditions aro. the same, treated and untreated
timbers of the same size dry at the same rate. Xecping the moisture con-
tent of thqe treated zones above normal values near the surface dous not
interfere with normal .dring as the controlling vn or pressure grAdient
is normal. But by reason of the higher moisture content of its surface
fiber the treated wood can be ra-pidly and safely dried in relative hu-
midities that cause untreated wood to chOk. For example, a large. un-
treated fLouglas fir timber will surface check when dried by relative
humidities as high as 90 percent; whereas if the s,-jne timber were treated
with sodium chloride the surface fibers would remain damp and unshrunken
in a relative humidity of 15 percent.
By way of emphasis it may b3 roll to repeat tnat the moistuTre-
retcntion principle of chemical seasoning operates most effectively when
the vapor pressure of the drying atmosphere is in equilibrium with the
vao pressure of tho tratiiW, solution. By following this technic the
untrted wood can be dried to a moisture content thaat is in equilibrium
rit th dringcodition while the surface fibers rem-ain in their green
dimnsin. 11hs racion of chemaically-trea~ted wood profoundl.y modifies
In nomal drying the surf ace fibers of a timber rr tece
the Mot and as a consequence chocks are initiated on the urfce
In drying chemioally-treatel. woo4 in proper; controlle
relative humidities the fibers which are stretce th mos ae lctdi
the untreated wood Just beneath the treated zones. Beingsrthd hs
fibers receive a tension set proportionate to the drir stress s ha s
the innate shrinkaege of the untreated fibers has been reduced bythi
being- stretched beyond their elastic limit. It rill be notedfrmige3
that the -point of departure from the normal moisture ditiuion cors
ponds to thie depth io which the salt has penetrated and also the zone
having the maximum tension set. The zone of maximum tension set is very
significant. Mhen the relative humidities are reduced enough to peritth
chemically-treated zone to dry, surface checking becomes possible. Howevr,
if the chemiCally-treated timber has been sufficiently dried in properly
regulated relative humidities the checks wll'l1 not penetrate Into side-cu
timbers beyond the zone of maximum tension set, that is, to a depth ofabu
The Chemical Diffuses Into the Wood
Because a piece of cloth or a sponge readily absorbs waer or an
aqueous solution, naturally one suspects that a piece of dry wood would
readily soak up or absorb a solution when submerged in it. Exerienceha
shown, however, that water and chemical solutions soak into wood very
slowly. For example, 4 -inch Douglas fir planks 7jeighed less than 50 pud
per cubic foot after soaking in water for 12 years whereas if copetel
saturated with water they would have weighed about 71 pounds per cuicfot
Fortunately for chemical seasoning,, the rate at which tetet
ing chemical moves into wood does not, depend primarily pon the forces of
capillarity. In fact the quantity of solution which is drawn into wvoo b
them is negligible. The force wThich cause-.s a chemical to penetrate int
the wood i ; mainly diffusion pressure, a force that is akin to osmotic
pressure. For aqueous solutions that are in equilibrium with 75 percet
relative humidity the osmotic -pressure is about 400 atmospheres (6,000
pounds per square inch). With forces of this magnitude at work duringth
process of cftemically treating w7ood it is evident that the rate of pene-
tration obtained by diffusion would not be stimulated materially b-y or-
dinary pressure treatments.
The chemical diffuses from the surrounding solution into the
'water in the w~ood. If the moisture content of the wrood is not high enoug
to provide a continuous film of water into which the chemical can diffuse,
the action is greatly impeded. Consequently, the a mout, rate, and depth
of penetration of chemical diffusion are directly associated with the
moisture content of the wood. The sapwood of a green Doi-las fir timber
has a hilgh enough moisture content to permit a chemical to diffuse into
it rapidly. The rate at which a chemical can diffuse into the hearwo
of thoroughly air-dried Douglas fir is no more rapiOd than the woodca
soak- p the solution which as previously shown is a very slow process.
Zven when gre,n, Douglas fir heartwood has but little more moisture than
is necessary to saturate the cell walls. Consequently fibers do not have
a I '7 -;
to ry er muh bfoe teylose heir fre water, leaving all the re-
mainng oisure n te cll alls.
Thus while Douglas fir is soaking in a chemical solution, the
checal diffuses into the water in the wood and the water just beneath
t created zone moves outward in response to the, vapor-pressure gradient
a is eventually lost to the solution. Owing to its original low mois-
ture content, the heartwood of Douglas fir, when steeped in a hot chemi-
cal solution having a comparatively low vapor pressure, soon dries
ugh to reduce materially the rate at which the chemical will diffuse
it. For this reason it is impossible, in a reasonable length of
time, to get a deep penetration of a chemical into the heartwood of a
Douglas fir timber. The green sapwood of all species on the other hand
is easily penetrated and deep penetration can be effected even in green
7 by 9 white oak ties by means of diffusion.
Douglas fir timbers dry in a chemical solution at about the same
rate as matched untreated timbers do when exposed to the attained bath
temperature and a relative humidity that is in equilibrium with the vapor
pressure of the treating solution. The bath temperature and the relative
vapor pressure of the treating solution need to be considered carefully in
7rpg out appropriate treating technic. If the items being treated can
be dried rapidly without end or surface checking, the temperature and the
concentration of the bath may be high. Timbers like boxed-heart 12 by
12's, however, cannot be treated so drastically.
Theoretically the rate at which a chemical will diffuse into
wood containing a given amount of free water is proportional to the abso-
lute temperature of the bath. Because the inherent properties of Douglas
fir prohibit bath temperatures much in excess of 1600 F. the range of
temperature encountered in chemical seasoning is comparatively small.
Within the temperature range of SO0 to 160 F. the osmotic pressure does
not vary much more than 14 percent. Hence, for practical purposes the
rate at which a chemical will diffuse into Douglas fir may be considered
a constant throughout the temperature range. With the exception of in-
c sing the drying rate during the soaking Process, elevated bath tem-
patures are not particularly useful and their use should be avoided in
t ting items that check easily.
A rbed Chemicals Affect
the Dimension of Wood
The foregoing presents a rough concept of the mechanism of im-
p ting wood by the methods used in chemical seasoning. But the story
wo not be complete without a discussion of the antishrink effects of
tecemical which, during the process of treating and subsequent drying,
fuses into the hygroscopic moisture (moisture in the cell wall, the
loss of which s shrinkage). In choosing a chemical seasoning agent
its antishrink as well as its water-retention properties should be
considered. Various chemical solutions will subsequently be compared wil
respect to these properties. At this point the discussion will be limits
to an explanation of how the dimension of wood is affected at various
stage of the chemical seasoning process.
In the process of absorbing a chemical, wood undergoes certain
physical changes. Within the penetrated zone, the chemical is present ij
the cell cavities and also in the fine wood structure in the cell walls.
In the outer fibers the hygroscopic moisture in the cell walls and the fi
water in the cell cavities attain a chemical concentration that is practi
cally equal to that of the treating solution. In diffusing into the hyg:
scopic moisture the chemical causes the cell walls to expand more or lesi
beyond their green dimension. The chemical concentration decreases from
the surface of the timber to the other side of the treated zone which col
tains practically no chemical. See figure 3. As previously explained, i
this region the chemical concentration gradient creates a vapor pressure
gradient and also causes different degrees of swelling.
From the standpoint of antishrink the absorbed chemical plays
its most significant role when the treated timbers are subsequently driet
When the vapor pressure of the surrounding air (relative humidity) is lo%
than the vapor pressure of the chemical solution in the wood, the treated
fibers gradually lose their moisture and the dry chemical is precipitate(
in both the cell walls and cell cavities. The .chemical that is deposited
in the cell cavity of course has no effect on the ultimate shrinkage of
the wood. The chemical that is deposited in the cell walls, however, aci
as chinks between the fibrils, thus reducing the shrink of treated
in proportion to the volume of the chemical in the fine wood structure.
The anti rink effect is modified by the concentration of the
treating solution.- In one experiment thin sections of wood were treated
in different concentrations of sodium chloride. The shrinkage of the
specimens treated with 36, 25, and 15 grams of sodium chloride per 100
grams of water was almost identical for each. Figure 4 shows that the
shrinkage of the specimen treated with 5 grams of salt per 100 grams of
water was slightly greater, and the shrinkage of the specimen treated wri
2 grams of salt per 100 grams of water was considerably greater. The shi
age of the latter specimen almost equalled the shrinkage of the untreated
control. The experiment shows that when the wood specimens are slowly i
the salt concentration of the unsaturated solution in the wood structure
creases and that the salt eventually diffuses into the cell wall.
Very often it is advantageous to chemically treat wood so that
chemical i-;ill continue to diffuse into the call wall during the drying pi
cess. This action cannot be obtained, however, if the moisture in the at
walls becomes saturated during the original treatment. There is thon no
mechanism by which the supply of chemical in the cell cavity can find it,
2Effects of Inorganic Salts Upon the Swelling and the Shrinking
of "ood" by Alfred J. Stamm.
way into the cell wall. By drying wood that has been treated-in a satu-
rated solution of a single chemical, the reserve supply of chemical so-
lution in the cell cavity will dry first and the dry chemical will be
precipitated in consequence. Then deposited in the cell cavities the
chemical has no antidhrink effects.
in order that the over-all absorbed chemical may have its maxi-
mum effect on the antishrink of the wood, partially saturated solutions
must be employed. As the concentration in the cell cavities gradually
increases, diffusion will continue; little if any free chemical remainin-~
in the cell cavity at the end of the drying process.
An excellent chemical seasoning agent would be one that (1) con-
tains a high percentage of solid without being too viscous; (2) has a
relative vapor pressure of 0.75 or 0.80 at time of treatment; (3) remains
in a liquid state in the cell cavities at relative humidities as low as
the wood will encounter in service. The over-all shrinkage of a timber
treated with such a solution would not be materially affected but it would
tend to reduce the shrinkage and consequently the tension stresses in the
With antishrink effects in mild a mixture of chemicals in so-
lution best suits oar purpose; a solution in which the same water acts as
a solvent for both chemicals.
The first chemical chosen would be highly soluble, would have a
relatively small molecule, and when dissolved in water would not be teQ
viscous and its saturated solution would have A relative vapor pressure of
0.75 to 0.80. This chemical would be added to a partially saturatted so-
lution of a chemical which would not crystallize out when the treated wood
was subsequently dried in usual relative humidities. The latter chemical
should preferably be highly soluble in water and its concentration should
be so adjusted that its relative vapor pressure will not materially affect
the vapor pressure of the saturated solution of the first chemical.
Glycerine, diethelene glycol, invert sugar, and hydrol are
examples of the second chemical and quite a number of chemicals would
serve reasonably well for the first. A typical treating solution anid per-
haps one of the best for water retention and antishrink alike is a 40 per-
cent solution of invert sugar to which is added a pound of area for every
pound of water in the sugar solution. Both chemicals diffuse into the wood
luring treatment and because they remain in solution in relative humidities
as low as 25 percent they continue to diffuse into the cell walls as the
concentration in the cell cavities increases during drying.
The~ s of Chemical Seasoning
Plus Antishrink Effect
In estimating the value of a chemical for chemical seasoning
purposes the 5-C yard. stick may be used which appraises the quality of the
laolo 2 was -Prepared with no thoght of finlity u us yW
of fir-It, focusing attention on the fact tha the list of hmclhai
proise of being suiltable for the chemical seasoning of Dougls firis s
prisin, y sort; second, indicating a techic whic ma be mlyai
aasa,-ing the vue of any chemical as an aid t seasoning.
Gos.--Wile the. uamty of chemical needed for chmialses
ing has not been definly establised a valu of 40O pods per houan
boad feet mayT be assued for esating thke cost of the ceia sdi
treating the wood. Hence everything~rtg elseB being he samethe nit ost
a cheracal is an imortazt fator in selecting~i a emicl eaonngaga
vommor chemicals, in the order of increasing cost up to 10 cent.-erpo
are listed in table 2.
But inladdition to the cost of a chemaic-:l osdeainms
piven the following propertes, some of r~hich hav bee, disuse pevo
Condensation problems.--The relative umidity rrth whic a ce
cal solution is in balance establishes the minimumreltiv huidiy t
whih the tLreted wood mwrJ be exosd wittholt cauingshrnkael ore
ove, if the treatted woo0d is exosd to a relative huidity in excess o
te couilirium relative 'hidity, it wi~ll tend to become wt by h mi
tluxe whih it condenses from the air. In tho interest of a rpddyn
rate nd minimum seasoning degrade the treating soluton shuldhav al
relative vaor p~ressure. T~en treated wd aith suc a chemicl, th eltv
humidiities usad in subsequent drying may be corrosndinglylow. n thi
respect, as will be seen from? column 4, sodiu dichroatewit it reat.
vaor nessiu,_ of 52 prcnt is suerior to sodium chloride, or xamle
rhn th~o former salt s used even the most refracto~y itemcanbe rie
a relative humidty of 52 percent, whfereas whn the lattrer sat isue
relative -humiditie less the 75 percent are apt to crras checking.
IBoviover, as atmospheric relative? humiditiesusaly xcef 5
percent, worod treated with sodium dicliom,-tc rll sweant in Usemuhoft
time. Treating solutions that are in eqilibrizi with reatvehuidti
ranging froL. 75 to 90 percent are preferale for ppO.-- s of chmia
seasnic. 15en treatled vrith theso chemicals Yooft~b will dr a arosoa
rapia rate in atmospheric hudities tha-t are not often excoodo. Henc
the treated wood will become dmp infrocuontl in usce.
h~or some reason yet understood, ome chemicals do not cause the rae
wood to sweat in relative hudities th~t exceed the rlative vapor
pressue of the soluion. For exape: while the relative vapor
-pressuresi of saturated soltios of soium cloride and urea respec-
tivelv a~o about the sam, salt -treat e -wood will1 br.come dam in a
relative 'hnidty of 75j per(-ent 7%,heeao erca-treated wod remais dr
to the touch in relative hu-miities as hit Jh as 90 percent.l :-
prvddtereltiehmdtsar maintandi accorda4nce with~ the
waterretetion; poetes of the solution employed. But the treating
that ha e lower vapor pressures permit the safe use of cor-
ringly lower drying humidities. Consequently from the standpoint of
d rate such chemicals are preferred to chemical seasoning agents hav-
i a higher relative vapor pressure.
Starting from lead nitrate which is almost useless as a chemical
s ning agent, the values of the chemicals with respect to drying rate
increase in proportion to their water-retention properties. Opposed to
this trend is a series of values which, due to condensation problems, be-
gnwith the chemical having the lowest relative vapor pressure and
steadily rises as the water-retention properties of the chemical decrease.
en, for chemical seasoning work, it is necessary to Choose a chemical
i is the best compromise between the desire for a rapid safe-drying
r and a minimum of condensation problems, the list of cheap chemicals
s urprisingly short. Of course, the water retention of an aqueous
u n can be modified by adjusting the concentration of a single
c or by combining two or more chemicals in solution. But flexi-
in this respect is not great because of other properties required
a chemical seasoning agnt.
Antishrink.--The more soluble chemicals in general reduce the
s n of wood to the greater extent. This would be expected fro the
h t is that the antishrink property is proportional to bulk of the
d emcal whch diffuses into the moisture contained by the fine
s i tructure of the wood. Assuming that the reduced shrinkage is
en.-yo due to this effect it is possible to calculate the antishrink
ct by means of the following equations:
Volume of chemical
which will dissolve Gras of chemical in 100 cc. of water
in 100 cc. of water = Density of dry chemical
at a given
-,,,m, of resultant
Volume of chemical in 100 cc. of water
Volume of the water -Lnd chemical
Reduction in shrikr.ge of chemically-
Atishrink exoresed treated w1od
in percent Shrinkage of untreated controls x 100
s correct because the water is slightly compressed
bs a itd with the processes of dissolvin the chemical.
The antishrink effects of saturated solutions of several of the
tested chemicals on completely impregnated wood have been determined exper
mentally. The expected antishrink was calculated under conditions of the
periment by means of the above Gquations. The other calculated antishrink
values are based on the solubility of the chemical at 32 F. In calculati
the antishrink of some of the chemicals it was necessary to use the density
values given for chemical plus water of crystallization. When calculated
this way, the antishrink values are too large, but even as calculated the
values indicate the relative antishrink effects of the various chemicals.
Where the expected antishrink values are calculated from con-
ditions of actual test they agree quite well with the actual measured
values. The exceptions to the general rule are those calculated for urca
and invert sugar.
In the case of these two chemicals the experimental values excee
the calculated values considerably.
The discrepancy in the case of the invert sugar can be explained
on the basis of a previously given hypothesis, viz., when wood, treated wi
a partially-saturated solution is subseuently dried slowly the concontrat:
of the solution in the cell cavities increases and the chemical eventually
diffuses into the cell wall. The hygroscopic moisture in Douglas fir occu
pies 0.175 of the total void volume so that when the moisture content of w
which has been completely filled with a partially-saturated solution is re-
duced to 29 percent the concentration of the solution is increased 5.7 tim,
This would saturate all but the most dilute solutions of invert sugar. On
solution basis concentrations of invert sugar greater than 75 percent are
viscous, however, that little diff sion could be expected. Assuming that,
in drying, a 40-percent sugar solution attained a concentration of 75 per-
cent, that is, 300 grams of sugar per 100 grams of water, the calculated a
tishrink value would be 65 percent. This value agrees reasonably well wit
the observed value of 61.5 percent. The calculation tends to confirm the
hypothesis regarding the mechanism by which absorbed partially-saturated
chemical solutions affect the shrinkage of wood.
The discrepancy between calculated and observed antishrink ef-
fects of urea is due to a different phenomenon. Urea is selectively ad-
sorbed by wood hence causes wood to swell considerably beyond its green or
water swollen dimension. If shrinkage and antishrink values are based on
the urea-swollen dimension the observed.antishrink value turns out to be
45.6 as against a calculated value of 43.9
The antishrink effects of various chemicals have been discussed
in detail (1) to present evidence that the chemical diffuses into the cell
walls; (2) to create reasonable confidence that the antishrink effects of
any chemical can be calculated quite accurately if the solution concen-
tration and density of the dry chemical are knorn; and (3) to show that
urea and invert sugar both have outstanding antishrink effects.
The significance of antishrink as far as chemical seasoning is
concerned is not that a treated timber will shrink less than an untreated
control. The significance lies in the ability of a chemical to minimize
the tendency of the treated wood to shrink when, as in air seasonin, the
relative humidity of the drying atmosphere is less than the relative vapor
pressure of the absorbed chemical.
1 7 -11-
Inthisconnectonit e realized that the full atisrink
invert sugar cannot be rEalized dun air drying. e cause of the low
water-retention properties of a 4O-percent invert sugar solution, the
te portion of the wood loses moisture in air drying- more rapidly than
e cal can diffuse from the cell cavity into the cell wall. Hence
in orer to make use of the antishrink properties of invert sugar solution
its water-retention properties must be increased by mixing it with other
When Douglas fir timber is treated in a suitable solution of in-
vert sugar and urea mixture, no shrinkage from green dimension occurs in
the surface fibers as long as the relative humidities during drying do not
fall below 50 percent. The total observed antishrink effect of a 40 part
by eight of invert sugar, 60 part of water, and 60 part of urea was 71.5
percent. So far as known, a chemical seasoning agent which imparts a lesser
degree of antishrink will not prevent 12 by 12-inch boxed-*h Ieart timbers
from checking during air drying. Even this high degree of antishrink is
Where refractory Douglas fir sizes are to be subsequently air
dried, they must first be treated with chemical solutions that impart a
high degree of antishrink. The antishrink aspects of a treatment, however,
axe not particularly significant if the timbrers are to be dried under
perly controlled conditions of relative humidity as in kiln drying.
,ater-retention property of the chemical, seasoning agent alone is suf-
ficient to protect the timber from checking provided the relative hu-
miities are properly controlled.
Corrosion.--Tdhen a chemical is imbibed by ..ood, it naturally
modifies the properties of the wuood. Sone salts arl-. particularly cor-
rosive to ferros fastening. Moreover, if during kiln drying it is necn's-
say to use relative humidities which are in excess of the relative vapor
pressure of the absorbed chemical solution, the timbers will drip salty
water on the metal parts of the kiln, thus corroding rails, track supports,
pipe, and so forth. If the items being dried will stand relative humidi-
ties which axe lower than the relative vapor -pressure of the salt solutions
corrosive treating solutions may be used'without material damage to the
kiln. But the presence of such chemical in thno wood after klryin,- still
creates a corrosion hazard to metal fastonin,- wT,,hen the wood is put in use.
Under certain circumstances and for certain conditions of use,
timbers treated'with corrosive chemicals may be satisfactory, Obviously,
however, regardless of cost, the use of a corrosive salt in chemical
seasoning work is limited. The corrosion hazard further reduces the list
of chemicals which can profitably be used.
Conductiity.--Salts on going into solutions ionize and thus the
eletrical conductivity of salt solution is greater than that of water
alone. Hehce, if high electrical insulating values are required as in the
caseof cross arms and poles, the wood should not be treated with a salt
slution. For most -purposes high electrical resistance is not required of
wood. But when it is required, a treatment with a orgaic chemical
tion is much preferable to a treatment with a salt s
chloride, monoammonium phosphate, etc.
Oolor.--Chemicals differ in their effect on the color of w
A distinctive color might be an advantage in selling chemically-seasone
Douglas fir dimension and cuttings. However, the chemical seasoning
should be chosen with the ultimate use of the wood in mind.
Other Properties.--Other properties to be considered besides those
previously mentioned are fire resistance, inflammability, insect resistance,
and decay resistance.
In table 2, the chemicals have been given an arbitrary ranking
with respect to most of these properties with the view of showing the ex
tent that the presence of the chemical in the wood may limit its use.
cording to the system employed, normal wood was given the ranking of one.
This scheme does not credit a chemical with the beneficial properties it
imparts to wood. For example, with the exception of the nitrates the
salts are, to some degree, fire retardants; ailumninum sulfate, ammonium
sulfate, sodium carbonate, ammonium chloride, manganese chloride, ammonium
phosphate, aluminum chloride and possibly calcium arsenate rank high as
f ire retardants.
In the concentrations used in chemical seasoning most of the
salts are somewhat decay resistant for a while and zinc sulfate is per
equal to zinc chloride. However, like zinc chloride, high concentrations
of zinc sulfate tend to hydrolize wood.
The ammonium salts, including urea, tend to separate into their
component parts at kiln temperature. For this reason urea controls sur-
face checiinp most effectively when the treated wood is dried at low tem-
peratures, that is, air dried. When wood treated with the other nmonium
salts is subsequently kiln dried, the corrosiveness as given in column 4
is apt to be increased. (See item 5 -- ammonium sulfate.) Accelerated
corrosion tests indicate that nails are corroded less when driven into
Douglas fir that has been treated with sodium dichromate and calcium ni-
trate than nails driven into matched untreated material.
A study of the table, in the light of what is known about the
properties that are required of a chemical seasoning agent, leads to the
conclusion that the use of each chemical is more or less restricted; some
chemicals cannot be employed if the timbers are to be subsequently air
dried other chemicals impart properties to the wood which make it more or
less unfit for certain uses. But of all the chemicals listed, urea has
the best chance of wide application.
By combining chemicals, undesirable water-retention properties
can be improved, antishrink properties increased and beneficial proper-
ties such as decay and fire resistance retained. As a general ru~e it
can be said that when two chemicals are mutually dissolved by the same
water, the relative vapor pressure of the solution is lower than the
vapor pressure of a solution of either chemical alone. Sometimes it is
not possible to anticipate the effects of a solution of two or more chemi-
cals. For example, experience has indicated that urea p'lus ammonium sul-
fate is not as good a chemical seasoning agent as urea alone.
General Statement About the Chemical
Seasoning of Douglas Fir
The previous observations thxnghout this report have been re-
dced to a few generalities with the view of showing the status of the
chemical seasoning study in bolder relief. Additional research may change
the picture somewhat, but in the light o present information this is the
way it appears.
Chemicals to be suitable for use as chemical seasoning agents
must be water-soluble and must possess one or both of the following proper-
ties in suitable degree.
Water retention.--The relative vapor pressure of a satisfactory
chemical seasoning agent generally should be in equilibrium with a rela-
tive humidity of about 75 percent.
Antishrink.--7hen large chemically-treated 3 by 12 and larger
Douglas fir timbers are air dried, a chemical seasoning agent should con-
siderably reduce the amount of shrinkage which occurs when the wood dries.
When large chemically-treated Doualas fir timbers are subsequently kiln
dried, the antishrink effect of the treating chemical is unimportant if it
has the proper water-retention properties.
Chemicals such as sodium chloride, ammonium sulfate, and mono-
ammonium phosphate have good water-retention properties but relatively
poor antishrink properties. Urea possesses good water-retention proper-
ties and very good antishrink properties. Mixtures of urea and invert
sugar possess good water-retention properties and excellent antishrink
Few chemicals cost as little as 10 cents a pound. If each low-
priced chemical is considered with respect to its effects on the color,
corrosiveness, fire resistance, and inflammability, decay resistance,
insect resistance, electrical resistance, nand hygroscopicity of the wood,
the resultant process of elimination will leave the list very short.
*ygroscopicity of Pertilizer Materials and Mixtures," Adams and 1ertz,
Industrial and Engineering Ohemistry, Vol. 21, No. 4, April 1929;
"Drying Gases by Absorption," Perry and Duus, Chemical and
etallurgical Engineering, Februiry 1934.
In appraising the commercial significane of chemical seasonn
a distinction mut be mad bewe htte prcs wil do unde ida o
ditions and what it will profitably do udr avrg comeca odtos
To say that timbers of a given size can be chemcaly seasoned witou su
face checking is quite different from sayingta treated timbers of th
same size can be safely or profitably kiln dried in the average run of cm
mercial kilns in a practical length of time.
With laboratory care and refined drying equipment Douglas fir
ranging in size from 1-inch lumber to boxed-heart 12 by 12 timbers can be
chemically seasoned to a low moisture content without surface checkin.
In kiln drying the more refractory items after chemical treatment, a hair-
trigger control of drying conditions is required which is not often attan
able in commercial kilns. Moreover, because of the drying periods therei
little promise of being able to profitably kiln dry timbers larger tha 6
by 12's after chemical treatment. We must look forward to the develomn
of chemical treatments which will permit large timbers to dry in place.
The commercial feasibility of kiln drying items of Douglas fir
after chemical treatment'generally depends on how well the kiln can be
regulated and on the period required to dry them to the desired moisture
content. FigurLing the kiln rental at 30 cents a thousand a day, dryjing
periods in excess of a week or i0.days ,are apt to be prohibitively expen-
sive. After a chemical treatment 3-inch planks can be kiln dried in less
than a week and 4-inch planks can be kiln dried in about 10 days. Side-cu
timbers up to 6 by 12's can be kiln dried enough in a week's time to stan
storage in the vicinity of Chicago. But by and large, the number of size
that may be profitably kiln dried after hemical treatment is limited.
The commercial feasibility of kiln drying- items of Douglas fir
after chemical treatment also depends on the likelihood of the treating
chemical setting up a corrosion hazard in the kiln.
In drying items treated with sodium chloride, for example, a cr
rosion hazard is set up in the kiln when the required hum~idities are in ex
cess of 73 percent. Unless the normal corrosive action of salt can be
materially reduced, salt-treated 12 by 12-inch boxed-herart timbers cannot
be expediently dried in a kiln because in drying this item relative humidi
ties in excess of 75 percent are required,. It is not necessary to create
corrosion hazard in the kiln when dryin 'g salt-treated boards, planks,an
side-cut timbers up to 6 by 121s. In drying these items the humidities re-
quired to prevent checking are 73 percent or less.
The feasibility of kiln drying items of Douglas fir after a chem
cal treatment is therefore limited by the cost of the proce ss, the suita-
bility of drying equipment, and by the corrosion hazar set up in the kl
when dring- large timbers that have been treated with a corrosive chemicl.
Choice of treating chemical influences not only the feasibility
of kiln dryring- but also the air dryinn, of the treated timbers.
31 -) 7 E
The good antishrink and water-retention properties of a saturated
urea solution make its use as a chemical seasoning agent more flexible than
any other single chemical thus far tried. Urea-treated Douglas fir can be
kiln dried just as satisfactorily as Douglas fir treated with any other
cheap chemical. Moreover, the danger of checking during subsequent air dry-
ing is less when urea is used than any other single chemical thus far tried.
Moreover, urea, which is noncorrosive, does not materially alter the natural
color of Douglas fir; in the concentration used in chemical seasoning urea
increases the decay resistance and reduces the inflammability of the wood
A solution consisting of a 40-percent solution of invert sugar to
which is added a pound of urea for every pound of water is technically the
best chemical seasoning agent so far tried. The invert sugar slightly im-
proves the water-retention properties of urea alone and greatly augments its
antishrink property. After being properly treated with the solution, it now
seems that 12 by 12-inch boxed-heart Douglas fir timbers will not surface
chepk badly when subsequently air dried or when permitted to dry in place
provided, they do not become wet while seasoning. Moreover, a treatment
with this combination of chemicals prior to 1iln drying has a, nmber of
advantages over a treatment with a single salt solution.
End coatings seem essential in order to minimize the end checking
of chemically-treated timbers larger than 3 by 12-inch planks.
We are on top of the job as far as the fundamentals of Douglas
fir chemical seasoning are concerned. It is no longer necessary to specu-
late as to the results of this or that process or the feasibility of using
this or that chemical. Further research undoubtedly will produce refine-
ments, but it will not alter the basic principles of the process as now
understood. Laboratory and field experiments show that the process has a
place in the commercial drying field. It permits the operator to dry
rapidly various sizes of Douglas fir that heretofore either had to be dried
slowly or could not be dried at all without checking. Rough calculations
indicate that certain sizes can now be profitably chemrically seasoned.
Further advances are in prospect when the project emerges from
the swaddling clothes of laboratory technic into a full-grown industrial
drying process. The essence of such advancement is simplification with the
view of reducing costs. The most forward step to be taken at present would
be the development of means of treating wood without the use of tanks or
similar equipment. The so-called dry and, sprinkling methods deserve a com-
mercial trial, but in adapting chemical seasoning to commercial needs the
basic principle of the process must be borne in mind. Chemical sepasoning
is a sharp tool that may cut the operator's throat if he doesn't know how
to handle it. Hence, a study of the primer is not a bad beginning, but
commercial processes can best be developed by the cautious experiments of
men who are not satisfied with the status quo of the present seasoning
R1 27 -16-
RAnk :Chemical : Prices : Water : Antishrink :
accord- : per : retention ------------:--------------Dr-- of----- permttd-y--i-- iid-- -poperties---------------
Ing to : pound :--------------: Ob- : Calcu-: CorrosI e- : Color Fire tonden-:leo- ox- : fire
price : Relative served: lated : nesa 1: :hasrd:sation strical :1sity: : ro.
: humidity in :aonduo- : : tard.
: equilibrium :tivity : ant
:with saturated: : : rat-
i: 68* P.o : :
(1) : () : (3) : (4) : (5) : (6) : (7) : (8) : (-) :11) : (12): (13) (14)
: Dollar : Percent : : t
aPrices are quotation$ given In Oil Paint and Drug Reporter, October 1938. (Most prices are in carload lose t.o.b. production plant.)
bVolues indicate minimum relative hamiity at which treated fibers will not shrink. Relative humidities higher than those indloated will cause
oAntishrink prperties are expressed as the percentage reduction in total shrinkage (green to o n dried) based on shrinkage of untreated
controls.' Effot of inorganio salt upon the swelling and shrinking of wood, by Dr. Stamv; unpublished data by Johnson and Loughborough;
S except when checking experimental values calculations are made on solubility at 32o F. calculated values marked C based on solubility of
dehydrated chemical. When antishrink of urea-treated wood is based on dimension of wood after treatment the antishrink is 45.6 obecking the
calculated value 43.9 very nicely.
Arbitrary ranking as to corrosiveness. *Indicate ranking based on accelerated %bt. **Fire-tube data. Values indicate weightose in percent
under conditions of standard test.
5 75 : $12.7 :
:... .... .... :. ...:
:L 81 :27.4 :
: 7 : 2 .6 :
: 8,75 F 8:
: 95 : . :
: 78 t: 1.6 :
Sodia sultate .............. :
Calcium acetate .............:
Sodium phosphate ...........
Bodium carbonate ............
Copper sultate ..............:
*3 1 1 : 2
2 3 or 4 1 ......:.......:
*2 : 1 :......t 1
4 1 :......:.......:
*2(Air dry) : 1 1 : 2
4(Kiln dry) :
*1 I 1 I 3 : 3 :
el 8 1 : 1 :1 :
*2 : 1 : 1:1
e1 ;81eaches: 3 t 2
2 : 1 : : 1
: 1 1 a 1
3 : 1
1 : 1 : 1:1 :
4 : 1 1 :2 I
: Al (0 ) t 20
S (Wg4)2 M 20
...... .. .. .. .
: M BPOtl2H 0 5 40
N ; 00 1026 38
N: M 0 ......
Invert sugar 79 percent :
solids 0.04: Dry...........:
Ammonium chloride........... :
Tribalo calcium phosphate..:
Sodium diahromate ...........
Potaesium nitrate ...........:
btrontium nitrate ...........:
Ammonium carbonate ..........:
Ammonium phosphate ..........:
Alumina chloride ...........
.05 :..............:.......:... .: ...... ........:...... .. .............. .. ....
......: 93 61.4 : 29.7 : 1 1 : 1 : 1 1 : 2 t .....................
.06 : 77.2 : 18.1 18.2 : 4 1 1 2 2 1 19
.065 :..............:....... o :..............:........:......:........ 2 1 s ................:......
.0675 :..............:.......: :..............:........:...... a....... : 1 s............... t.. ..
.07 : 2 :.......: 29.9 : *1 :3 or 4 1 1 : : 2 1 : "00207220 t 36
: 5,86* F. c. : 33.3 : *1 : 1 : 1 2 : 1 : unso4a o a 9
54 : -19.6 19.9 : 3 : : 3 s 3 2 : 1 8 Knti
9 :.......: 5*9 : 2 1 : 3 1 : : 1 : 67o:
:..............:.......: 2).1 1 2 : 1 3 : .......: 2 : 1 8Br( ROP H....
:.............. :...............:...................... ....,: ,.. ,. .....,,:... . ,,,......,.. ,,..
: 86 :.......:
: 88 :.......:
1 98 : ......:
Ranking given In
1. Very exten
2. Extended u
4. Very limit
z M as870 I
ans 7 to 12 is arbitrary but indloates:
Be (presence of ohbical practically no handicap to use).
some Iteme of wood and soonial uses).
I "30s7 0
a BaC1 0
: 01, e
F b No 2302)0
: P b s 0 2 8 8
: Normal wood
Typical methods for obeically seasoning Douglas fir
Schedule Biase I Chemical treatment t Drying treatment
----------------------:------------------ -------------------------------- ---------
a Days I Concentration I Temperature t Days I Dry bulb t Relative
a : I t humidity
-- - ----------- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -
go. OF. I Or. Percent
SSoak in Bodium Chloride Solution I Kiln Schedule
1-A a 3 by 12-inch : 1/2 Saturation : 41 to 160 i 2 160 70
: side out or : 1-1/2 Saturation : 160 1 : 170 : 0
i boxed heart : t a 1 180 t 50
Remarks: Dried to 14 percent moisture content. Dripped water on damp days: Only sap corners
attracted moisture from air when planks dressed to standard dimensions.
S Soak in Bodium Chloride Solution I Kiln Schedule
1-B : 3 by 12.-inch :
aide out or a 2 & Saturation 40 a 2 5 160 3
boxed heart s : 1 170
: 1 : 180 50
Remarks: Dried to 16 percent without degrade: No condensation problem after dressing to
* : Soak in 40 Percent Solution of Invert Sugar: Store on Sticks in a Hot Dry Room
; to Which Was Added 1 Pound Urea Per Pound i ilatin aot ry mte
I of Water in Solution :
1-C : 3 by 12-inch :
aide out or : 4 o 0to 60 2-1/12 I to 120 25
:boxed heart a
Remarks: Planks in a perfect condition after attaining a moisture content of 13 percent.
Their color was natural and they did not condense moisture from the air. Full
length planks after having been given the 1-C chemical treatment could probably
be put into service any place in the United States without further drying.
: : Soaki.n Sodiuma Chloride Solution Kiln Schedule
2-A a 6 by 6-inch : 4 : Saturation 40 to 60 1 2 160 : 73
: side out 2 170 70
S:: 2 175 a 70
Sa : 180 : 55
Remarks: The treatment will dry 6 by 6-inch timbers to a moisture content of 18 to 20 percent.
The drying time seems too long to be practical. Danger of corroding iron in the
kiln if humidities range about 73 percent.
I Soak in Urea Solution : Kiln Schedule
2-B : 6 by 6-inch s :
side out a 3 5 Saturation 4o0 to 60 2 I 160 71
5 a a a 2 170
: 1 175 2
: 1-2/3 180 56
Remarks Without an end coating timbers dry perfectly to 18 to 20 percent moisture content and
will not subsequently check in relative humidities as low as 25 percent.
Soak in Urea Solution : Kiln Schedule
2-c 6 by 6-inch : 5 : Saturation : 4 0 60 9-3/8 120 70
a boxed heart: & : :
Remarks: When so treated and dried 6 by 6-inch boxed-heart timbers will not degrade in a
relative humidity of 25 percent.
: Soak in Sodium Chloride Solution a Kiln Schedule
2-D : 6 by 6-inch 5 Saturation a 4o0 to 60 t 3 140 a 77
t boxed heart 2 : 140 : 75
a 2 : 140 70
: 2 160 0
S 2 160 0
Remarks: Will dry to moisture content of 16 to 18 percent. The 11 day period in the kiln
may not be economical. Also danger of corroding kiln trucks, kiln rails,
track supports, etc.
Soak in a 40 Percent Solution of Invert Air Dry in Hmidities not Less Than
I Sugar Saturating the Solution with Urea erent in an Unheated ed for
b to 90 Days
2-Z8 6 by 6-inch a 5 : 40 to 60
a boxed heart :
1 36532 F (Continued)
Typical methods for cheicallly season Dalas fir (Contnued)
Schedule: sizse Chemical treatment a Drying treatment
: Days Concentration : Temperature a Days Dry bulb I Relative
: :a a : busidity
No. : t : : o Prc
a Soak in Bodium Chloride Solution tiln Sahedule
3-A : 6 by 12-inoh : 3 : Saturated 120 5 160 73
: side-out : a or a
I 160 5 73
Remarks: Five-day dryin period will season side out timbers to stand storage in sheds in the
vicinity of oChiago. Eighteen days of drying required to reduce their moisture
content to 16 to 18 percent.
: Soak in Sodium Chloride Solution Kiln Schedule
3-B : 6 by 12-inch : 6 : Saturation 120 8 160 73
Sboxed heart : : a : or :
:: : 24 a 160 7]
Remarks: As always kiln control must be exact relative humidities in excess of 75 percent
will cause timbers to sweat and drip salt water on tracks, pipes, etc. Eight-
day drying will season boxed-heart timber enough to stand storage in unheated
shed in vicinity of Chicago: 24 days will dry timbers to a moisture content of
14 to 16 percent when they can be used in dry climate.
Soak in Urea Solution : Air Dry Preferably in a Shed
3-C : 6 by 12-inch : 6 : Saturated 40 to so80 a
: side out
i Soak in 40 Percent Bolution of Invert Air Dry PreTferably in a Bhed
SSBugar: Saturate With Urea
3-D : 6 by 12-inch : 8 40 to 80 :
: boxed heart : : a
Remarks: Either 3-C or 3-D may permit the use of respective timbers directly from the bath.
: Soak in 40 Percent Solution of Invert Air Dry in a Shed
: ugar: Saturate Solution With Urea
4-A :12 by 12-inch : 15 40 to 80 :
: boxed heart : :
: and side-cut:
Remarks: It seems that planks up to 4 inches in thickness can be chemically treated and
subsequently kiln dried. Timbers 6 inches and thicker, for economy's sake should
be chemically treated and either air dried or allowed to dry in place. Timbers
or planks thicker than ) inches should be end coated.
Z 1 36533 F
PARrIAL VAPOR PRE55SURE INCHES5 OF
Z /Y\3 5 01
THE Mo0157-RE1 CONTENT OF 51ITKA SPRUCE
AT- EOVILIB/R/UM WITH THc INvPlc,4 TEP TEMPERATURE, P49TAL 04,009 Pcfm5uRE,
Amp IFELATIr HumlaITY
-d *re 2
0 0.10 0.20 0.30 a40 0.50 0.60 a.70 0.80 0.90 /.00
RELATIVE VAOR PRESSURE OF THE AIR
EQUIL13R/UM MOISTURE CONTENT Or NATURAL WOOD
AND WOOD T'RE,4 TED WITH A SATUR.4TED SOLUTION OF rODI1UM CHLORIDE.
30OTH CUR VES FOR A4 TEMPERA TURE OF 70 'F
M K 36871 jr
V16 4VERAGOE MOISTURE CONTEND T 212
__ AVERAGE SALT CONTENT = /.64 __ K
CENTER OF SECTION
SI I TENSION SET DISTRIBUTION
01 1I I I r I t r I s 10
0 0.2 04. 0.6 0.8 /.0 1.2 1.4 1.6 1.8 2.0 2.Z 2.4 2.6 2.8 3.0
DISTANCE FROM FACE OF TIMBER INCHES )
TENSION SET (IN PERCENT) TOTAL NORMAL SHRINKAGE TOTAL OBSERYVD SHRINKAGE x/OO
TOTAL NORMAL SHRINKAGE
U II 36872 7
Moisture, salt, and tension set.
Distribution in a 6 by 12
Douglas-fir timber after 5 days
of drying at 160' F. and 72 per-
cent relative humidity.
4~N o salt
(D 2 grams sodium chloride per 100 grams of water
11 4) 5 Frams sodium chloride per 100 grams of water
g)5 grams sodium chloride per 100 grams of water
101 036 grarms sodium chloride per 100 grams of water
O 0.1 0.) 0.5 .4 0., 0.6 0.7 0 .9 1.0
Figure 4.--(1) Relative vapor pressure-shrinkage and (2) moisture
content-shrinkage relationships for white pine treated
with different concentrations of sodium chloride
Fi ure 4
UNIVERSITY OF FLORIDA
3 1262 08927 3303