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(Released for publication on November 16, 1932)
Moisture Content, Specific Gravity, and Air
Space in Wood
BY J. D. MACLEAN,1 MADISON, WIS.
contains volatile substances that distill at the drying tempera­
ture, the term moisture by definition will also include these dis­
tilled volatile substances in addition to the water. In the fol­
lowing discussion, therefore, the terms “water in the wood” and
“moisture content” include these volatile materials as well as
the water.
When the weight of the oven-dry wood is used as a base, the
moisture percentage shows the number of parts of water by
weight to the number of parts of oven-dry wood by weight. For
example, if a piece of wood has a moisture percentage M of 75,
there are by weight 75 parts of water to 100 parts of oven-dry
wood, or the water weighs three-fourths as much as the ovendry wood. Again, if a timber has a moisture content of 100 per
cent, the weight of the water is equal to the weight of the ovendry wood. If the moisture content is 125 per cent, the water
weighs one and one-fourth times as much as the oven-dry wood,
or in other words the moisture is five-ninths and the oven-dry
wood is four-ninths the total weight.
Since seasoning takes place most rapidly longitudinally, samples
taken from near the ends of timbers that have been seasoning
may show a moisture content considerably lower than samples
taken at a point where end-drying would not affect the results.
Furthermore, samples containing more or less sapwood may have
a moisture content widely different from that of the heartwood.
Samples should be weighed immediately after they are cut from
the timber and as soon as they are taken from the drying oven.
Plants impregnating wood with preservatives treat both
round and sawed timbers, and it is therefore desirable to know
the moisture content of the sapwood separate from that of the
heartwood. Table 1 has been prepared with that in view. The
moisture content values given in Table 1 were obtained at the
Forest Products Laboratory and are average values determined
as soon as possible after the trees were cut. These values prob­
ably show very closely the average moisture content of the living
trees from which the samples were obtained. Individual timbers
of any particular species, or timbers obtained from a given lo­
cality, however, may have a moisture content varying consider­
ably from the average value given in Table 1. Variability in
moisture content will also be found in wood from different parts
of the sapwood and of the heartwood of the same tree.
Table 1 shows in general that the differences in the moisture
content of the sapwood and heartwood are much greater in the
softwoods, or conifers, than in the hardwoods, mainly because the
heartwood is usually drier in the conifers than in the hardwoods.
An exposition of the physics of wood is given in the paper.
It describes the cell structure of wood and the physical
changes that take place as it is dried to oven-dry condition
or
impregnated
to
maximum
absorption. Formulas
and
graphs give the relation that exists between percentage of
moisture
content,
specific
gravity,
water
content,
total
weight, and percentage of air space. The data are of in­
terest to those engaged in wood research or wood physics
or in kiln drying, wood preserving, or fireproofing.
T
HE material wood contains wood substance, moisture, and
air or gas. The wood substance includes both the cellular
structure and extractives of the wood and usually has a
variable quantity of water absorbed in the cell walls. When wood
is green or wet, the water also occupies a part or all of the cell cavi­
ties. In addition there is usually more or less air in the wood
cells depending on the density of the wood and the moisture con­
tent. In the present discussion the volume occupied by air or
gas will be designated as air space.
The purpose of this article is to present graphic methods and
simple formulas worked out at the Forest Products Laboratory
from which the relations among moisture content, specific gravity,
and air space in wood may be better understood and applied by
the wood user to the many practical problems arising daily in the
routine treating and seasoning of wood. For example, the
amount of preservative that can be forced into the wood will be
limited by the amount of air space, and when the moisture con­
tent and specific gravity are known, the approximate amount of
air space can be computed. When this air space is known, it
becomes possible to compute within reasonable limits about how
much preservative can be absorbed as a maximum.
MOISTURE CONTENT
The amount of moisture in wood, which is designated as mois­
ture content, is commonly expressed as a percentage of the weight
of the oven-dry wood. For example, if W is the weight of a
sample before oven drying and D is the weight after oven drying,
then the moisture percentage M is (W - D)/D × 100. In
order to obtain the weight of the wood when in an oven-dry condi­
tion, a small sample of the wood is heated at approximately
100 C (212 F) until the weight becomes constant. If the wood
1 Senior Engineer,
Forest Products Laboratory, Branch of Re­
search, Forest Service, U. S. Department of Agriculture. (The Forest
Products Laboratory is maintained at Madison, Wis., in cooperation
with the University of Wisconsin.) The author is a graduate of the
University of Wisconsin in mechanical engineering. His degrees re­
ceived at the university include M.E., M.S. in mathematics, and
Ph.D. in engineering and mathematics. During the last 13 years
he has been at the Forest Products Laboratory engaged in research
work on various problems, such as a study of the relation of treating
variables to the penetration and absorption of preservatives into
wood, heat conduction in wood, and related subjects. He is the
author of a number of publications relating to wood preservation,
heat conduction, and similar investigations.
For presentation at the meeting of the Wood Industries Division
of THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS, Jamestown,
N. Y., November 15 and 16,1932. All papers are subject to revision.
NOTE: Statements and opinions advanced in papers are to be
understood as individual expressions of their authors, and not those
of the Society.
SHRINKAGE
AND
FIBER-SATURATION POINT
Water contained in the cell cavities of wood is commonly called
free water, whereas that in the fiber walls is known as imbibed
or hygroscopic moisture. The cell walls remain saturated until
the free water has been removed. Any reduction of moisture
in the cell walls causes the wood to shrink, and the amount of
shrinkage will depend upon how much of the hygroscopic moisture
is removed by seasoning.
The fiber-saturation point is defined as the moisture content of
the wood when the cell walls are still saturated but no free water
is present in the cavities. This moisture value varies for different
1
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ing date of presentation. Address a11 communications to Editorial Department, A.S.M.E., 29 West
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2
TRANSACTIONS OF THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS
TABLE 1
AVERAGE SPECIFIC GRAVITY AND MOISTURE CONTENT VALUES FOR 28 SPECIES
species and is also influenced by temperature, but for practical
purposes it may be assumed as about 25 per cent.
The shrinkage2 of wood occurs largely in the radial and tangential directions-thatis, at right angles to the growth rings
and parallel to them, respectively-andis generally negligible
in the longitudinal direction. Tangential shrinkage is greater
than radial. The difference in shrinkage in the two directions
varies for different species and even in different boards or timbers
of the same species. An average ratio of tangential to radial
shrinkage for the various species is about 1.8 to 1. The shrinkage in the volume of wood going from the green to the oven-dry
condition is usually about 12 to 18 per cent for the hardwoods and
8 to 14 per cent for the softwoods. If seasoned wood is soaked
in water until all the fibers are wet, it will swell until the fibersaturation point is reached, or in other words, it will regain the
dimensions it had when green.
During the process of seasoning there is a moisture gradient
from the surface to the interior part of the timber. The interior
of a timber may have a moisture content well above the fiber-saturation point while the outer portion is below. The distribution
of moisture will depend on variables, such as species, size of
timber, seasoning conditions, original moisture content, and
whether sapwood or heartwood. On account of these variable
factors the moisture content at different distances from the surface may vary considerably from the average value for the whole
cross-section, and most timbers, particularly the larger sizes,
will not shrink uniformly from the fiber-saturation point. In
many cases, however, the average moisture content is of particular
interest and is the moisture value most easily found.
SPECIFIC GRAVITY
Specific gravity is the ratio between the density of the material
and the density of pure water at 4 C (39.1 F). In the C.G.S.
system the numerical values for density and specific gravity are
the same. For the English system the density of water is 62.4
lb per cu ft, while its specific gravity is still unity.
2 Data on the volumetric shrinkage from green to oven-dry condition, average weights per cubic foot when green and air dry, and
specific gravities based on the weight of the oven dry wood and vol­
ume when green, may be found for various species in U. S. Department of Agriculture Tech. Bull. 174, “The Air Seasoning of Wood.”
Since wood normally contains a variable quantity of water, the
specific gravity of wood is commonly based on the weight of the
water-free wood and the volume at the moisture content under
consideration. In other words, the specific gravity is the ratio
of the weight when oven-dry of a given volume of wood at the
current moisture content to the weight of an equal volume of
water at its maximum density.
When the volume of the wood is in cubic centimeters and the
weight is in grams, the specific gravity S is equal to the weight
of the oven-dry wood per cubic centimeter. If the volume is
measured in cubic inches and the weight is in pounds, S is the
weight of the oven-dry wood per cubic inch divided by the ap­
proximate weight of a cubic inch of water at maximum density,
which is 0.0361 lb. Weights of wood are commonly expressed
in the United States in pounds per cubic foot, and the specific
gravity based on this unit is therefore the weight of the oven-dry
wood in pounds per cubic foot divided by 62.4. The value of S
will of course be the same whichever system of units is used, since
it is the ratio of the weights of unit volumes of wood and
water.
Wood is at its maximum volume for any moisture content at
or above that corresponding to the fiber-saturation point. The
specific gravity based on the weight of the oven-dry wood and
volume when green, therefore, will apply for all moisture-content
values above the fiber-saturation point regardless of the amount
of water present. As wood seasons, however, the weight of the
oven-dry wood per unit volume will increase in proportion to the
moisture lost below the fiber-saturation point owing to shrinkage
and the accompanying increase in the amount of wood substance
per unit volume. For this reason the specific gravity of wood
will increase as the moisture content decreases below the fibersaturation point, and the difference in specific gravity for any
two moisture values will depend upon the degree of shrinkage.
Obviously, then, the term specific gravity when applied to wood
has little significance unless the moisture conditions at which
the volume was measured are stated.
Since volume changes occur between zero moisture content
and the fiber-saturation point, the specific gravity of any timber
will vary between that based on the weight of the oven-dry wood
and the volume when oven-dry and that based on the weight of the oven-dry wood and the volume when green. Averagefigures
3
WOOD INDUSTRIES
for these maximum and minimum values are given in Table 1
for the species listed.
Since the specific gravity is computed from the weight of the
oven-dry wood, based on unit volume at the current moisture
content, the weight of the water in the wood has no bearing
whatever on the specific gravity as herein defined. The only
effect the water has is its influence on the shrinkage or swelling
which occurs between the condition when the wood is oven-dry
and that when the fiber-saturation point is reached.
In addition to changes in specific gravity caused by shrinkage
or normal moisture changes below the fiber-saturation point
there is a condition which sometimes develops during seasoning
known as collapse. In this case the cell walls become collapsed
to a greater or less extent, thereby causing a reduction in the
amount of air space in the lumena or cell cavities. This condi­
tion would tend to increase the specific gravity, since there would
be a greater amount of wood substance per unit volume.
It must be clearly understood that the specific gravity of wood
may vary to a greater or less extent for different timbers of the
same species. This is mainly because of differences in growth
conditions, but may be due also to variations in the amount of
shrinkage if the timbers are partially seasoned. Even samples
taken from different parts of the same timber may show more or
less variation in specific gravity values. It is therefore evident
that specific gravity values, like those of moisture content, are
only approximate even when the determinations are made from
wood samples taken from the timber under consideration.
Specific Gravity of Wood Substance. In addition to the spe­
cific gravity of wood based on the weight of the oven-dry wood
and volume at the moisture content under consideration it is of
interest in making certain computations to know the specific
gravity of the wood substance itself-thatis, the specific gravity
of the oven-dry wood without any air space or water in it. A
given volume of oven-dry wood normally contains a large propor­
tion of air space in the cell cavities and cell walls, and the volume
occupied by the wood substance itself is considerably less than the
total volume. The specific gravity of wood substance has been
found to be fairly constant for all species, and various methods
of measurement indicate that it is commonly between 1.50 and
1.58. This indicates that if oven-dry wood could be com­
pressed so that no air space were present, a given volume of the
wood substance would weigh something over one and a half times
as much as an equal volume of water. An average value of
1.55 for the specific gravity of wood substance is used for com­
putations given in this paper.
The specific gravity of wood substance should not be confused
with the specific gravity of wood as commonly known and which
is designated as S in the present discussion. In the specific
gravity of wood, the wood is assumed to have its normal cellular
structure, and as a result a given volume of the oven-dry wood
contains both air space and wood substance. The specific
gravity S will therefore always be less than the specific gravity
of wood substance alone.
Relation of Specific Gravity and Moisture Content. Although
it is not always true that the change in specific gravity S with
change in the volume of the wood going from a green to an
oven-dry condition is directly proportional to the moisture loss
below the fiber-saturation point, it is sufficiently close to assume
that this proportional relation exists. If then it is desired to
find the specific gravity at some moisture content K below the
fiber-saturation point, the specific gravity Sk at the moisture
content K can be computed, as indicated by formula [10] given
in the Appendix. For example, from Table 1 the average specific
gravity Sg (based on the weight of the oven-dry wood and volume
when green) is shown for loblolly pine to be 0.50, and Sd (the
specific gravity based on weight of the oven-dry wood and volume
when oven-dry) is 0.57.
The computed specific gravity at 10
per cent moisture content would be,
or approximately 0.54.
The percentage
shrinkage in volume to this moisture content would be (formula
Appendix)
or approximately 7.4 per cent.
Fig. 1 has been prepared so that approximate values of Sk can
be determined directly for the various species given in Table 1
and is based on the values of Sg and Sd given in the table.
FIG. 1 RELATION
OF
AVERAGE SPECIFIC GRAVITY AND MOISTURE
CONTENT
Specific Gravity Measurements. The specific gravity for any
particular timber may be determined within reasonable limits
from samples cut from the piece. Where this is not practicable,
a hole about 1 in. in diameter and 2 in. deep can be bored to obtain
the sample. The volume of wood removed can be computed from
the diameter and depth of the hole, and the boring chips can be
oven-dried in a paper bag. Samples cut from the timber should
not be much over 1 in. in length (in the direction of the grain),
as longer pieces require a considerable time for seasoning to the
oven-dry condition. The samples should be taken far enough
from the ends to avoid the influence of end-seasoning. Samples
or borings must be weighed immediately after they are obtained
and immediately after oven-drying to avoid unobserved moisture
4
TRANSACTIONS OF THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS
changes. The moisture content and specific gravity may be
determined in the same operation. For example, assume the
original weight of a sample with dimensions 3 by 8 by 1 in. is
0.53 lb and the weight when oven-dry is 0.49 lb, the moisture
content is then 0.53 minus 0.49 divided by 0.49, or approximately
only average values, the indicated weights would of course be
only approximate.
The maximum weight of water the wet wood can hold per
cubic foot and the maximum percentage of moisture possible
are found from Fig. 3 by reading the weight and moisture con­
tent corresponding to the point where a vertical line from the
given specific gravity intersects the maximum moisture line which
slopes downward-to the right.
To illustrate the use of Figs. 2 and 3 by a concrete example,
assume the species under consideration is loblolly pine, for which
Table 1 shows an average specific gravity of 0.50 based on the
weight of the oven-dry wood and the volume when green. For
this specific gravity Fig. 2, or formula [2a] of the Appendix,
shows the weight of the oven-dry wood to be 31.2 lb per cu ft
of the green volume. At fiber-saturation point (25 per cent
moisture) Fig. 3, or formula [5a], Appendix, shows that with the
assumed gravity of 0.50 the wood has about 7.8 lb of water per
cubic foot, and the maximum amount of water the wood could
hold, assuming all the air space filled, would be about 42 lb per
FIG. 2 RELATION BETWEEN WEIGHT OF OVEN-DRY WOOD PER
CUBIC FOOT AND SPECIFIC GRAVITY
(The volume in each case being taken at the current moisture content.)
8.2 per cent, and the specific gravity, based on the weight of the
oven-dry wood and volume at 8.2 per cent moisture content, is
0.49 divided by the weight of 24 cu in. of water (0.49
24 times
0.0361), or approximately 0.57.
If the volume of the specimen is measured after oven-drying,
the specific gravity based on the weight when oven-dry and
volume when oven-dry can also be determined. For example,
if the volume of the specimen, which was 24 cu in. at 8.2 per cent
moisture content, is found to be 23 cu in. ,after oven-drying, the
specific gravity based on this volume will be 0.49 divided by 23
multiplied by 0.0361, or approximately 0.59. By substituting
0.57 for Sk, 8.2 for K, and 0.59 for Sd in formula [10] of the Ap­
pendix, the specific gravity Sg, based on the weight of the ovendry wood and the volume when green, is computed as about 0.53.
Relations Between Specific Gravity, Weight of Oven-Dry Wood,
and Weight of Water in Wood. Figs. 2 and 3 have been prepared
so that the weight of oven-dry wood per cubic foot and the
pounds of water per cubic foot can be readily determined for
wood having a known percentage of moisture and a given specific
gravity. Since both moisture content and specific gravity are
FIG. 3 RELATIONS BETWEEN WEIGHT OF WATER IN WOOD, SPECIFIC GRAVITY, AND PERCENTAGE OF MOISTURE (Specific gravity based on the weight of the oven-dry wood and volume at current moisture content.)
cu ft. The volume occupied by the water (25 per cent moisture
content) is found from formula [6] of the Appendix to be about
12.5 per cent. From Fig. 3, or formula [4] of the Appendix,
it is also found that the maximum possible moisture content is
about 135 per cent. The total weight of wood and water (formula
[7a], Appendix) is the weight of the oven-dry wood per cubic
foot found from Fig. 2, plus the weight of water per cubic foot
found from Fig. 3.
WOOD INDUSTRIES
A previous computation showed that if this species having a
specific gravity of 0.50 based on the weight when oven-dry and
volume when green were seasoned to 10 per cent moisture the
specific gravity would be about 0.54. With this specific gravity
and moisture content it is readily found, as in the preceding ex­
amples, that the weight of the oven-dry wood (based on the vol­
ume at 10 per cent moisture) would be about 33.7 lb per cu ft;
the weight of water, 3.4 lb per cu ft; the volume occupied by the
water, 5.4 per cent; and the total weight of wood and water, 37.1
lb per cu ft.
If in making moisture-content computations the weight of the
oven-dry wood is computed from the specific gravity (formula
[2] or [2a], Appendix), it is important that the proper specific
gravity figure be used. This specific gravity must be based on
the weight of the oven-dry wood and the volume of the wood at
5
content is above the fiber-saturation point or not. This can be
done by using the average specific gravity based on green volume
(Table 1), assuming the moisture content at fiber-saturation
point to be about 25 per cent, and computing the total weight
by formula [7a] of the Appendix. With the specific gravity
figure given for red oak, the weight at the fiber-saturation point
is found to be 0.56 × 62.4 × 1.25, or about 44 lb per cu f t .
With a weight of 50 lb per cu ft, the average moisture content
is evidently well above the fiber-saturation point, and is found
by means of formula [3a] of the Appendix to be about 43 per cent.
Fig. 4 makes it convenient to determine approximately these
moisture percentage values for variations from the fiber-satura­
tion point to the maximum amount of water the wood can hold.
The maximum weight line is the greatest weight (in pounds per
cubic foot) the wood can have for the specific gravity given.
LEGEND
that is, the volume at the mois­
NO
NO.
GR NO
NO.
W H I T E OAK
059 6- SHORTLEAF P I N E
0.49 11- SOUTHERN
LOWLAND WHITE F l R . 0 . 3 7
ture conditions when the sample
....
....
15IsmA
is taken.
.
16-WESTERN WHITE
BEECH
_ _ _ _ _0.56 A M E R I C A N E L M
.
For example, assume a green
timber weighs 65 lb per cu ft, the
specific gravity based on the
weight when oven-dry and the
volume when green is 0.53 and
the specific gravity after air
seasoning is 0.59. The weight
of the oven-dry wood based on
the volume when green is then
(0.53 × 62.4), or 33.1 lb per
cu ft, as can also be found from
Fig. 2. From Equation [3a] of
the Appendix the moisture con­
tent based on the weight of the
oven-dry wood is found to be
about 96 percent. This can also
be checked very closely from
Fig. 3. Subtracting 33.1 from
65 gives 31.9 lb of water per
cubic foot. By reading across
from 31.9 on the vertical scale
(Fig. 3) to the specific gravity
FIG 4 RELATION OF WEIGHT OF GREEN WOOD AND MOISTURE CONTENT
0.53, the chart shows that the
The specific gravity figures in Fig. 4 are based on the weight of
moisture-content scale gives approximately 96 per cent.
If, on the other hand, the specific gravity based on the weight the oven-dry wood and the volume when green, since the wood is
of the oven-dry wood and volume when air dry of this timber had assumed at or above the fiber-saturation point.
Fig. 4 shows weights and moisture content values for differences
been taken, the weight when oven-dry would be 0.59 × 62.4,
or 36.8 lb, and the computed mooisture content based on this in specific gravity of 0.05 and can be used for species other than
weight would be about 77 per cant. The latter is an incorrect those listed by interpolating between the lines connecting the
value, as it does not give, as in the first example, the same mois­ different specific-gravity values shown on the maximum weight
ture content that would be shown if a sample were cut from the line. The dotted lines are for species that may have specific
green timber and the moisture content computed as indicated gravities higher than those of the species listed. If, for example,
a wood had a specific gravity of 0.63 based on the weight of the
in formula [3] of the Appendix, where
oven-dry wood and the volume when green and weighed 60 lb
Relation of Moisture Content and Total Weight of Wood. Fre­ per cu ft, the average moisture content is found to be approxi­
quently it is inconvenient to take samples for making a computa­ mately 54 per cent, a value lying between the lines for species
tion of moisture content and specific gravity. In such cases a having specific gravities of 0.60 and 0.65, respectively.
If weights of sample timbers are taken at intervals, Fig. 4 will
rough estimate of the average mositure content can be made for
moisture content values above the fiber-saturation point by also be found useful for estimating how fast the moisture content
weighing representative timbers, finding the weight per unit is changing during the seasoning period. The moisture content
volume, and computing the average moisture conten t from the indicated from the weight of the timber will of course be only
specific gravity data based on the weight of the oven-dry wood an approximate average value because of the uncertain influence
and the volume when green given in Table 1 for the species under of such variables as size and shape of the timber and amount of
consideration. For example, if red-oak timbers have an average shrinkage at the surface where the moisture content may be be­
weight of 50 lb per cu ft, it might be of interest to know about low the fiber-saturation point. Furthermore, the specific gravity
what the average moisture content would be for this weight. It of the sample timbers may depart more or less from the average
would first be necessary to estimate whether the aberage moisture for the species.
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TRANSACTIONS OF THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS
The change in weight of oven-dry wood per unit volume below
the fiber-saturation point offsets to some extent weight variations
caused by changes in moisture content. On this account a
considerable change in moisture content below the fiber-satura­
tion point may cause only a relatively small variation in the total
weight per unit volume.
AIR SPACE IN WOOD
In a given volume of wood the air space is the proportion of the
total volume that is left after subtracting the volume occupied by
the wood substance and the water which the wood contains.
The volume occupied by the wood substance will be the weight
of the oven-dry wood divided by the weight of an equal volume
of the wood substance. If, for example, a cubic foot of wood at
15 per cent moisture content weighs 36 lb when the wood is in an
oven-dry condition, the volume occupied by the wood substance
FIG. 5 MAXIMUM PER CENT OF AIR SPACE IN A GIVEN VOLUME OF
WOOD HAVING A MOISTURE CONTENT M AND SPECIFIC GRAVITY S
(Specific gravity based on the weight of the oven-dry wood and volume at
current moisture content.)
is 36 divided by (62.4 × 1.55), or about 0.372 cu ft. From for­
mula [5] of the Appendix the weight of water equals 36 X 0.15,
or 5.40 lb. The volume occupied by water is 5.40 divided by
62.4, or about 0.087 cu ft. The air space is then, by formula [8]
of the Appendix, equal to [1 - (0.372 + 0.087)], or about 0.541
cu ft, or 54.1 per cent.
Fig. 5 has been prepared so that the percentage of air space can
be found directly for a given moisture content and specific gravity.
For example, with the loblolly pine having a specific gravity of
0.50 and moisture content of 25 per cent, the air space is found to
be about 55 per cent, while at 10 per cent moisture (specific
gravity = 0.54) the wood has about 59 per cent air space. The
air space for intermediate moisture values can be found suf­
ficiently close by interpolating.
Fig. 5 can also be used for finding the maximum percentage
moisture content the wood can have, which is equal to P/S,
where the value of Pis the percentage air space, the value of which
is found opposite the zero moisture content line, and S is the
specific gravity based on the weight of the oven-dry wood and
the volume when green. For example, when the specific gravity
equals 0.50, the maximum air space is found from Fig. 5 to be
67.5 per cent at zero moisture content. In this case the green
volume condition is assumed without any water in the wood.
Dividing 67.5 by 0.50 gives 135 per cent moisture, which as pre­
viously noted was the moisture content shown in Fig. 3 as the
maximum for this specific gravity. It may be of interest to ob­
serve that if, in formula [8a] of the Appendix, the moisture per­
centage M is zero and both sides of the equation are multiplied by
1/S, the result gives formula [4] for finding the maximum possible
moisture content. The point where a given moisture-percentage
line intersects the abscissa (Fig. 5) is the average specific-gravity
value of the species (based on the weight of the oven-dry wood and
the volume when green), which would have the moisture content
shown on the intersecting line as a maximum. For example,
wood having a specific gravity of 1.0 could not have, at most, a
moisture content of more than about 35.5 per cent even if com­
pletely saturated. In most growing trees, however, the heart­
wood has more or less air space, and this in many cases is also
true of the sapwood.
In the wood-preserving industry computations based on the
amount of air space may be of assistance in determining whether
the wood has been sufficiently seasoned to obtain a given absorp­
tion of preservative or whether the specified absorption can be
obtained, considering the moisture conditions at which the wood
must be treated and the proportion of the total volume that can
be impregnated.
If, for example, the loblolly pine timber having 10 per cent
moisture content (specific gravity = 0.54 at 10 per cent moisture
content) were treated with a preservative oil, the moisture con­
tent would not change, assuming the preservative does not con­
tain water, and as far as the effect of the preservative is concerned
there would be no appreciable change in volume, since the swelling
caused by the oil absorbed is very slight. For this reason the
absorption of preservative would depend on the amount of air
space and how much of this air space was filled in treatment.
Assume for illustration that the specific gravity of the preserva­
tive oil is 1 at the treating temperature, that all of the wood
volume can be impregnated, and that all of the air space can be
filled with preservative. When the air space (59 per cent) is
completely filled, the maximum absorption that can be obtained
is 62.4 X 0.59 = 36.8 lb per cu ft. On the other hand, if the
wood at the same moisture content (10 per cent) were treated
with a water solution, water would be absorbed in the cell walls
as well as in the air space, and the volume would increase up to
the fiber-saturation point, while the specific gravity would de­
crease to that based on the weight of the oven-dry wood and
volume when green. The increase in volume would be about pro­
0.54 - 0.50
portional to the change in specific gravity; that is,
=
0.54
0.074, or roughly 7 per cent. Since a cubic foot of the wood at
10 per cent moisture was found from Fig. 3 to have about 3.4 lb.
of water, the amount of original water per cubic foot after treat­
ment would be 3.4/1.07, or about 3.2 lb. From Fig. 3 it was
found that the maximum quantity of water the wood could hold
would be about 42 lb per cu ft. Then 42.0 minus 3.2 gives 38.8 lb
of solution that could be absorbed as a maximum when the specific
gravity of the solution is 1, or the same as that assumed for the
preservative oil.
As another illustration, assume that at the time of treatment
the sapwood in a charge of Southern pine poles averages about
60 per cent of the total volume, the average moisture content of
the sapwood is 50 per cent, and the average specific gravity based
on the volume when green is 0.50. A specification calls for a
WOOD INDUSTRIES
net absorption of 18 lb of coal-tar creosote per cubic foot, and it
is assumed that only the sapwood is penetrated. The question
arises whether it is possible to obtain this absorption without
further seasoning.
From Fig. 5 it is found that with a moisture content of 50 per
cent and a specific gravity of 0.50, the sapwood has about 43 per
cent air space, but since only 60 per cent of the timber is to be
treated, the air space in the treated portion is equivalent to
0.6 × 43, or approximately 26 per cent based on the entire volume.
The absorption is in pounds per cubic foot, and the specific gravity
of the preservative at the treating temperature will be assumed
as 1.03 for the purpose of computation.
The maximum gross absorption possible if the air space were
filled would be 1.03 X 62.4 X 0.26, or about 16.7 lb per cu ft.
Furthermore, the net absorption is generally less than the gross
absorption, even when a so-called full-cell treatment is employed,
and under ordinary treating conditions not all of the air space in
the treated portion could be completely filled. This is particu­
larly true in the case of preservative oils which penetrate the
cell walls only to a very slight extent. With the conditions as­
sumed it is evident that it would be impossible to meet the speci­
fication requiring an absorption of 18 lb per cu ft.
If it is assumed that 80 per cent of the air space is filled with
preservative, the percentage of air space necessary, based on the
total volume, is evidently (18 X 100) divided by (1.03 X 62.4 X
0.80) = 35 per cent. Since only the sapwood comprising 60
per cent of the total volume is impregnated, the required air space
in the treated portion is 35 divided by 0.6, which equals approxi­
mately 58 per cent. Fig. 5 shows that to obtain this amount of
air space the timber would require seasoning below the fibersaturation point if the specific gravity is 0.50. A close estimate
of the moisture content and specific gravity giving this air space
can be made by using Fig. 1. Assuming the species is loblolly
pine and that the moisture content is 15 per cent, Fig. 1 shows the
corresponding specific gravity for this moisture content to be
nearly 0.53. With this specific gravity and moisture content,
Fig. 5 indicates that the air space would be about 58 per cent.
Except in the case of sapwood or easily treated heartwood, only
the outer part of the timber is impregnated. A simple calcula­
tion like the foregoing, however, shows the limitations of gross
absorption which sometimes are not considered in the preparation
of treating specifications.
It may also be of interest to know how much of the air space
is filled with preservative for a specified treatment. For example,
assume that a loblolly pine tie (all sapwood) has an average
moisture content of 20 per cent and is to be impregnated by an
empty-cell process with a net retention of 6 lb of creosote per cubic
foot. At 20 per cent moisture content formula [10] of the
Appendix, or Fig. 1, shows that the specific gravity would be
about 0.514, or approximately .0.51. Fig. 5 shows that with a
specific gravity of 0.51 and a moisture content of 20 per cent,
the wood has about 56 per cent air space. If this space were all
filled with preservative having a specific gravity of 1.03, the gross
absorption would be 62.4 × 1.03 × 0.56, or about 36.0 lb per
cu ft. Since the tie is to be impregnated by means of an emptycell method, there will be a "kickback,"that is, a quantity of
preservative rejected by the wood after pressure is released.
This kickback may vary from about 50 to 65 per cent of the gross
absorption. Assuming the kickback to be 60 per cent, the gross
absorption would need to be 6 divided by 1 minus 0.6, or 15 lb
per cu ft. That is, only 15 divided by 36, or about 42 per cent
of the available air space would be filled during the treatment.
Similarly only one-sixth, or less than 17 per cent, of the air space
would be filled when the treatment was completed.
Computations such as the foregoing, of course, apply whether
a full-cell or an empty-cell process is used.
7
Appendix
FORMULAS FOR COMPUTING RELATION OF MOISTURE
CONTENT, SPECIFIC GRAVITY, AND AIR SPACE IN WOOD