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The Mechanical Properties of Wood by Samuel J. Record

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The term _hardness_ is used in two senses, namely: (1)
resistance to indentation, and (2) resistance to abrasion or
scratching. In the latter sense hardness combined with toughness
is a measure of the wearing ability of wood and is an important
consideration in the use of wood for floors, paving blocks,
bearings, and rollers. While resistance to indentation is
dependent mostly upon the density of the wood, the wearing
qualities may be governed by other factors such as toughness,
and the size, cohesion, and arrangement of the fibres. In use
for floors, some woods tend to compact and wear smooth, while
others become splintery and rough. This feature is affected to
some extent by the manner in which the wood is sawed; thus
edge-grain pine flooring is much better than flat-sawn for
uniformity of wear.

| (Forest Service Cir. 213) |
| COMMON NAME OF SPECIES | Average | End | Radial | Tangential |
| | | surface | surface | surface |
| | Pounds | Pounds | Pounds | Pounds |
| | | | | |
| Hardwoods | | | | |
| | | | | |
| 1 Osage orange | 1,971 | 1,838 | 2,312 | 1,762 |
| 2 Honey locust | 1,851 | 1,862 | 1,860 | 1,832 |
| 3 Swamp white oak | 1,174 | 1,205 | 1,217 | 1,099 |
| 4 White oak | 1,164 | 1,183 | 1,163 | 1,147 |
| 5 Post oak | 1,099 | 1,139 | 1,068 | 1,081 |
| 6 Black oak | 1,069 | 1,093 | 1,083 | 1,031 |
| 7 Red oak | 1,043 | 1,107 | 1,020 | 1,002 |
| 8 White ash | 1,046 | 1,121 | 1,000 | 1,017 |
| 9 Beech | 942 | 1,012 | 897 | 918 |
| 10 Sugar maple | 937 | 992 | 918 | 901 |
| 11 Rock elm | 910 | 954 | 883 | 893 |
| 12 Hackberry | 799 | 829 | 795 | 773 |
| 13 Slippery elm | 788 | 919 | 757 | 687 |
| 14 Yellow birch | 778 | 827 | 768 | 739 |
| 15 Tupelo | 738 | 814 | 666 | 733 |
| 16 Red maple | 671 | 766 | 621 | 626 |
| 17 Sycamore | 608 | 664 | 560 | 599 |
| 18 Black ash | 551 | 565 | 542 | 546 |
| 19 White elm | 496 | 536 | 456 | 497 |
| 20 Basswood | 239 | 273 | 226 | 217 |
| | | | | |
| Conifers | | | | |
| | | | | |
| 1 Longleaf pine | 532 | 574 | 502 | 521 |
| 2 Douglas fir | 410 | 415 | 399 | 416 |
| 3 Bald cypress | 390 | 460 | 355 | 354 |
| 4 Hemlock | 384 | 463 | 354 | 334 |
| 5 Tamarack | 384 | 401 | 380 | 370 |
| 6 Red pine | 347 | 355 | 345 | 340 |
| 7 White fir | 346 | 381 | 322 | 334 |
| 8 Western yellow pine | 328 | 334 | 307 | 342 |
| 9 Lodgepole pine | 318 | 316 | 318 | 319 |
| 10 White pine | 299 | 304 | 294 | 299 |
| 11 Engelmann pine | 266 | 272 | 253 | 274 |
| 12 Alpine fir | 241 | 284 | 203 | 235 |
| NOTE.--Black locust and hickory are not included in this table, |
| but their position would be near the head of the list. |

Tests for either form of hardness are of comparative value only.
Tests for indentation are commonly made by penetrations of the
material with a steel punch or ball.[16] Tests for abrasion are
made by wearing down wood with sandpaper or by means of a sand

[Footnote 16: See articles by Gabriel Janka listed in
bibliography, pages 151-152.]


_Cleavability_ is the term used to denote the facility with
which wood is split. A splitting stress is one in which the
forces act normally like a wedge. (See Fig. 21.) The plane of
cleavage is parallel to the grain, either radially or

[Illustration: FIG. 21.--Cleavage of highly elastic wood. The
cleft runs far ahead of the wedge.]

This property of wood is very important in certain uses such as
firewood, fence rails, billets, and squares. Resistance to
splitting or low cleavability is desirable where wood must hold
nails or screws, as in box-making. Wood usually splits more
readily along the radius than parallel to the growth rings
though exceptions occur, as in the case of cross grain.

Splitting involves transverse tension, but only a portion of the
fibres are under stress at a time. A wood of little stiffness
and strong cohesion across the grain is difficult to split,
while one with great stiffness, such as longleaf pine, is easily
split. The form of the grain and the presence of knots greatly
affect this quality.

| (Forest Service Cir. 213) |
| | When | When |
| COMMON NAME | surface of | surface of |
| OF SPECIES | failure is | failure is |
| | radial | tangential |
| | Lbs. per | Lbs. per |
| | sq. inch | sq. inch |
| | | |
| Hardwoods | | |
| | | |
| Ash, black | 275 | 260 |
| white | 333 | 346 |
| Bashwood | 130 | 168 |
| Beech | 339 | 527 |
| Birch, yellow | 294 | 287 |
| Elm, slippery | 401 | 424 |
| white | 210 | 270 |
| Hackberry | 422 | 436 |
| Locust, honey | 552 | 610 |
| Maple, red | 297 | 330 |
| sugar | 376 | 513 |
| Oak, post | 354 | 487 |
| red | 380 | 470 |
| swamp white | 428 | 536 |
| white | 382 | 457 |
| yellow | 379 | 470 |
| Sycamore | 265 | 425 |
| Tupelo | 277 | 380 |
| | | |
| Conifers | | |
| | | |
| Arborvitae | 148 | 139 |
| Cypress, bald | 167 | 154 |
| Fir, alpine | 130 | 133 |
| Douglas | 139 | 127 |
| white | 145 | 187 |
| Hemlock | 168 | 151 |
| Pine, lodgepole | 142 | 140 |
| longleaf | 187 | 180 |
| red | 161 | 154 |
| sugar | 168 | 189 |
| western yellow | 162 | 187 |
| white | 144 | 160 |
| Spruce, Engelmann | 110 | 135 |
| Tamarack | 167 | 159 |



Wood is an organic product--a structure of infinite variation of
detail and design.[17] It is on this account that no two woods
are alike--in reality no two specimens from the same log are
identical. There are certain properties that characterize each
species, but they are subject to considerable variation. Oak,
for example, is considered hard, heavy, and strong, but some
pieces, even of the same species of oak, are much harder,
heavier, and stronger than others. With hickory are associated
the properties of great strength, toughness, and resilience, but
some pieces are comparatively weak and brash and ill-suited for
the exacting demands for which good hickory is peculiarly

[Footnote 17: For details regarding the structure of wood see
Record, Samuel J.: Identification of the economic woods of the
United States. New York, John Wiley & Sons, 1912.]

It follows that no definite value can be assigned to the
properties of any wood and that tables giving average results of
tests may not be directly applicable to any individual stick.
With sufficient knowledge of the intrinsic factors affecting the
results it becomes possible to infer from the appearance of
material its probable variation from the average. As yet too
little is known of the relation of structure and chemical
composition to the mechanical and physical properties to permit
more than general conclusions.


To understand the effect of variations in the rate of growth it
is first necessary to know how wood is formed. A tree increases
in diameter by the formation, between the old wood and the inner
bark, of new woody layers which envelop the entire stem, living
branches, and roots. Under ordinary conditions one layer is
formed each year and in cross section as on the end of a log
they appear as rings--often spoken of as _annual rings_. These
growth layers are made up of wood cells of various kinds, but
for the most part fibrous. In timbers like pine, spruce,
hemlock, and other coniferous or softwood species the wood cells
are mostly of one kind, and as a result the material is much
more uniform in structure than that of most hardwoods. (See
Frontispiece.) There are no vessels or pores in coniferous wood
such as one sees so prominently in oak and ash, for example.
(See Fig. 22.)

[Illustration: FIG. 22.--Cross sections of a ring-porous
hardwood (white ash), a diffuse-porous hardwood (red gum), and a
non-porous or coniferous wood (eastern hemlock). X 30.
_Photomicrographs by the author._]

The structure of the hardwoods is more complex. They are more or
less filled with vessels, in some cases (oak, chestnut, ash)
quite large and distinct, in others (buckeye, poplar, gum) too
small to be seen plainly without a small hand lens. In
discussing such woods it is customary to divide them into two
large classes--_ring-porous_ and _diffuse-porous_. (See Fig.
22.) In ring-porous species, such as oak, chestnut, ash, black
locust, catalpa, mulberry, hickory, and elm, the larger vessels
or pores (as cross sections of vessels are called) become
localized in one part of the growth ring, thus forming a region
of more or less open and porous tissue. The rest of the ring is
made up of smaller vessels and a much greater proportion of wood
fibres. These fibres are the elements which give strength and
toughness to wood, while the vessels are a source of weakness.

In diffuse-porous woods the pores are scattered throughout the
growth ring instead of being collected in a band or row.
Examples of this kind of wood are gum, yellow poplar, birch,
maple, cottonwood, basswood, buckeye, and willow. Some species,
such as walnut and cherry, are on the border between the two
classes, forming a sort of intermediate group.

If one examines the smoothly cut end of a stick of almost any
kind of wood, he will note that each growth ring is made up of
two more or less well-defined parts. That originally nearest the
centre of the tree is more open textured and almost invariably
lighter in color than that near the outer portion of the ring.
The inner portion was formed early in the season, when growth
was comparatively rapid and is known as _early wood_ (also
spring wood); the outer portion is the _late wood_, being
produced in the summer or early fall. In soft pines there is not
much contrast in the different parts of the ring, and as a
result the wood is very uniform in texture and is easy to work.
In hard pine, on the other hand, the late wood is very dense and
is deep-colored, presenting a very decided contrast to the soft,
straw-colored early wood. (See Fig. 23.) In ring-porous woods
each season's growth is always well defined, because the large
pores of the spring abut on the denser tissue of the fall
before. In the diffuse-porous, the demarcation between rings is
not always so clear and in not a few cases is almost, if not
entirely, invisible to the unaided eye. (See Fig. 22.)

[Illustration: FIG. 23.--Cross section of longleaf pine showing
several growth rings with variations in the width of the
dark-colored late wood. Seven resin ducts are visible. X 33.
_Photomicrograph by U.S. Forest Service_]

If one compares a heavy piece of pine with a light specimen it
will be seen at once that the heavier one contains a larger
proportion of late wood than the other, and is therefore
considerably darker. The late wood of all species is denser than
that formed early in the season, hence the greater the
proportion of late wood the greater the density and strength.
When examined under a microscope the cells of the late wood are
seen to be very thick-walled and with very small cavities, while
those formed first in the season have thin walls and large
cavities. The strength is in the walls, not the cavities. In
choosing a piece of pine where strength or stiffness is the
important consideration, the principal thing to observe is the
comparative amounts of early and late wood. The width of ring,
that is, the number per inch, is not nearly so important as the
proportion of the late wood in the ring.

It is not only the proportion of late wood, but also its
quality, that counts. In specimens that show a very large
proportion of late wood it may be noticeably more porous and
weigh considerably less than the late wood in pieces that
contain but little. One can judge comparative density, and
therefore to some extent weight and strength, by visual

The conclusions of the U.S. Forest Service regarding the effect
of rate of growth on the properties of Douglas fir are
summarized as follows:

"1. In general, rapidly grown wood (less than eight rings per
inch) is relatively weak. A study of the individual tests upon
which the average points are based shows, however, that when it
is not associated with light weight and a small proportion of
summer wood, rapid growth is not indicative of weak wood.

"2. An average rate of growth, indicated by from 12 to 16 rings
per inch, seems to produce the best material.

"3. In rates of growths lower than 16 rings per inch, the
average strength of the material decreases, apparently
approaching a uniform condition above 24 rings per inch. In such
slow rates of growth the texture of the wood is very uniform,
and naturally there is little variation in weight or strength.

"An analysis of tests on large beams was made to ascertain if
average rate of growth has any relation to the mechanical
properties of the beams. The analysis indicated conclusively
that there was no such relation. Average rate of growth [without
consideration also of density], therefore, has little
significance in grading structural timber."[18] This is because
of the wide variation in the percentage of late wood in
different parts of the cross section.

[Footnote 18: Bul. 88: Properties and uses of Douglas fir, p.

Experiments seem to indicate that for most species there is a
rate of growth which, in general, is associated with the
greatest strength, especially in small specimens. For eight
conifers it is as follows:[19]

[Footnote 19: Bul. 108, U. S. Forest Service: Tests of
structural timbers, p. 37.]

Rings per inch
Douglas fir 24
Shortleaf pine 12
Loblolly pine 6
Western larch 18
Western hemlock 14
Tamarack 20
Norway pine 18
Redwood 30

No satisfactory explanation can as yet be given for the real
causes underlying the formation of early and late wood. Several
factors may be involved. In conifers, at least, rate of growth
alone does not determine the proportion of the two portions of
the ring, for in some cases the wood of slow growth is very hard
and heavy, while in others the opposite is true. The quality of
the site where the tree grows undoubtedly affects the character
of the wood formed, though it is not possible to formulate a
rule governing it. In general, however, it may be said that
where strength or ease of working is essential, woods of
moderate to slow growth should be chosen. But in choosing a
particular specimen it is not the width of ring, but the
proportion and character of the late wood which should govern.

In the case of the ring-porous hardwoods there seems to exist a
pretty definite relation between the rate of growth of timber
and its properties. This may be briefly summed up in the general
statement that the more rapid the growth or the wider the rings
of growth, the heavier, harder, stronger, and stiffer the wood.
This, it must be remembered, applies only to ring-porous woods
such as oak, ash, hickory, and others of the same group, and is,
of course, subject to some exceptions and limitations.

In ring-porous woods of good growth it is usually the middle
portion of the ring in which the thick-walled, strength-giving
fibres are most abundant. As the breadth of ring diminishes,
this middle portion is reduced so that very slow growth produces
comparatively light, porous wood composed of thin-walled vessels
and wood parenchyma. In good oak these large vessels of the
early wood occupy from 6 to 10 per cent of the volume of the
log, while in inferior material they may make up 25 per cent or
more. The late wood of good oak, except for radial grayish
patches of small pores, is dark colored and firm, and consists
of thick-walled fibres which form one-half or more of the wood.
In inferior oak, such fibre areas are much reduced both in
quantity and quality. Such variation is very largely the result
of rate of growth.

Wide-ringed wood is often called "second-growth," because the
growth of the young timber in open stands after the old trees
have been removed is more rapid than in trees in the forest, and
in the manufacture of articles where strength is an important
consideration such "second-growth" hardwood material is
preferred. This is particularly the case in the choice of
hickory for handles and spokes. Here not only strength, but
toughness and resilience are important. The results of a series
of tests on hickory by the U.S. Forest Service show that "the
work or shock-resisting ability is greatest in wide-ringed wood
that has from 5 to 14 rings per inch, is fairly constant from 14
to 38 rings, and decreases rapidly from 38 to 47 rings. The
strength at maximum load is not so great with the most
rapid-growing wood; it is maximum with from 14 to 20 rings per
inch, and again becomes less as the wood becomes more closely
ringed. The natural deduction is that wood of first-class
mechanical value shows from 5 to 20 rings per inch and that
slower growth yields poorer stock. Thus the inspector or buyer
of hickory should discriminate against timber that has more than
20 rings per inch. Exceptions exist, however, in the case of
normal growth upon dry situations, in which the slow-growing
material may be strong and tough."[20]

[Footnote 20: Bul. 80: The commercial hickories, pp. 48-50.]

The effect of rate of growth on the qualities of chestnut wood
is summarized by the same authority as follows: "When the rings
are wide, the transition from spring wood to summer wood is
gradual, while in the narrow rings the spring wood passes into
summer wood abruptly. The width of the spring wood changes but
little with the width of the annual ring, so that the narrowing
or broadening of the annual ring is always at the expense of the
summer wood. The narrow vessels of the summer wood make it
richer in wood substance than the spring wood composed of wide
vessels. Therefore, rapid-growing specimens with wide rings have
more wood substance than slow-growing trees with narrow rings.
Since the more the wood substance the greater the weight, and
the greater the weight the stronger the wood, chestnuts with
wide rings must have stronger wood than chestnuts with narrow
rings. This agrees with the accepted view that sprouts (which
always have wide rings) yield better and stronger wood than
seedling chestnuts, which grow more slowly in diameter."[21]

[Footnote 21: Bul. 53: Chestnut in southern Maryland, pp.

In diffuse-porous woods, as has been stated, the vessels or
pores are scattered throughout the ring instead of collected in
the early wood. The effect of rate of growth is, therefore, not
the same as in the ring-porous woods, approaching more nearly
the conditions in the conifers. In general it may be stated that
such woods of medium growth afford stronger material than when
very rapidly or very slowly grown. In many uses of wood,
strength is not the main consideration. If ease of working is
prized, wood should be chosen with regard to its uniformity of
texture and straightness of grain, which will in most cases
occur when there is little contrast between the late wood of one
season's growth and the early wood of the next.


Examination of the end of a log of many species reveals a
darker-colored inner portion--the _heartwood_, surrounded by a
lighter-colored zone--the _sapwood_. In some instances this
distinction in color is very marked; in others, the contrast is
slight, so that it is not always easy to tell where one leaves
off and the other begins. The color of fresh sapwood is always
light, sometimes pure white, but more often with a decided tinge
of green or brown.

Sapwood is comparatively new wood. There is a time in the early
history of every tree when its wood is all sapwood. Its
principal functions are to conduct water from the roots to the
leaves and to store up and give back according to the season the
food prepared in the leaves. The more leaves a tree bears and
the more thrifty its growth, the larger the volume of sapwood
required, hence trees making rapid growth in the open have
thicker sapwood for their size than trees of the same species
growing in dense forests. Sometimes trees grown in the open may
become of considerable size, a foot or more in diameter, before
any heartwood begins to form, for example, in second-growth
hickory, or field-grown white and loblolly pines.

As a tree increases in age and diameter an inner portion of the
sapwood becomes inactive and finally ceases to function. This
inert or dead portion is called heartwood, deriving its name
solely from its position and not from any vital importance to
the tree, as is shown by the fact that a tree can thrive with
its heart completely decayed. Some, species begin to form
heartwood very early in life, while in others the change comes
slowly. Thin sapwood is characteristic of such trees as
chestnut, black locust, mulberry, Osage orange, and sassafras,
while in maple, ash, gum, hickory, hackberry, beech, and
loblolly pine, thick sapwood is the rule.

There is no definite relation between the annual rings of growth
and the amount of sapwood. Within the same species the
cross-sectional area of the sapwood is roughly proportional to
the size of the crown of the tree. If the rings are narrow, more
of them are required than where they are wide. As the tree gets
larger, the sapwood must necessarily become thinner or increase
materially in volume. Sapwood is thicker in the upper portion of
the trunk of a tree than near the base, because the age and the
diameter of the upper sections are less.

When a tree is very young it is covered with limbs almost, if
not entirely, to the ground, but as it grows older some or all
of them will eventually die and be broken off. Subsequent growth
of wood may completely conceal the stubs which, however, will
remain as knots. No matter how smooth and clear a log is on the
outside, it is more or less knotty near the middle. Consequently
the sapwood of an old tree, and particularly of a forest-grown
tree, will be freer from knots than the heartwood. Since in most
uses of wood, knots are defects that weaken the timber and
interfere with its ease of working and other properties, it
follows that sapwood, because of its position in the tree, may
have certain advantages over heartwood.

It is really remarkable that the inner heartwood of old trees
remains as sound as it usually does, since in many cases it is
hundreds of years, and in a few instances thousands of years,
old. Every broken limb or root, or deep wound from fire,
insects, or falling timber, may afford an entrance for decay,
which, once started, may penetrate to all parts of the trunk.
The larvae of many insects bore into the trees and their tunnels
remain indefinitely as sources of weakness. Whatever advantages,
however, that sapwood may have in this connection are due solely
to its relative age and position.

If a tree grows all its life in the open and the conditions of
soil and site remain unchanged, it will make its most rapid
growth in youth, and gradually decline. The annual rings of
growth are for many years quite wide, but later they become
narrower and narrower. Since each succeeding ring is laid down
on the outside of the wood previously formed, it follows that
unless a tree materially increases its production of wood from
year to year, the rings must necessarily become thinner. As a
tree reaches maturity its crown becomes more open and the annual
wood production is lessened, thereby reducing still more the
width of the growth rings. In the case of forest-grown trees so
much depends upon the competition of the trees in their struggle
for light and nourishment that periods of rapid and slow growth
may alternate. Some trees, such as southern oaks, maintain the
same width of ring for hundreds of years. Upon the whole,
however, as a tree gets larger in diameter the width of the
growth rings decreases.

It is evident that there may be decided differences in the grain
of heartwood and sapwood cut from a large tree, particularly one
that is overmature. The relationship between width of growth
rings and the mechanical properties of wood is discussed under
Rate of Growth. In this connection, however, it may be stated
that as a general rule the wood laid on late in the life of a
tree is softer, lighter, weaker, and more even-textured than
that produced earlier. It follows that in a large log the
sapwood, because of the time in the life of the tree when it was
grown, may be inferior in hardness, strength, and toughness to
equally sound heartwood from the same log.

After exhaustive tests on a number of different woods the U.S.
Forest Service concludes as follows: "Sapwood, except that from
old, overmature trees, is as strong as heartwood, other things
being equal, and so far as the mechanical properties go should
not be regarded as a defect."[22] Careful inspection of the
individual tests made in the investigation fails to reveal any
relation between the proportion of sapwood and the breaking
strength of timber.

[Footnote 22: Bul. 108: Tests of structural timber, p. 35.]

In the study of the hickories the conclusion was: "There is an
unfounded prejudice against the heartwood. Specifications place
white hickory, or sapwood, in a higher grade than red hickory,
or heartwood, though there is no inherent difference in
strength. In fact, in the case of large and old hickory trees,
the sapwood nearest the bark is comparatively weak, and the best
wood is in the heart, though in young trees of thrifty growth
the best wood is in the sap."[23] The results of tests from
selected pieces lying side by side in the same tree, and also
the average values for heartwood and sapwood in shipments of the
commercial hickories without selection, show conclusively that
"the transformation of sapwood into heartwood does not affect
either the strength or toughness of the wood.... It is true,
however, that sapwood is usually more free from latent defects
than heartwood."[24]

[Footnote 23: Bul. 80: The commercial hickories, p. 50.]

[Footnote 24: _Loc. cit._]

Specifications for paving blocks often require that longleaf
pine be 90 per cent heart. This is on the belief that sapwood is
not only more subject to decay, but is also weaker than
heartwood. In reality there is no sound basis for discrimination
against sapwood on account of strength, provided other
conditions are equal. It is true that sapwood will not resist
decay as long as heartwood, if both are untreated with
preservatives. It is especially so of woods with deep-colored
heartwood, and is due to infiltrations of tannins, oils, and
resins, which make the wood more or less obnoxious to
decay-producing fungi. If, however, the timbers are to be
treated, sapwood is not a defect; in fact, because of the
relative ease with which it can be impregnated with
preservatives it may be made more desirable than heartwood.[25]

[Footnote 25: Although the factor of heart or sapwood does not
influence the mechanical properties of the wood and there is
usually no difference in structure observable under the
microscope, nevertheless sapwood is generally decidedly
different from heartwood in its physical properties. It dries
better and more easily than heartwood, usually with less
shrinkage and little checking or honeycombing. This is
especially the case with the more refractory woods, such as
white oaks and _Eucalyptus globulus_ and _viminalis_. It is
usually much more permeable to air, even in green wood, notably
so in loblolly pine and even in white oak. As already stated, it
is much more subject to decay. The sapwood of white oak may be
impregnated with creosote with comparative ease, while the
heartwood is practically impenetrable. These facts indicate a
difference in its chemical nature.--H.D. Tiemann.]

In specifications for structural timbers reference is sometimes
made to "boxheart," meaning the inclusion of the pith or centre
of the tree within a cross section of the timber. From numerous
experiments it appears that the position of the pith does not
bear any relation to the strength of the material. Since most
season checks, however, are radial, the position of the pith may
influence the resistance of a seasoned beam to horizontal shear,
being greatest when the pith is located in the middle half of
the section.[26]

[Footnote 26: Bul. 108, U.S. Forest Service, p. 36.]


From data obtained from a large number of tests on the strength
of different woods it appears that, other things being equal,
the crushing strength parallel to the grain, fibre stress at
elastic limit in bending, and shearing strength along the grain
of wood vary in direct proportion to the weight of dry wood per
unit of volume when green. Other strength values follow
different laws. The hardness varies in a slightly greater ratio
than the square of the density. The work to the breaking point
increases even more rapidly than the cube of density. The
modulus of rupture in bending lies between the first power and
the square of the density. This, of course, is true only in case
the greater weight is due to increase in the amount of wood
substance. A wood heavy with resin or other infiltrated
substance is not necessarily stronger than a similar specimen
free from such materials. If differences in weight are due to
degree of seasoning, in other words, to the relative amounts of
water contained, the rules given above will of course not hold,
since strength increases with dryness. But of given specimens of
pine or of oak, for example, in the green condition, the
comparative strength may be inferred from the weight. It is not
permissible, however, to compare such widely different woods as
oak and pine on a basis of their weights.[27]

[Footnote 27: The oaks for some unknown reason fall below the
normal strength for weight, whereas the hickories rise above.
Certain other woods also are somewhat exceptional to the normal
relation of strength and density.]

The weight of wood substance, that is, the material which
composes the walls of the fibres and other cells, is practically
the same in all species, whether pine, hickory, or cottonwood,
being a little greater than half again as heavy as water. It
varies slightly from beech sapwood, 1.50, to Douglas fir
heartwood, 1.57, averaging about 1.55 at 30 deg. to 35 deg. C., in terms
of water at its greatest density 4 deg. C. The reason any wood
floats is that the air imprisoned in its cavities buoys it up.
When this is displaced by water the wood becomes water-logged
and sinks. Leaving out of consideration infiltrated substances,
the reason a cubic foot of one kind of dry wood is heavier than
that of another is because it contains a greater amount of wood
substance. ~Density~ is merely the weight of a unit of volume,
as 35 pounds per cubic foot, or 0.56 grams per cubic centimetre.
~Specific gravity~ or relative density is the ratio of the
density of any material to the density of distilled water at 4 deg.
C. (39.2 deg. F.). A cubic foot of distilled water at 4 deg. C. weighs
62.43 pounds. Hence the specific gravity of a piece of wood with
a density of 35 pounds is 35 / 62.43 = 0.561. To find the weight
per cubic foot when the specific gravity is given, simply
multiply by 62.43. Thus, 0.561 X 62.43 = 35. In the metric
system, since the weight of a cubic centimetre of pure water is
one gram, the density in grams per cubic centimetre has the same
numerical value as the specific gravity.

Since the amount of water in wood is extremely variable it
usually is not satisfactory to refer to the density of green
wood. For scientific purposes the density of "oven-dry" wood is
used; that is, the wood is dried in an oven at a temperature of
100 deg.C. (212 deg.F.) until a constant weight is attained. For
commercial purposes the weight or density of air-dry or
"shipping-dry" wood is used. This is usually expressed in pounds
per thousand board feet, a board foot being considered as
one-twelfth of a cubic foot.

Wood shrinks greatly in drying from the green to the oven-dry
condition. (See Table XIV.) Consequently a block of wood
measuring a cubic foot when green will measure considerably less
when oven-dry. It follows that the density of oven-dry wood does
not represent the weight of the dry wood substance in a cubic
foot of green wood. In other words, it is not the weight of a
cubic foot of green wood minus the weight of the water which it
contains. Since the latter is often a more convenient figure to
use and much easier to obtain than the weight of oven-dry wood,
it is commonly expressed in tables of "specific gravity or
density of dry wood."

| (Forest Service Cir. 213) |
| | | Specific gravity | Shrinkage from green to |
| | Mois- | oven-dry, based on | oven-dry condition |
| COMMON NAME | ture |--------------------+---------------------------|
| OF SPECIES | content | Volume | Volume | In | | Tangen- |
| | | when | when | volume | Radial | tial |
| | | green | oven-dry | | | |
| | Per | | | Per | Per | Per |
| | cent | | | cent | cent | cent |
| | | | | | | |
| Hardwoods | | | | | | |
| | | | | | | |
| Ash, black | 77 | 0.466 | | | | |
| white | 38 | .550 | 0.640 | 12.6 | 4.3 | 6.4 |
| " | 47 | .516 | .590 | 11.7 | | |
| Basswood | 110 | .315 | .374 | 14.5 | 6.2 | 8.4 |
| Beech | 61 | .556 | .669 | 16.5 | 4.6 | 10.5 |
| Birch, yellow | 72 | .545 | .661 | 17.0 | 7.9 | 9.0 |
| Elm, rock | 46 | .578 | | | | |
| slippery | 57 | .541 | .639 | 15.5 | 5.1 | 9.9 |
| white | 66 | .430 | | | | |
| Gum, red | 71 | .434 | | | | |
| Hackberry | 50 | .504 | .576 | 14.0 | 4.2 | 8.9 |
| Hickory, | | | | | | |
| big shellbark | 64 | .601 | | 17.6 | 7.4 | 11.2 |
| " " | 55 | .666 | | 20.9 | 7.9 | 14.2 |
| bitternut | 65 | .624 | | | | |
| mockernut | 64 | .606 | | 16.5 | 6.9 | 10.4 |
| " | 57 | .662 | | 18.9 | 8.4 | 11.4 |
| " | 48 | .666 | | | | |
| nutmeg | 76 | .558 | | | | |
| pignut | 59 | .627 | | 15.0 | 5.6 | 9.8 |
| " | 54 | .667 | | 15.3 | 6.3 | 9.5 |
| " | 55 | .667 | | 16.9 | 6.8 | 10.9 |
| " | 52 | .667 | | 21.2 | 8.5 | 13.8 |
| shagbark | 65 | .608 | | 16.0 | 6.5 | 10.2 |
| " | 58 | .646 | | 18.4 | 7.9 | 11.4 |
| " | 64 | .617 | | | | |
| " | 60 | .653 | | 15.5 | 6.5 | 9.7 |
| water | 74 | .630 | | | | |
| Locust, honey | 53 | .695 | .759 | 8.6 | | |
| Maple, red | 69 | .512 | | | | |
| sugar | 57 | .546 | .643 | 14.3 | 4.9 | 9.1 |
| " | 56 | .577 | | | | |
| Oak, post | 64 | .590 | .732 | 16.0 | 5.7 | 10.6 |
| red | 80 | .568 | .660 | 13.1 | 3.7 | 8.3 |
| swamp white | 74 | .637 | .792 | 17.7 | 5.5 | 10.6 |
| tanbark | 88 | .585 | | | | |
| white | 58 | .594 | .704 | 15.8 | 6.2 | 8.3 |
| " | 62 | .603 | .696 | 14.3 | 4.9 | 9.0 |
| " | 78 | .600 | .708 | 16.0 | 4.8 | 9.2 |
| yellow | 77 | .573 | .669 | 14.2 | 4.5 | 9.7 |
| " | 80 | .550 | | | | |
| Osage orange | 31 | .761 | .838 | 8.9 | | |
| Sycamore | 81 | .454 | .526 | 13.5 | 5.0 | 7.3 |
| Tupelo | 121 | .475 | .545 | 12.4 | 4.4 | 7.9 |

| (Forest Service Cir. 213) |
| | | Specific gravity | Shrinkage from green to |
| | | oven-dry, based on | oven-dry condition |
| COMMON NAME | |--------------------+---------------------------|
| OF SPECIES | Mois- | Volume | Volume | In | | Tangen- |
| | ture | when | when | volume | Radial | tial |
| | content | green | oven-dry | | | |
| | Per | | | Per | Per | Per |
| | cent | | | cent | cent | cent |
| | | | | | | |
| Conifers | | | | | | |
| | | | | | | |
| Arborvitae | 55 | .293 | .315 | 7.0 | 2.1 | 4.9 |
| Cedar, incense | 80 | .363 | | | | |
| Cypress, bald | 79 | .452 | .513 | 11.5 | 3.8 | 6.0 |
| Fir, alpine | 47 | .306 | .321 | 9.0 | 2.5 | 7.1 |
| amabilis | 117 | .383 | | | | |
| Douglas | 32 | .418 | .458 | 10.9 | 3.7 | 6.6 |
| white | 156 | .350 | .437 | 10.2 | 3.4 | 7.0 |
| Hemlock (east.) | 129 | .340 | .394 | 9.2 | 2.3 | 5.0 |
| Pine, lodgepole | 44 | .370 | .415 | 11.3 | 4.2 | 7.1 |
| " | 58 | .371 | .407 | 10.1 | 3.6 | 5.9 |
| longleaf | 63 | .528 | .599 | 12.8 | 6.0 | 7.6 |
| red or Nor | 54 | .440 | .507 | 11.5 | 4.5 | 7.2 |
| shortleaf | 52 | .447 | | | | |
| sugar | 123 | .360 | .386 | 8.4 | 2.9 | 5.6 |
| west yellow | 98 | .353 | .395 | 9.2 | 4.1 | 6.4 |
| " " | 125 | .377 | .433 | 11.5 | 4.3 | 7.3 |
| " " | 93 | .391 | .435 | 9.9 | 3.8 | 5.8 |
| white | 74 | .363 | .391 | 7.8 | 2.2 | 5.9 |
| Redwood | 81 | .334 | | | | |
| " | 69 | .366 | | | | |
| Spruce, | | | | | | |
| Engelmann | 45 | .325 | .359 | 10.5 | 3.7 | 6.9 |
| " | 156 | .299 | .335 | 10.3 | 3.0 | 6.2 |
| red | 31 | .396 | | | | |
| white | 41 | .318 | | | | |
| Tamarack | 52 | .491 | .558 | 13.6 | 3.7 | 7.4 |

This weight divided by 62.43 gives the specific gravity per
green volume. It is purely a fictitious quantity. To convert
this figure into actual density or specific gravity of the dry
wood, it is necessary to know the amount of shrinkage in volume.
If S is the percentage of shrinkage from the green to the
oven-dry condition, based on the green volume; D, the density of
the dry wood per cubic foot while green; and d the actual
density of oven-dry wood, then ---------- = d.
1 - .0 S

This relation becomes clearer from the following analysis:
Taking V and W as the volume and weight, respectively, when
green, and v and w as the corresponding volume and weight when
w W V - v
oven-dry, then, d = --- ; D = --- ; S = ------- X 100, and
v V V
V - v
s = ------- X 100, in which S is the percentage of shrinkage
from the green to the oven-dry condition, based on the green
volume, and s the same based on the oven-dry volume.

In tables of specific gravity or density of wood it should
always be stated whether the dry weight per unit of volume when
green or the dry weight per unit of volume when dry is intended,
since the shrinkage in volume may vary from 6 to 50 per cent,
though in conifers it is usually about 10 per cent, and in
hardwoods nearer 15 per cent. (See Table XIV.)


In species which show a distinct difference between heartwood
and sapwood the natural color of heartwood is invariably darker
than that of the sapwood, and very frequently the contrast is
conspicuous. This is produced by deposits in the heartwood of
various materials resulting from the process of growth,
increased possibly by oxidation and other chemical changes,
which usually have little or no appreciable effect on the
mechanical properties of the wood. (See HEARTWOOD AND SAPWOOD,
above.) Some experiments[28] on very resinous longleaf pine
specimens, however, indicate an increase in strength. This is
due to the resin which increases the strength when dry. Spruce
impregnated with crude resin and dried is greatly increased in
strength thereby.

[Footnote 28: Bul. 70, U.S. Forest Service, p. 92; also p. 126,

Since the late wood of a growth ring is usually darker in color
than the early wood, this fact may be used in judging the
density, and therefore the hardness and strength of the
material. This is particularly the case with coniferous woods.
In ring-porous woods the vessels of the early wood not
infrequently appear on a finished surface as darker than the
denser late wood, though on cross sections of heartwood the
reverse is commonly true. Except in the manner just stated the
color of wood is no indication of strength.

Abnormal discoloration of wood often denotes a diseased
condition, indicating unsoundness. The black check in western
hemlock is the result of insect attacks.[29] The reddish-brown
streaks so common in hickory and certain other woods are mostly
the result of injury by birds.[30] The discoloration is merely
an indication of an injury, and in all probability does not of
itself affect the properties of the wood. Certain rot-producing
fungi impart to wood characteristic colors which thus become
criterions of weakness. Ordinary sap-staining is due to fungous
growth, but does not necessarily produce a weakening effect.[31]

[Footnote 29: See Burke, H.E.: Black check in western hemlock.
Cir. No. 61, U.S. Bu. Entomology, 1905.]

[Footnote 30: See McAtee, W.L.: Woodpeckers in relation to trees
and wood products. Bul. No. 39, U.S. Biol. Survey, 1911.]

[Footnote 31: See Von Schrenck, Hermann: The "bluing" and the
"red rot" of the western yellow pine, with special reference to
the Black Hills forest reserve. Bul. No. 36, U.S. Bu. Plant
Industry, Washington, 1903, pp. 13-14.

Weiss, Howard, and Barnum, Charles T.: The prevention of
sapstain in lumber. Cir. 192, U.S. Forest Service, Washington,
1911, pp. 16-17.]


_Cross grain_ is a very common defect in timber. One form of it
is produced in lumber by the method of sawing and has no
reference to the natural arrangement of the wood elements. Thus
if the plane of the saw is not approximately parallel to the
axis of the log the grain of the lumber cut is not parallel to
the edges and is termed diagonal. This is likely to occur where
the logs have considerable taper, and in this case may be
produced if sawed parallel to the axis of growth instead of
parallel to the growth rings.

Lumber and timber with diagonal grain is always weaker than
straight-grained material, the extent of the defect varying with
the degree of the angle the fibres make with the axis of the
stick. In the vicinity of large knots the grain is likely to be
cross. The defect is most serious where wood is subjected to
flexure, as in beams.

_Spiral grain_ is a very common defect in a tree, and when
excessive renders the timber valueless for use except in the
round. It is produced by the arrangement of the wood fibres in a
spiral direction about the axis instead of exactly vertical.
Timber with spiral grain is also known as "torse wood." Spiral
grain usually cannot be detected by casual inspection of a
stick, since it does not show in the so-called visible grain of
the wood, by which is commonly meant a sectional view of the
annual rings of growth cut longitudinally. It is accordingly
very easy to allow spiral-grained material to pass inspection,
thereby introducing an element of weakness in a structure.

There are methods for readily detecting spiral grain. The
simplest is that of splitting a small piece radially. It is
necessary, of course, that the split be radial, that is, in a
plane passing through the axis of the log, and not tangentially.
In the latter case it is quite probable that the wood would
split straight, the line of cleavage being between the growth

In inspection, the elements to examine are the rays. In the case
of oak and certain other hardwoods these rays are so large that
they are readily seen not only on a radial surface, but on the
tangential as well. On the former they appear as flakes, on the
latter as short lines. Since these rays are between the fibres
it naturally follows that they will be vertical or inclined
according as the tree is straight-grained or spiral-grained.
While they are not conspicuous in the softwoods, they can be
seen upon close scrutiny, and particularly so if a small hand
magnifier is used.

When wood has begun to dry and check it is very easy to see
whether or not it is straight- or spiral-grained, since the
checks will for the most part follow along the rays. If one
examines a row of telephone poles, for example, he will probably
find that most of them have checks running spirally around them.
If boards were sawed from such a pole after it was badly checked
they would fall to pieces of their own weight. The only way to
get straight material would be to split it out.

It is for this reason that split billets and squares are
stronger than most sawed material. The presence of the spiral
grain has little, if any, effect on the timber when it is used
in the round, but in sawed material the greater the pitch of the
spiral the greater is the defect.


_Knots_ are portions of branches included in the wood of the
stem or larger branch. Branches originate as a rule from the
central axis of a stem, and while living increase in size by the
addition of annual woody layers which are a continuation of
those of the stem. The included portion is irregularly conical
in shape with the tip at the pith. The direction of the fibre is
at right angles or oblique to the grain of the stem, thus
producing local cross grain.

During the development of a tree most of the limbs, especially
the lower ones, die, but persist for a time--often for years.
Subsequent layers of growth of the stem are no longer intimately
joined with the dead limb, but are laid around it. Hence dead
branches produce knots which are nothing more than pegs in a
hole, and likely to drop out after the tree has been sawed into
lumber. In grading lumber and structural timber, knots are
classified according to their form, size, soundness, and the
firmness with which they are held in place.[32]

[Footnote 32: See Standard classification of structural timber.
Yearbook Am. Soc. for Testing Materials, 1913, pp. 300-303.
Contains three plates showing standard defects.]

Knots materially affect checking and warping, ease in working,
and cleavability of timber. They are defects which weaken timber
and depreciate its value for structural purposes where strength
is an important consideration. The weakening effect is much more
serious where timber is subjected to bending and tension than
where under compression. The extent to which knots affect the
strength of a beam depends upon their position, size, number,
direction of fibre, and condition. A knot on the upper side is
compressed, while one on the lower side is subjected to tension.
The knot, especially (as is often the case) if there is a season
check in it, offers little resistance to this tensile stress.
Small, knots, however, may be so located in a beam along the
neutral plane as actually to increase the strength by tending to
prevent longitudinal shearing. Knots in a board or plank are
least injurious when they extend through it at right angles to
its broadest surface. Knots which occur near the ends of a beam
do not weaken it. Sound knots which occur in the central portion
one-fourth the height of the beam from either edge are not
serious defects.

Extensive experiments by the U.S. Forest Service[33] indicate
the following effects of knots on structural timbers:

[Footnote 33: Bul. 108, pp. 52 _et seq._]

(1) Knots do not materially influence the stiffness of
structural timber.

(2) Only defects of the most serious character affect the
elastic limit of beams. Stiffness and elastic strength are more
dependent upon the quality of the wood fibre than upon defects
in the beam.

(3) The effect of knots is to reduce the difference between the
fibre stress at elastic limit and the modulus of rupture of
beams. The breaking strength is very susceptible to defects.

(4) Sound knots do not weaken wood when subject to compression
parallel to the grain.[34]

[Footnote 34: Bul. 115, U.S. Forest Service: Mechanical
properties of western hemlock, p. 20.]


A common defect in standing timber results from radial splits
which extend inward from the periphery of the tree, and almost,
if not always, near the base. It is most common in trees which
split readily, and those with large rays and thin bark. The
primary cause of the splitting is frost, and various theories
have been advanced to explain the action.

R. Hartig[35] believes that freezing forces out a part of the
imbibition water of the cell walls, thereby causing the wood to
shrink, and if the interior layers have not yet been cooled,
tangential strains arise which finally produce radial clefts.

[Footnote 35: Hartig, R.: The diseases of trees (trans. by
Somerville and Ward), London and New York, 1894, pp. 282-294.]

Another theory holds that the water is not driven out of the
cell walls, but that difference in temperature conditions of
inner and outer layers is itself sufficient to set up the
strains, resulting in splitting. An air temperature of 14 deg.F. or
less is considered necessary to produce frost splits.

A still more recent theory is that of Busse[36] who considers
the mechanical action of the wind a very important factor. He
observed: (_a_) Frost splits sometimes occur at higher
temperatures than 14 deg.F. (_b_) Most splits take place shortly
before sunrise, _i.e._, at the time of lowest air and soil
temperature; they are never heard to take place at noon,
afternoon, or evening. (_c_) They always occur between two roots
or between the collars of two roots, (_d_) They are most
frequent in old, stout-rooted, broad-crowned trees; in younger
stands it is always the stoutest members that are found with
frost splits, while in quite young stands they are altogether
absent, (_e_) Trees on wet sites are most liable to splits, due
to difference in wood structure, just as difference in wood
structure makes different species vary in this regard. (_f_)
Frost splits are most numerous less than three feet above the

[Footnote 36: Busse, W.: Frost-, Ring- und Kernrisse. Forstwiss.
Centralb., XXXII, 2, 1910, pp. 74-81.]

When a tree is swayed by the wind the roots are counteracting
forces, and the wood fibres are tested in tension and
compression by the opposing forces; where the roots exercise
tension stresses most effectively the effect of compression
stresses is at a minimum; only where the pressure is in excess
of the tension, _i.e._, between the roots, can a separation of
the fibre result. Hence, when by frost a tension on the entire
periphery is established, and the wind localizes additional
strains, failure occurs. The stronger the compression and
tension, the severer the strains and the oftener failures occur.
The occurrence of reports of frost splits on wind-still days is
believed by Busse to be due to the opening of old frost splits
where the tension produced by the frost alone is sufficient.

Frost splits may heal over temporarily, but usually open up
again during the following winter. The presence of old splits is
often indicated by a ridge of callous, the result of the
cambium's effort to occlude the wound. Frost splits not only
affect the value of lumber, but also afford an entrance into the
living tree for disease and decay.


_Heart shake_ occurs in nearly all overmature timber, being more
frequent in hardwoods (especially oak) than in conifers. In
typical heart shake the centre of the hole shows indications of
becoming hollow and radial clefts of varying size extend outward
from the pith, being widest inward. It frequently affects only
the butt log, but may extend to the entire hole and even the
larger branches. It usually results from a shrinkage of the
heartwood due probably to chemical changes in the wood.

When it consists of a single cleft extending across the pith it
is termed _simple heart shake_. Shake of this character in
straight-grained trees affects only one or two central boards
when cut into lumber, but in spiral-grained timber the damage is
much greater. When shake consists of several radial clefts it is
termed _star shake_. In some instances one or more of these
clefts may extend nearly to the bark. In felled or converted
timber clefts due to heart shake may be distinguished from
seasoning cracks by the darker color of the exposed surfaces.
Such clefts, however, tend to open up more and more as the
timber seasons.

_Cup_ or _ring shake_ results from the pulling apart of two or
more growth rings. It is one of the most serious defects to
which sound timber is subject, as it seriously reduces the
technical properties of wood. It is very common in sycamore and
in western larch, particularly in the butt portion. Its
occurrence is most frequent at the junction of two growth layers
of very unequal thickness. Consequently it is likely to occur in
trees which have grown slowly for a time, then abruptly
increased, due to improved conditions of light and food, as in
thinning. Old timber is more subject to it than young trees. The
damage is largely confined to the butt log. Cup shake is often
associated with other forms of shake, and not infrequently shows
traces of decay.

The causes of cup shake are uncertain. The swaying action of the
wind may result in shearing apart the growth layers, especially
in trees growing in exposed places. Frost may in some instances
be responsible for cup shake or at least a contributing factor,
although trees growing in regions free from frost often have
ring shake. Shrinkage of the heartwood may be concentric as well
as radial in its action, thus producing cup shake instead of, or
in connection with, heart shake.

A local defect somewhat similar in effect to cup shake is known
as _rind gall_. If the cambium layer is exposed by the removal
of the entire bark or rind it will die. Subsequent growth over
the damaged portion does not cohere with the wood previously
formed by the old cambium. The defect resulting is termed rind
gall. The most common causes of it are fire, gnawing, blazing,
chipping, sun scald, lightning, and abrasions.

_Heart break_ is a term applied to areas of compression failure
along the grain found in occasional logs. Sometimes these breaks
are invisible until the wood is manufactured into the finished
article. The occurrence of this defect is mostly limited to the
dense hardwoods, such as hickory and to heavy tropical species.
It is the source of considerable loss in the fancy veneer
industry, as the veneer from valuable logs so affected drops to

The cause of heart break is not positively known. It is highly
probable, however, that when the tree is felled the trunk
strikes across a rock or another log, and the impact causes
actual failure in the log as in a beam.

_Resin_ or _pitch pockets_ are of common occurrence in the wood
of larch, spruce, fir, and especially of longleaf and other hard
pines. They are due to accumulations of resin in openings
between adjacent layers of growth. They are more frequent in
trees growing alone than in those of dense stands. The pockets
are usually a few inches in greatest dimension and affect only
one or two growth layers. They are hidden until exposed by the
saw, rendering it impossible to cut lumber with reference to
their position. Often several boards are damaged by a single
pocket. In grading lumber, pitch pockets are classified as
small, standard, and large, depending upon their width and


[Footnote 37: For detailed information regarding insect
injuries, the reader is referred to the various publications of
the U.S. Bureau of Entomology, Washington, D.C.]

The larvae of many insects are destructive to wood. Some attack
the wood of living trees, others only that of felled or
converted material. Every hole breaks the continuity of the
fibres and impairs the strength, and if there are very many of
them the material may be ruined for all purposes where strength
is required.

Some of the most common insects attacking the wood of living
trees are the oak timber worm, the chestnut timber worm,
carpenter worms, ambrosia beetles, the locust borer, turpentine
beetles and turpentine borers, and the white pine weevil.

The insect injuries to forest products may be classed according
to the stage of manufacture of the material. Thus round timber
with the bark on, such as poles, posts, mine props, and sawlogs,
is subject to serious damage by the same class of insects as
those mentioned above, particularly by the round-headed borers,
timber worms, and ambrosia beetles. Manufactured unseasoned
products are subject to damage from ambrosia beetles and other
wood borers. Seasoned hardwood lumber of all kinds, rough
handles, wagon stock, etc., made partially or entirely of
sapwood, are often reduced in value from 10 to 90 per cent by a
class of insects known as powder-post beetles. Finished hardwood
products such as handles, wagon, carriage and machinery stock,
especially if ash or hickory, are often destroyed by the
powder-post beetles. Construction timbers in buildings, bridges
and trestles, cross-ties, poles, mine props, fence posts, etc.,
are sometimes seriously injured by wood-boring larvae, termites,
black ants, carpenter bees, and powder-post beetles, and
sometimes reduced in value from 10 to 100 per cent. In tropical
countries termites are a very serious pest in this respect.


Vast amounts of timber used for piles in wharves and other
marine structures are constantly being destroyed or seriously
injured by marine borers. Almost invariably they are confined to
salt water, and all the woods commonly used for piling are
subject to their attacks. There are two genera of mollusks,
_Xylotrya_ and _Teredo_, and three of crustaceans, _Limnoria,
Chelura_, and _Sphoeroma_, that do serious damage in many places
along both the Atlantic and Pacific coasts.

These mollusks, which are popularly known as "shipworms," are
much alike in structure and mode of life. They attack the
exposed surface of the wood and immediately begin to bore. The
tunnels, often as large as a lead pencil, extend usually in a
longitudinal direction and follow a very irregular, tangled
course. Hard woods are apparently penetrated as readily as soft
woods, though in the same timber the softer parts are preferred.
The food consists of infusoria and is not obtained from the wood
substance. The sole object of boring into the wood is to obtain

Although shipworms can live in cold water they thrive best and
are most destructive in warm water. The length of time required
to destroy an average barked, unprotected pine pile on the
Atlantic coast south from Chesapeake Bay and along the entire
Pacific coast varies from but one to three years.

Of the crustacean borers, _Limnoria_, or the "wood louse," is
the only one of great importance, although _Sphoeroma_ is
reported destructive in places. _Limnoria_ is about the size of
a grain of rice and tunnels into the wood for both food and
shelter. The galleries extend inward radially, side by side, in
countless numbers, to the depth of about one-half inch. The thin
wood partitions remaining are destroyed by wave action, so that
a fresh surface is exposed to attack. Both hard and soft woods
are damaged, but the rate is faster in the soft woods or softer
portions of a wood.

Timbers seriously attacked by marine borers are badly weakened
or completely destroyed. If the original strength of the
material is to be preserved it is necessary to protect the wood
from the borers. This is sometimes accomplished by proper
injection of creosote oil, and more or less successfully by the
use of various kinds of external coatings.[38] No treatment,
however, has proved entirely satisfactory.

[Footnote 38: See Smith, C. Stowell: Preservation of piling
against marine wood borers. Cir. 128, U.S. Forest Service, 1908,
pp. 15.]


[Footnote 39: See Von Schrenck, H.: The decay of timber and
methods of preventing it. Bul. 14, U.S. Bu. Plant Industry,
Washington, D.C., 1902. Also Buls. 32, 114, 214, 266.

Meineoke, E.P.: Forest tree diseases common in California and
Nevada, U.S. Forest Service, Washington, D.C., 1914.

Hartig, R.: The diseases of trees. London and New York, 1894.]

Fungi are responsible for almost all decay of wood. So far as
known, all decay is produced by living organisms, either fungi
or bacteria. Some species attack living trees, sometimes killing
them, or making them hollow, or in the case of pecky cypress and
incense cedar filling the wood with galleries like those of
boring insects. A much larger variety work only in felled or
dead wood, even after it is placed in buildings or manufactured
articles. In any case the process of destruction is the same.
The mycelial threads penetrate the walls of the cells in search
of food, which they find either in the cell contents (starches,
sugars, etc.), or in the cell wall itself. The breaking down of
the cell walls through the chemical action of so-called
"enzymes" secreted by the fungi follows, and the eventual
product is a rotten, moist substance crumbling readily under the
slightest pressure. Some species remove the ligneous matter and
leave almost pure cellulose, which is white, like cotton; others
dissolve the cellulose, leaving a brittle, dark brown mass of
ligno-cellulose. Fungi (such as the bluing fungus) which merely
stain wood usually do not affect its mechanical properties
unless the attacks are excessive.

It is evident, then, that the action of rot-causing fungi is to
decrease the strength of wood, rendering it unsound, brittle,
and dangerous to use. The most dangerous kinds are the so-called
"dry-rot" fungi which work in many kinds of lumber after it is
placed in the buildings. They are particularly to be dreaded
because unseen, working as they do within the walls or inside of
casings. Several serious wrecks of large buildings have been
attributed to this cause. It is stated[40] that in the three
years (1911-1913) more than $100,000 was required to repair
damage due to dry rot.

[Footnote 40: Dry rot in factory timbers, by Inspection Dept.
Associated Factory Mutual Fire Insurance Cos., 31 Milk Street,
Boston, 1913.]

Dry rot develops best at 75 deg.F. and is said to be killed by a
temperature of 110 deg.F.[41] Fully 70 per cent humidity is
necessary in the air in which a timber is surrounded for the
growth of this fungus, and probably the wood must be quite near
its fibre saturation condition. Nevertheless _Merulius
lacrymans_ (one of the most important species) has been found to
live four years and eight months in a dry condition.[42]
Thorough kiln-drying will kill this fungus, but will not prevent
its redevelopment. Antiseptic treatment, such as creosoting, is
the best prevention.

[Footnote 41: Falck, Richard: Die Meruliusfauele des Bauholzes,
Hausschwammforschungen, 6. Heft., Jena, 1912.]

[Footnote 42: Mez, Carl: Der Hausschwamm. Dresden, 1908, p. 63.]

All fungi require moisture and air[43] for their growth.
Deprived of either of these the fungus dies or ceases to
develop. Just what degree of moisture in wood is necessary for
the "dry-rot" fungus has not been determined, but it is
evidently considerably above that of thoroughly air-dry timber,
probably more than 15 per cent moisture. Hence the importance of
free circulation of air about all timbers in a building.

[Footnote 43: A culture of fungus placed in a glass jar and the
air pumped out ceases to grow, but will start again as soon as
oxygen is admitted.]

Warmth is also conducive to the growth of fungi, the most
favorable temperature being about 90 deg.F. They cannot grow in
extreme cold, although no degree of cold such as occurs
naturally will kill them. On the other hand, high temperature
will kill them, but the spores may survive even the boiling
temperature. Mould fungus has been observed to develop rapidly
at 130 deg.F. in a dry kiln in moist air, a condition under which an
animal cannot live more than a few minutes. This fungus was
killed, however, at about 140 deg. or 145 deg.F.[44]

[Footnote 44: Experiments in kiln-drying _Eucalyptus_ in
Berkeley, U.S. Forest Service.]

The fungus (_Endothia parasitica_ And.) which causes the
chestnut blight kills the trees by girdling them and has no
direct effect upon the wood save possibly the four or five
growth rings of the sapwood.[45]

[Footnote 45: See Anderson, Paul J.: The morphology and life
history of the chestnut blight fungus. Bul. No. 7, Penna.
Chestnut Tree Blight Com., Harrisburg, 1914, p. 17.]


[Footnote 46: See York, Harlan H.: The anatomy and some of the
biological aspects of the "American mistletoe." Bul. 120, Sci.
Ser. No. 13, Univ. of Texas, Austin, 1909.

Bray, Wm. L.: The mistletoe pest in the Southwest. Bul. 166,
U.S. Bu. Plant Ind., Washington, 1910.

Meinecke, E.P.: Forest tree diseases common in California and
Nevada. U.S. Forest Service, Washington, 1914, pp. 54-58.]

The most common of the higher parasitic plants damaging timber
trees are mistletoes. Many species of deciduous trees are
attacked by the common mistletoe (_Phoradendron flavescens_). It
is very prevalent in the South and Southwest and when present in
sufficient quantity does considerable damage. There is also a
considerable number of smaller mistletoes belonging to the genus
_Razoumofskya (Arceuthobium)_ which are widely distributed
throughout the country, and several of them are common on
coniferous trees in the Rocky Mountains and along the Pacific

One effect of the common mistletoe is the formation of large
swellings or tumors. Often the entire tree may become stunted or
distorted. The western mistletoe is most common on the branches,
where it produces "witches' broom." It frequently attacks the
trunk as well, and boards cut from such trees are filled with
long, radial holes which seriously damage or destroy the value
of the timber affected.


The data available regarding the effect of the locality of
growth upon the properties of wood are not sufficient to warrant
definite conclusions. The subject has, however, been kept in
mind in many of the U.S. Forest Service timber tests and the
following quotations are assembled from various reports:

"In both the Cuban and longleaf pine the locality where grown
appears to have but little influence on weight or strength, and
there is no reason to believe that the longleaf pine from one
State is better than that from any other, since such variations
as are claimed can be found on any 40-acre lot of timber in any
State. But with loblolly and still more with shortleaf this
seems not to be the case. Being widely distributed over many
localities different in soil and climate, the growth of the
shortleaf pine seems materially influenced by location. The wood
from the southern coast and gulf region and even Arkansas is
generally heavier than the wood from localities farther north.
Very light and fine-grained wood is seldom met near the southern
limit of the range, while it is almost the rule in Missouri,
where forms resembling the Norway pine are by no means rare. The
loblolly, occupying both wet and dry soils, varies accordingly."
Cir. No. 12, p. 6.

" ... It is clear that as all localities have their heavy and
their light timber, so they all share in strong and weak, hard
and soft material, and the difference in quality of material is
evidently far more a matter of individual variation than of soil
or climate." _Ibid._, p.22

"A representative committee of the Carriage Builders'
Association had publicly declared that this important industry
could not depend upon the supplies of southern timber, as the
oak grown in the South lacked the necessary qualities demanded
in carriage construction. Without experiment this statement
could be little better than a guess, and was doubly unwarranted,
since it condemned an enormous amount of material, and one
produced under a great variety of conditions and by at least a
dozen species of trees, involving, therefore, a complexity of
problems difficult enough for the careful investigator, and
entirely beyond the few unsystematic observations of the members
of a committee on a flying trip through one of the greatest
timber regions of the world.

"A number of samples were at once collected (part of them
supplied by the carriage builders' committee), and the fallacy
of the broad statement mentioned was fully demonstrated by a
short series of tests and a more extensive study into structure
and weight of these materials. From these tests it appears that
pieces of white oak from Arkansas excelled well-selected pieces
from Connecticut, both in stiffness and endwise compression (the
two most important forms of resistance)." Report upon the
forestry investigations of the U.S.D.A. 1877-1898, p. 331. See
also Rep. of Div. of For., 1890, p. 209.

"In some regions there are many small, stunted hickories, which
most users will not touch. They have narrow sap, are likely to
be birdpecked, and show very slow growth. Yet five of these
trees from a steep, dry south slope in West Virginia had an
average strength fully equal to that of the pignut from the
better situation, and were superior in toughness, the work to
maximum load being 36.8 as against 31.2 for pignut. The trees
had about twice as many rings per inch as others from better

"This, however, is not very significant, as trees of the same
species, age, and size, growing side by side under the same
conditions of soil and situation, show great variation in their
technical value. It is hard to account for this difference, but
it seems that trees growing in wet or moist situations are
rather inferior to those growing on fresher soil; also, it is
claimed by many hickory users that the wood from limestone soils
is superior to that from sandy soils.

"One of the moot questions among hickory men is the relative
value of northern and southern hickory. The impression prevails
that southern hickory is more porous and brash than hickory from
the north. The tests ... indicate that southern hickory is as
tough and strong as northern hickory of the same age. But the
southern hickories have a greater tendency to be shaky, and this
results in much waste. In trees from southern river bottoms the
loss through shakes and grub-holes in many cases amounts to as
much as 50 per cent.

"It is clear, therefore, that the difference in northern and
southern hickory is not due to geographic location, but rather
to the character of timber that is being cut. Nearly all of that
from southern river bottoms and from the Cumberland Mountains is
from large, old-growth trees; that from the north is from
younger trees which are grown under more favorable conditions,
and it is due simply to the greater age of the southern trees
that hickory from that region is lighter and more brash than
that from the north." Bul. 80, pp. 52-55.


It is generally believed that winter-felled timber has decided
advantages over that cut at other seasons of the year, and to
that cause alone are frequently ascribed much greater
durability, less liability to check and split, better color, and
even increased strength and toughness. The conclusion from the
various experiments made on the subject is that while the time
of felling may, and often does, affect the properties of wood,
such result is due to the weather conditions rather than to the
condition of the wood.

There are two phases of this question. One is concerned with the
physiological changes which might take place during the year in
the wood of a living tree. The other deals with the purely
physical results due to the weather, as differences in
temperature, humidity, moisture, and other features to be
mentioned later.

Those who adhere to the first view maintain that wood cut in
summer is quite different in composition from that cut in
winter. One opinion is that in summer the "sap is up," while in
winter it is "down," consequently winter-felled timber is drier.
A variation of this belief is that in summer the sap contains
certain chemicals which affect the properties of wood and does
not contain them in winter. Again it is sometimes asserted that
wood is actually denser in winter than in summer, as part of the
wood substance is dissolved out in the spring and used for plant
food, being restored in the fall.

It is obvious that such views could apply only to sapwood, since
it alone is in living condition at the time of cutting.
Heartwood is dead wood and has almost no function in the
existence of the tree other than the purely mechanical one of
support. Heartwood does undergo changes, but they are gradual
and almost entirely independent of the seasons.

Sapwood might reasonably be expected to respond to seasonal
changes, and to some extent it does. Just beneath the bark there
is a thin layer of cells which during the growing season have
not attained their greatest density. With the exception of this
one annual ring, or portion of one, the density of the wood
substance of the sapwood is nearly the same the year round.
Slight variations may occur due to impregnation with sugar and
starch in the winter and its dissolution in the growing season.
The time of cutting can have no material effect on the inherent
strength and other mechanical properties of wood except in the
outermost annual ring of growth.

The popular belief that sap is up in the spring and summer and
is down in the winter has not been substantiated by experiment.
There are seasonal differences in the composition of sap, but so
far as the amount of sap in a tree is concerned there is fully
as much, if not more, during the winter than in summer.
Winter-cut wood is not drier, to begin with, than
summer-felled--in reality, it is likely to be wetter.[47]

[Footnote 47: See Record, S.J.: Sap in relation to the
properties of wood. Proc. Am. Wood Preservers' Assn., Baltimore,
Md., 1913, pp. 160-166.

Kempfer, Wm. H.: The air-seasoning of timber. In Bul. 161, Am.
Ry. Eng. Assn., 1913, p. 214.]

The important consideration in regard to this question is the
series of circumstances attending the handling of the timber
after it is felled. Wood dries more rapidly in summer than in
winter, not because there is less moisture at one time than
another, but because of the higher temperature in summer. This
greater heat is often accompanied by low humidity, and
conditions are favorable for the rapid removal of moisture from
the exposed portions of wood. Wood dries by evaporation, and
other things being equal, this will proceed much faster in hot
weather than in cold.

It is a matter of common observation that when wood dries it
shrinks, and if shrinkage is not uniform in all directions the
material pulls apart, causing season checks. (See Fig. 27.) If
evaporation proceeds more rapidly on the outside than inside,
the greater shrinkage of the outer portions is bound to result
in many checks, the number and size increasing with the degree
of inequality of drying.

In cold weather, drying proceeds slowly but uniformly, thus
allowing the wood elements to adjust themselves with the least
amount of rupturing. In summer, drying proceeds rapidly and
irregularly, so that material seasoned at that time is more
likely to split and check.

There is less danger of sap rot when trees are felled in winter
because the fungus does not grow in the very cold weather, and
the lumber has a chance to season to below the danger point
before the fungus gets a chance to attack it. If the logs in
each case could be cut into lumber immediately after felling and
given exactly the same treatment, for example, kiln-dried, no
difference due to the season of cutting would be noted.


[Footnote 48: See Tiemann, H.D.: Effect of moisture upon the
strength and stiffness of wood. Bul. 70, U.S. Forest Service,
Washington, D.C., 1906; also Cir. 108, 1907.]

Water occurs in living wood in three conditions, namely: (1) in
the cell walls, (2) in the protoplasmic contents of the cells,
and (3) as free water in the cell cavities and spaces. In
heartwood it occurs only in the first and last forms. Wood that
is thoroughly air-dried retains from 8 to 16 per cent of water
in the cell walls, and none, or practically none, in the other
forms. Even oven-dried wood retains a small percentage of
moisture, but for all except chemical purposes, may be
considered absolutely dry.

The general effect of the water content upon the wood substance
is to render it softer and more pliable. A similar effect of
common observation is in the softening action of water on
rawhide, paper, or cloth. Within certain limits the greater the
water content the greater its softening effect.

Drying produces a decided increase in the strength of wood,
particularly in small specimens. An extreme example is the case
of a completely dry spruce block two inches in section, which
will sustain a permanent load four times as great as that which
a green block of the same size will support.

The greatest increase due to drying is in the ultimate crushing
strength, and strength at elastic limit in endwise compression;
these are followed by the modulus of rupture, and stress at
elastic limit in cross-bending, while the modulus of elasticity
is least affected. These ratios are shown in Table XV, but it is
to be noted that they apply only to wood in a much drier
condition than is used in practice. For air-dry wood the ratios
are considerably lower, particularly in the case of the ultimate
strength and the elastic limit. Stiffness (within the elastic
limit), while following a similar law, is less affected. In the
case of shear parallel to the grain, the general effect of
drying is to increase the strength, but this is often offset by
small splits and checks caused by shrinkage.

| (Forest Service Bul. 70, p. 89) |
| KIND OF STRENGTH | Longleaf | Spruce | Chestnut |
| | pine | | |
| | (1) (2) | (1) (2) | (1) (2) |
| | | | |
| Crushing strength parallel | | | |
| to grain | 2.89 2.60 | 3.71 3.41 | 2.83 2.55 |
| Elastic limit in | | | |
| compression | | | |
| parallel to grain | 2.60 2.34 | 3.80 3.49 | 2.40 2.26 |
| Modulus of rupture in | | | |
| bending | 2.50 2.20 | 2.81 2.50 | 2.09 1.82 |
| Stress at elastic limit in | | | |
| bending | 2.90 2.55 | 2.90 2.58 | 2.30 2.00 |
| Crushing strength at right | | | |
| angles to grain | | 2.58 2.48 | |
| Shearing strength parallel | | | |
| to grain | 2.01 1.91 | 2.03 1.95 | 1.55 1.47 |
| Modulus of elasticity in | | | |
| compression parallel to | | | |
| grain | 1.63 1.47 | 2.26 2.08 | 1.43 1.29 |
| Modulus of elasticity in | | | |
| bending | 1.59 1.35 | 1.43 1.23 | 1.44 1.21 |
| NOTE.--The figures in the first column show the relative increase in |
| strength between a green specimen and a kiln-dry specimen of equal |
| size. The figures in the second column show the relative increase of |
| strength of the same block after being dried from a green condition |
| to 3.5 per cent moisture, correction having been made for shrinkage. |
| That is, in the first column the strength values per actual unit of |
| area are used; in the second the values per unit of area of green |
| wood which shrinks to smaller size when dried. |
| |
| See also Cir. 108, Fig. 1, p. 8. |

The moisture content has a decided bearing also upon the manner
in which wood fails. In compression tests on very dry specimens
the entire piece splits suddenly into pieces before any buckling
takes place (see Fig. 9.), while with wet material the block
gives way gradually, due to the buckling or bending of the walls
of the fibres along one or more shearing planes. (See Fig. 14.)
In bending tests on wet beams, first failure occurs by
compression on top of the beam, gradually extending downward
toward the neutral axis. Finally the beam ruptures at the
bottom. In the case of very dry beams the failure is usually by
splitting or tension on the under side (see Fig. 17.), without
compression on the upper, and is often sudden and without
warning, and even while the load is still increasing. The effect
varies somewhat with different species, chestnut, for example,
becoming more brittle upon drying than do ash, hemlock, and
longleaf pine. The tensile strength of wood is least affected by
drying, as a rule.

In drying wood no increase in strength results until the free
water is evaporated and the cell walls begin to dry[49]. This
critical point has been called the _fibre-saturation point_.
(See Fig. 24.) Conversely, after the cell walls are saturated
with water, any increase in the amount of water absorbed merely
fills the cavities and intercellular spaces, and has no effect
on the mechanical properties. Hence, soaking green wood does not
lessen its strength unless the water is heated, whereupon a
decided weakening results.

[Footnote 49: The wood of _Eucalyptus globulus_ (blue gum)
appears to be an exception to this rule. Tiemann says: "The wood
of blue gum begins to shrink immediately from the green
condition, even at 70 to 90 per cent moisture content, instead
of from 30 or 25 per cent as in other species of hardwoods."
Proc. Soc. Am. For., Washington, Vol. VIII, No. 3, Oct., 1913,
p. 313.]

[Illustration: FIG. 24.--Relation of the moisture content to the
various strength values of spruce. FSP = fibre-saturation

The strengthening effects of drying, while very marked in the
case of small pieces, may be fully offset in structural timbers
by inherent weakening effects due to the splitting apart of the
wood elements as a result of irregular shrinkage, and in some
cases also to the slitting of the cell walls (see Fig. 25).
Consequently with large timbers in commercial use it is unsafe
to count upon any greater strength, even after seasoning, than
that of the green or fresh condition.

[Illustration: FIG. 25.--Cross section of the wood of western
larch showing fissures in the thick-walled cells of the late
wood. Highly magnified. _Photo by U. S. Forest Service._]

In green wood the cells are all intimately joined together and
are at their natural or normal size when saturated with water.
The cell walls may be considered as made up of little particles
with water between them. When wood is dried the films of water
between the particles become thinner and thinner until almost
entirely gone. As a result the cell walls grow thinner with loss
of moisture,--in other words, the cell shrinks.

It is at once evident that if drying does not take place
uniformly throughout an entire piece of timber, the shrinkage as
a whole cannot be uniform. The process of drying is from the
outside inward, and if the loss of moisture at the surface is
met by a steady capillary current of water from the inside, the
shrinkage, so far as the degree of moisture affected it, would
be uniform. In the best type of dry kilns this condition is
approximated by first heating the wood thoroughly in a moist
atmosphere before allowing drying to begin.

In air-seasoning and in ordinary dry kilns this condition too
often is not attained, and the result is that a dry shell is
formed which encloses a moist interior. (See Fig. 26.)
Subsequent drying out of the inner portion is rendered more
difficult by this "case-hardened" condition. As the outer part
dries it is prevented from shrinking by the wet interior, which
is still at its greatest volume. This outer portion must either
check open or the fibres become strained in tension. If this
outer shell dries while the fibres are thus strained they become
"set" in this condition, and are no longer in tension. Later
when the inner part dries, it tends to shrink away from the
hardened outer shell, so that the inner fibres are now strained
in tension and the outer fibres are in compression. If the
stress exceeds the cohesion, numerous cracks open up, producing
a "honey-combed" condition, or "hollow-horning," as it is
called. If such a case-hardened stick of wood be resawed, the
two halves will cup from the internal tension and external
compression, with the concave surface inward.

[Illustration: FIG. 26.--Progress of drying throughout the
length of a chestnut beam, the black spots indicating the
presence of free water in the wood. The first section at the
left was cut one-fourth inch from the end, the next one-half
inch, the next one inch, and all the others one inch apart. The
illustration shows case-hardening very clearly. _Photo by U. S.
Forest Service._]

For a given surface area the loss of water from wood is always
greater from the ends than from the sides, due to the fact that
the vessels and other water-carriers are cut across, allowing
ready entrance of drying air and outlet for the water vapor.
Water does not flow out of boards and timbers of its own accord,
but must be evaporated, though it may be forced out of very
sappy specimens by heat. In drying a log or pole with the bark
on, most of the water must be evaporated through the ends, but
in the case of peeled timbers and sawn boards the loss is
greatest from the surface because the area exposed is so much

The more rapid drying of the ends causes local shrinkage, and
were the material sufficiently plastic the ends would become
bluntly tapering. The rigidity of the wood substance prevents
this and the fibres are split apart. Later, as the remainder of
the stick dries many of the checks will come together, though
some of the largest will remain and even increase in size as the
drying proceeds. (See Fig. 27.)

[Illustration: FIG. 27.--Excessive season checking. _Photo by U.
S. Forest Service._]

A wood cell shrinks very little lengthwise. A dry wood cell is,
therefore, practically of the same length as it was in a green
or saturated condition, but is smaller in cross section, has
thinner walls, and a larger cavity. It is at once evident that
this fact makes shrinkage more irregular, for wherever cells
cross each other at a decided angle they will tend to pull apart
upon drying. This occurs wherever pith rays and wood fibres
meet. A considerable portion of every wood is made up of these
rays, which for the most part have their cells lying in a radial
direction instead of longitudinally. (See Frontispiece.) In
pine, over 15,000 of these occur on a square inch of a
tangential section, and even in oak the very large rays which
are readily visible to the eye as flakes on quarter-sawed
material represent scarcely one per cent of the number which the
microscope reveals.

A pith ray shrinks in height and width, that is, vertically and
tangentially as applied to the position in a standing tree, but
very little in length or radially. The other elements of the
wood shrink radially and tangentially, but almost none
lengthwise or vertically as applied to the tree. Here, then, we
find the shrinkage of the rays tending to shorten a stick of
wood, while the other cells resist it, and the tendency of a
stick to get smaller in circumference is resisted by the endwise
reaction or thrust of the rays. Only in a tangential direction,
or around the stick in direction of the annual rings of growth,
do the two forces coincide. Another factor to the same end is
that the denser bands of late wood are continuous in a
tangential direction, while radially they are separated by
alternate zones of less dense early wood. Consequently the
shrinkage along the rings (tangential) is fully twice as much as
toward the centre (radial). (See Table XIV.) This explains why
some cracks open more and more as drying advances. (See Fig.

Although actual shrinkage in length is small, nevertheless the
tendency of the rays to shorten a stick produces strains which
are responsible for some of the splitting open of ties, posts,
and sawed timbers with box heart. At the very centre of a tree
the wood is light and weak, while farther out it becomes denser
and stronger. Longitudinal shrinkage is accordingly least at the
centre and greater toward the outside, tending to become
greatest in the sapwood. When a round or a box-heart timber
dries fast it splits radially, and as drying continues the cleft
widens partly on account of the greater tangential shrinkage and
also because the greater contraction of the outer fibres warps
the sections apart. If a small hardwood stem is split while
green for a short distance at the end and placed where it can
dry out rapidly, the sections will become bow-shaped with the
concave sides out. These various facts, taken together, explain
why, for example, an oak tie, pole, or log may split open its
entire length if drying proceeds rapidly and far enough. Initial
stresses in the living trees produce a similar effect when the
log is sawn into boards. This is especially so in _Eucalyptus
globulus_ and to a less extent with any rapidly grown wood.

The use of S-shaped thin steel clamps to prevent large checks
and splits is now a common practice in this country with
crossties and poles as it has been for a long time in European
countries. These devices are driven into the butts of the
timbers so as to cross incipient checks and prevent their
widening. In place of the regular S-hook another of crimped iron
has been devised. (See Fig. 28.) Thin straps of iron with one
tapered edge are run between intermeshing cogs and crimped,
after which they may be cut off any length desired. The time for
driving S-irons of either form is when the cracks first appear.

[Illustration: FIG. 28.--Control of season checking by the use
of S-irons. _Photo by U. S. Forest Service._]

The tendency of logs to split emphasizes the importance of
converting them into planks or timbers while in a green
condition. Otherwise the presence of large checks may render
much lumber worthless which might have been cut out in good
condition. The loss would not be so great if logs were perfectly
straight-grained, but this is seldom the case, most trees
growing more or less spirally or irregularly. Large pieces crack
more than smaller ones, quartered lumber less than that sawed
through and through, thin pieces, especially veneers, less than
thicker boards.

In order to prevent cracks at the ends of boards, small straps
of wood may be nailed on them or they may be painted. This
method is usually considered too expensive, except in the case
of valuable material. Squares used for shuttles, furniture,
gun-stocks, and tool handles should always be protected at the
ends. One of the best means is to dip them into melted
paraffine, which seals the ends and prevents loss of moisture
there. Another method is to glue paper on the ends. In some
cases abroad paper is glued on to all the surfaces of valuable
exotic balks. Other substances sometimes employed for the
purpose of sealing the wood are grease, carbolineum, wax, clay,
petroleum, linseed oil, tar, and soluble glass. In place of
solid beams, built-up material is often preferable, as the
disastrous results of season checks are thereby largely overcome
or minimized.


The effect of temperature on wood depends very largely upon the
moisture content of the wood and the surrounding medium. If
absolutely dry wood is heated in absolutely dry air the wood
expands. The extent of this expansion is denoted by a
coefficient corresponding to the increase in length or other
dimensions for each degree rise in temperature divided by the
original length or other dimension of the specimen. The
coefficient of linear expansion of oak has been found to be
.00000492; radial expansion, .0000544, or about eleven times the
longitudinal. Spruce expands less than oak, the ratio of radial
to longitudinal expansion being about six to one. Metals and
glass expand equally in all directions, since they are
homogeneous substances, while wood is a complicated structure.
The coefficient of expansion of iron is .0000285, or nearly six
times the coefficient of linear expansion of oak and seven times
that of spruce[50].

[Footnote 50: See Schlich's Manual of Forestry, Vol. V. (rev.
ed.), p. 75.]

Under ordinary conditions wood contains more or less moisture,
so that the application of heat has a drying effect which is
accompanied by shrinkage. This shrinkage completely obscures the
expansion due to the heating.

Experiments made at the Yale Forest School revealed the effect
of temperature on the crushing strength of wet wood. In the case
of wet chestnut wood the strength decreases 0.42 per cent for
each degree the water is heated above 60 deg. F.; in the case of
spruce the decrease is 0.32 per cent.

The effects of high temperature on wet wood are very marked.
Boiling produces a condition of great pliability, especially in
the case of hardwoods. If wood in this condition is bent and
allowed to dry, it rigidly retains the shape of the bend, though
its strength may be somewhat reduced. Except in the case of very
dry wood the effect of cold is to increase the strength and
stiffness of wood. The freezing of any free water in the pores
of the wood will augment these conditions.

The effect of steaming upon the strength of cross-ties was
investigated by the U.S. Forest Service in 1904. The conclusions
were summarized as follows:

"(1) The steam at pressure up to 40 pounds applied for 4 hours,
or at a pressure of 20 pounds up to 20 hours, increases the
weight of ties. At 40 pounds' pressure applied for 4 hours and
at 20 pounds for 5 hours the wood began to be scorched.

"(2) The steamed and saturated wood, when tested immediately
after treatment, exhibited weaknesses in proportion to the
pressure and duration of steaming. (See Table XVI.) If allowed
to air-dry subsequently the specimens regained the greater part
of their strength, provided the pressure and duration had not
exceeded those cited under (1). Subsequent immersion in water of
the steamed wood and dried specimens showed that they were
weaker than natural wood similarly dried and resoaked."[51]

[Footnote 51: Cir. 39. Experiments on the strength of treated
timber, p. 18.]

| (Forest Service, Cir. 39) |
| | Cylinder conditions | Strength |
| |---------------------------------+--------------------------------------------|
| | Steaming | Static | Impact | |

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