PART II FACTORS AFFECTING THE MECHANICAL PROPERTIES OF WOOD_The Mechanical Properties of Wood Including a Discussion of the Factors Affecting the Mechanical Properties, and Methods of Timber Testing

INTRODUCTION

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

adapted.

[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.

RATE OF GROWTH

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

inspection.

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.

29.]

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.

20-21.]

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.

HEARTWOOD AND SAPWOOD

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 larvæ 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.]

WEIGHT, DENSITY, AND SPECIFIC GRAVITY

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° to 35° C., in terms

of water at its greatest density 4° 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°

C. (39.2° F.). A cubic foot of distilled water at 4° 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°C. (212°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."

|----------------------------------------------------------------------------|

| TABLE XIV |

|----------------------------------------------------------------------------|

| SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN WOODS |

| (Forest Service Cir. 213) |

|----------------------------------------------------------------------------|

| COMMON NAME | ture |--------------------+---------------------------|

|-----------------+---------+---------+----------+--------+--------+---------|

| | Per | | | Per | Per | Per |

| | | | | | | |

| | | | | | | |

| 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 |

| 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 |

|----------------------------------------------------------------------------|

| TABLE XIV (CONT.) |

|----------------------------------------------------------------------------|

| SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN WOODS |

| (Forest Service Cir. 213) |

|----------------------------------------------------------------------------|

| COMMON NAME | |--------------------+---------------------------|

|-----------------+---------+---------+----------+--------+--------+---------|

| | Per | | | Per | Per | Per |

| | | | | | | |

| | | | | | | |

| Arborvitæ | 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 | | | | |

| 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.)

COLOR

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,

appendix.]

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

_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

rings.

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

_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.]

FROST SPLITS

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°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°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

ground.

[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.

SHAKES, GALLS, PITCH POCKETS

_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

pieces.

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

length.

INSECT INJURIES[37]

[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 larvæ 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 larvæ, 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.

MARINE WOOD-BORER INJURIES

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

shelter.

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.]

FUNGOUS INJURIES[39]

[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°F. and is said to be killed by a

temperature of 110°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 Meruliusfaüle 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°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°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° or 145°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.]

PARASITIC PLANT INJURIES.[46]

[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

coast.

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.

LOCALITY OF GROWTH

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

situations.

"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.

SEASON OF CUTTING

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.

WATER CONTENT[48]

[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.

|----------------------------------------------------------------------|

| TABLE XV |

|----------------------------------------------------------------------|

| EFFECT OF DRYING ON THE MECHANICAL PROPERTIES OF WOOD, SHOWN IN |

| RATIO OF INCREASE DUE TO REDUCING MOISTURE CONTENT FROM |

| THE GREEN CONDITION TO KILN-DRY (3.5 PER CENT) |

| (Forest Service Bul. 70, p. 89) |

|----------------------------------------------------------------------|

|----------------------------+-------------+-------------+-------------|

| | (1) (2) | (1) (2) | (1) (2) |

| | | | |

| to grain | 2.89 2.60 | 3.71 3.41 | 2.83 2.55 |

| parallel to grain | 2.60 2.34 | 3.80 3.49 | 2.40 2.26 |

| bending | 2.50 2.20 | 2.81 2.50 | 2.09 1.82 |

| bending | 2.90 2.55 | 2.90 2.58 | 2.30 2.00 |

| angles to grain | | 2.58 2.48 | |

| to grain | 2.01 1.91 | 2.03 1.95 | 1.55 1.47 |

| grain | 1.63 1.47 | 2.26 2.08 | 1.43 1.29 |

| 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

point.]

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

greater.

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.

27.)

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.

TEMPERATURE

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° 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.]

|------------------------------------------------------------------------------------------|

| TABLE XVI |

|------------------------------------------------------------------------------------------|

| EFFECT OF STEAMING ON THE STRENGTH OF GREEN LOBLOLLY PINE |

| (Forest Service, Cir. 39) |

|------------------------------------------------------------------------------------------|

| | Cylinder conditions | Strength |

| |---------------------------------+--------------------------------------------|

| |---------------------------------+---------------------+----------| Average |

|-----------+--------+----------+-------------+----------+----------+----------+-----------|

| | Hrs. | sq. inch | °F. | | | | |

| | | | | | | | |

| Steam, | 4 | | 230[a] | 91.3 | 79.1 | 96.4 | 88.9 |

| at | 4 | 10 | 238 | 78.2 | 93.7 | 93.3 | 88.4 |

| various | 4 | 20 | 253 | 83.3 | 84.2 | 91.4 | 80.8 |

| pressures | 4 | 30 | 269 | 80.4 | 78.4 | 89.8 | 82.9 |

| | 4 | 40 | 283 | 78.1 | 74.4 | 74.0 | 75.5 |

| | 4 | 50 | 292 | 75.8 | 71.5 | 63.9 | 70.4 |

| | 4 | 100 | 337 | 41.4 | 65.0 | 55.2 | 53.9 |

|-----------+--------+----------+-------------+----------+----------+----------+-----------|

| Steam, | 1 | 20 | 257 | 100.6 | 98.6 | 86.7 | 95.3 |

| for | 2 | 20 | 267 | 88.4 | 93.0 | 107.0 | 96.1 |

| various | 3 | 20 | 260 | 90.0 | 93.6 | 84.1 | 89.2 |

| periods | 4 | 20 | 253 | 83.3 | 84.2 | 91.4 | 86.3 |

| | 5 | 20 | 253 | 85.0 | 78.1 | 84.2 | 82.4 |

| | 6 | 20 | 242 | 95.2 | 89.8 | 76.0 | 87.0 |

| | 10 | 20 | 255 | 73.7 | 82.0 | 76.0 | 77.2 |

| | 20 | 20 | 258 | 67.5 | 65.0 | 99.0 | 77.2 |

|------------------------------------------------------------------------------------------|

| [Footnote a: It will be noted that the temperature was 230°. This is the maximum |

| temperature by the maximum-temperature recording thermometer, and is due to the handling |

| of the exhaust valve. The average temperature was that of exhaust steam.] |

|------------------------------------------------------------------------------------------|

"(3) A high degree of steaming is injurious to wood in strength

and spike-holding power. The degree of steaming at which

pronounced harm results will depend upon the quality of the wood

and its degree of seasoning, and upon the pressure (temperature)

of steam and the duration of its application. For loblolly pine

the limit of safety is certainly 30 pounds for 4 hours, or 20

pounds for 6 hours."[52]

[Footnote 52: _Ibid._, p. 21. See also Cir. 108, p. 19, table

5.]

Experiments made at the Yale Forest School showed that steaming

above 30 pounds' gauge pressure reduces the strength of wood

permanently while wet from 25 to 75 per cent.

PRESERVATIVES

The exact effects of chemical impregnation upon the mechanical

properties of wood have not been fully determined, though they

have been the subject of considerable investigation.[53] More

depends upon the method of treatment than upon the preservatives

used. Thus preliminary steaming at too high pressure or for too

long a period will materially weaken the wood, (See TEMPERATURE,

above.)

[Footnote 53: Hatt, W. K.: Experiments on the strength of

treated timber. Cir. 39, U.S. Forest Service, 1906, p. 31.]

The presence of zinc chloride does not weaken wood under static

loading, although the indications are that the wood becomes

brittle under impact. If the solution is too strong it will

decompose the wood.

Soaking in creosote oil causes wood to swell, and accordingly

decreases the strength to some extent, but not nearly so much so

as soaking in water.[54]

[Footnote 54: Teesdale, Clyde II.: The absorption of creosote by

the cell walls of wood. Cir. 200, U. S. Forest Service, 1912, p.

7.]

Soaking in kerosene seems to have no significant weakening

effect.[55]

[Footnote 55: Tiemann, H.D.: Effect of moisture upon the

strength and stiffness of wood. Bul. 70, U. S. Forest Service,

1907, pp. 122-123, tables 43-44.]

The Mechanical Properties of Wood Including a Discussion of the Factors Affecting the Mechanical Properties, and Methods of Timber Testing · 第 2 / 4 章