HARDNESS
The term _hardness_ is used in two senses, namely: (1) resistance to indentation, and (2) resistance to abrasion or scratching. In the latter sense hardness combined with toughness is a measure of the wearing ability of wood and is an important consideration in the use of wood for floors, paving blocks, bearings, and rollers. While resistance to indentation is dependent mostly upon the density of the wood, the wearing qualities may be governed by other factors such as toughness, and the size, cohesion, and arrangement of the fibres. In use for floors, some woods tend to compact and wear smooth, while others become splintery and rough. This feature is affected to some extent by the manner in which the wood is sawed; thus edge-grain pine flooring is much better than flat-sawn for uniformity of wear.
|——————————————————————-| | TABLE XII | |——————————————————————-| | HARDNESS OF 32 WOODS IN GREEN CONDITION, | | AS INDICATED BY THE LOAD REQUIRED TO IMBED | | A 0.444-INCH STEEL BALL TO ONE-HALF ITS DIAMETER | | (Forest Service Cir. 213) | |——————————————————————-| | COMMON NAME OF SPECIES | Average | End | Radial | Tangential | | | | surface | surface | surface | |————————+———+———+———+————| | | Pounds | Pounds | Pounds | Pounds | | | | | | |
| Hardwoods | | | | | | | | | | |
| 1 Osage orange | 1,971 | 1,838 | 2,312 | 1,762 | | 2 Honey locust | 1,851 | 1,862 | 1,860 | 1,832 | | 3 Swamp white oak | 1,174 | 1,205 | 1,217 | 1,099 | | 4 White oak | 1,164 | 1,183 | 1,163 | 1,147 | | 5 Post oak | 1,099 | 1,139 | 1,068 | 1,081 | | 6 Black oak | 1,069 | 1,093 | 1,083 | 1,031 | | 7 Red oak | 1,043 | 1,107 | 1,020 | 1,002 | | 8 White ash | 1,046 | 1,121 | 1,000 | 1,017 | | 9 Beech | 942 | 1,012 | 897 | 918 | | 10 Sugar maple | 937 | 992 | 918 | 901 | | 11 Rock elm | 910 | 954 | 883 | 893 | | 12 Hackberry | 799 | 829 | 795 | 773 | | 13 Slippery elm | 788 | 919 | 757 | 687 | | 14 Yellow birch | 778 | 827 | 768 | 739 | | 15 Tupelo | 738 | 814 | 666 | 733 | | 16 Red maple | 671 | 766 | 621 | 626 | | 17 Sycamore | 608 | 664 | 560 | 599 | | 18 Black ash | 551 | 565 | 542 | 546 | | 19 White elm | 496 | 536 | 456 | 497 | | 20 Basswood | 239 | 273 | 226 | 217 | | | | | | |
| Conifers | | | | | | | | | | |
| 1 Longleaf pine | 532 | 574 | 502 | 521 | | 2 Douglas fir | 410 | 415 | 399 | 416 | | 3 Bald cypress | 390 | 460 | 355 | 354 | | 4 Hemlock | 384 | 463 | 354 | 334 | | 5 Tamarack | 384 | 401 | 380 | 370 | | 6 Red pine | 347 | 355 | 345 | 340 | | 7 White fir | 346 | 381 | 322 | 334 | | 8 Western yellow pine | 328 | 334 | 307 | 342 | | 9 Lodgepole pine | 318 | 316 | 318 | 319 | | 10 White pine | 299 | 304 | 294 | 299 | | 11 Engelmann pine | 266 | 272 | 253 | 274 | | 12 Alpine fir | 241 | 284 | 203 | 235 | |——————————————————————-| | NOTE.–Black locust and hickory are not included in this table, | | but their position would be near the head of the list. | |——————————————————————-|
Tests for either form of hardness are of comparative value only. Tests for indentation are commonly made by penetrations of the material with a steel punch or ball.[16] Tests for abrasion are made by wearing down wood with sandpaper or by means of a sand blast.
[Footnote 16: See articles by Gabriel Janka listed in bibliography, pages 151-152.]
CLEAVABILITY
_Cleavability_ is the term used to denote the facility with which wood is split. A splitting stress is one in which the forces act normally like a wedge. (See Fig. 21.) The plane of cleavage is parallel to the grain, either radially or tangentially.
[Illustration: FIG. 21.–Cleavage of highly elastic wood. The cleft runs far ahead of the wedge.]
This property of wood is very important in certain uses such as firewood, fence rails, billets, and squares. Resistance to splitting or low cleavability is desirable where wood must hold nails or screws, as in box-making. Wood usually splits more readily along the radius than parallel to the growth rings though exceptions occur, as in the case of cross grain.
Splitting involves transverse tension, but only a portion of the fibres are under stress at a time. A wood of little stiffness and strong cohesion across the grain is difficult to split, while one with great stiffness, such as longleaf pine, is easily split. The form of the grain and the presence of knots greatly affect this quality.
|———————————————| | TABLE XIII |
|———————————————| | CLEAVAGE STRENGTH OF SMALL CLEAR PIECES OF | | 32 WOODS IN GREEN CONDITION |
| (Forest Service Cir. 213) | |———————————————| | | When | When |
| COMMON NAME | surface of | surface of | | OF SPECIES | failure is | failure is | | | radial | tangential |
|——————-+————+————| | | Lbs. per | Lbs. per |
| | sq. inch | sq. inch |
| | | |
| Hardwoods | | |
| | | |
| Ash, black | 275 | 260 | | white | 333 | 346 |
| Bashwood | 130 | 168 |
| Beech | 339 | 527 |
| Birch, yellow | 294 | 287 | | Elm, slippery | 401 | 424 |
| white | 210 | 270 |
| Hackberry | 422 | 436 | | Locust, honey | 552 | 610 |
| Maple, red | 297 | 330 | | sugar | 376 | 513 |
| Oak, post | 354 | 487 | | red | 380 | 470 |
| swamp white | 428 | 536 | | white | 382 | 457 |
| yellow | 379 | 470 |
| Sycamore | 265 | 425 |
| Tupelo | 277 | 380 |
| | | |
| Conifers | | |
| | | |
| Arborvitae | 148 | 139 | | Cypress, bald | 167 | 154 |
| Fir, alpine | 130 | 133 | | Douglas | 139 | 127 |
| white | 145 | 187 |
| Hemlock | 168 | 151 |
| Pine, lodgepole | 142 | 140 | | longleaf | 187 | 180 |
| red | 161 | 154 |
| sugar | 168 | 189 |
| western yellow | 162 | 187 | | white | 144 | 160 |
| Spruce, Engelmann | 110 | 135 | | Tamarack | 167 | 159 |
|———————————————|
PART II FACTORS AFFECTING THE MECHANICAL PROPERTIES OF WOOD
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 larvae of many insects bore into the trees and their tunnels remain indefinitely as sources of weakness. Whatever advantages, however, that sapwood may have in this connection are due solely to its relative age and position.
If a tree grows all its life in the open and the conditions of soil and site remain unchanged, it will make its most rapid growth in youth, and gradually decline. The annual rings of growth are for many years quite wide, but later they become narrower and narrower. Since each succeeding ring is laid down on the outside of the wood previously formed, it follows that unless a tree materially increases its production of wood from year to year, the rings must necessarily become thinner. As a tree reaches maturity its crown becomes more open and the annual wood production is lessened, thereby reducing still more the width of the growth rings. In the case of forest-grown trees so much depends upon the competition of the trees in their struggle for light and nourishment that periods of rapid and slow growth may alternate. Some trees, such as southern oaks, maintain the same width of ring for hundreds of years. Upon the whole, however, as a tree gets larger in diameter the width of the growth rings decreases.
It is evident that there may be decided differences in the grain of heartwood and sapwood cut from a large tree, particularly one that is overmature. The relationship between width of growth rings and the mechanical properties of wood is discussed under Rate of Growth. In this connection, however, it may be stated that as a general rule the wood laid on late in the life of a tree is softer, lighter, weaker, and more even-textured than that produced earlier. It follows that in a large log the sapwood, because of the time in the life of the tree when it was grown, may be inferior in hardness, strength, and toughness to equally sound heartwood from the same log.
After exhaustive tests on a number of different woods the U.S. Forest Service concludes as follows: “Sapwood, except that from old, overmature trees, is as strong as heartwood, other things being equal, and so far as the mechanical properties go should not be regarded as a defect.”[22] Careful inspection of the individual tests made in the investigation fails to reveal any relation between the proportion of sapwood and the breaking strength of timber.
[Footnote 22: Bul. 108: Tests of structural timber, p. 35.]
In the study of the hickories the conclusion was: “There is an unfounded prejudice against the heartwood. Specifications place white hickory, or sapwood, in a higher grade than red hickory, or heartwood, though there is no inherent difference in strength. In fact, in the case of large and old hickory trees, the sapwood nearest the bark is comparatively weak, and the best wood is in the heart, though in young trees of thrifty growth the best wood is in the sap.”[23] The results of tests from selected pieces lying side by side in the same tree, and also the average values for heartwood and sapwood in shipments of the commercial hickories without selection, show conclusively that “the transformation of sapwood into heartwood does not affect either the strength or toughness of the wood…. It is true, however, that sapwood is usually more free from latent defects than heartwood.”[24]
[Footnote 23: Bul. 80: The commercial hickories, p. 50.]
[Footnote 24: _Loc. cit._]
Specifications for paving blocks often require that longleaf pine be 90 per cent heart. This is on the belief that sapwood is not only more subject to decay, but is also weaker than heartwood. In reality there is no sound basis for discrimination against sapwood on account of strength, provided other conditions are equal. It is true that sapwood will not resist decay as long as heartwood, if both are untreated with preservatives. It is especially so of woods with deep-colored heartwood, and is due to infiltrations of tannins, oils, and resins, which make the wood more or less obnoxious to decay-producing fungi. If, however, the timbers are to be treated, sapwood is not a defect; in fact, because of the relative ease with which it can be impregnated with preservatives it may be made more desirable than heartwood.[25]
[Footnote 25: Although the factor of heart or sapwood does not influence the mechanical properties of the wood and there is usually no difference in structure observable under the microscope, nevertheless sapwood is generally decidedly different from heartwood in its physical properties. It dries better and more easily than heartwood, usually with less shrinkage and little checking or honeycombing. This is especially the case with the more refractory woods, such as white oaks and _Eucalyptus globulus_ and _viminalis_. It is usually much more permeable to air, even in green wood, notably so in loblolly pine and even in white oak. As already stated, it is much more subject to decay. The sapwood of white oak may be impregnated with creosote with comparative ease, while the heartwood is practically impenetrable. These facts indicate a difference in its chemical nature.–H.D. Tiemann.]
In specifications for structural timbers reference is sometimes made to “boxheart,” meaning the inclusion of the pith or centre of the tree within a cross section of the timber. From numerous experiments it appears that the position of the pith does not bear any relation to the strength of the material. Since most season checks, however, are radial, the position of the pith may influence the resistance of a seasoned beam to horizontal shear, being greatest when the pith is located in the middle half of the section.[26]
[Footnote 26: Bul. 108, U.S. Forest Service, p. 36.]
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 deg. to 35 deg. C., in terms of water at its greatest density 4 deg. C. The reason any wood floats is that the air imprisoned in its cavities buoys it up. When this is displaced by water the wood becomes water-logged and sinks. Leaving out of consideration infiltrated substances, the reason a cubic foot of one kind of dry wood is heavier than that of another is because it contains a greater amount of wood substance. ~Density~ is merely the weight of a unit of volume, as 35 pounds per cubic foot, or 0.56 grams per cubic centimetre. ~Specific gravity~ or relative density is the ratio of the density of any material to the density of distilled water at 4 deg. C. (39.2 deg. F.). A cubic foot of distilled water at 4 deg. C. weighs 62.43 pounds. Hence the specific gravity of a piece of wood with a density of 35 pounds is 35 / 62.43 = 0.561. To find the weight per cubic foot when the specific gravity is given, simply multiply by 62.43. Thus, 0.561 X 62.43 = 35. In the metric system, since the weight of a cubic centimetre of pure water is one gram, the density in grams per cubic centimetre has the same numerical value as the specific gravity.
Since the amount of water in wood is extremely variable it usually is not satisfactory to refer to the density of green wood. For scientific purposes the density of “oven-dry” wood is used; that is, the wood is dried in an oven at a temperature of 100 deg.C. (212 deg.F.) until a constant weight is attained. For commercial purposes the weight or density of air-dry or “shipping-dry” wood is used. This is usually expressed in pounds per thousand board feet, a board foot being considered as one-twelfth of a cubic foot.
Wood shrinks greatly in drying from the green to the oven-dry condition. (See Table XIV.) Consequently a block of wood measuring a cubic foot when green will measure considerably less when oven-dry. It follows that the density of oven-dry wood does not represent the weight of the dry wood substance in a cubic foot of green wood. In other words, it is not the weight of a cubic foot of green wood minus the weight of the water which it contains. Since the latter is often a more convenient figure to use and much easier to obtain than the weight of oven-dry wood, it is commonly expressed in tables of “specific gravity or density of dry wood.”
|—————————————————————————-| | TABLE XIV | |—————————————————————————-| | SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN WOODS | | (Forest Service Cir. 213) | |—————————————————————————-| | | | Specific gravity | Shrinkage from green to | | | Mois- | oven-dry, based on | oven-dry condition | | COMMON NAME | ture |——————–+—————————| | OF SPECIES | content | Volume | Volume | In | | Tangen- | | | | when | when | volume | Radial | tial | | | | green | oven-dry | | | | |—————–+———+———+———-+——–+——–+———| | | Per | | | Per | Per | Per | | | cent | | | cent | cent | cent | | | | | | | | | | Hardwoods | | | | | | | | | | | | | | | | Ash, black | 77 | 0.466 | | | | | | white | 38 | .550 | 0.640 | 12.6 | 4.3 | 6.4 | | ” | 47 | .516 | .590 | 11.7 | | | | Basswood | 110 | .315 | .374 | 14.5 | 6.2 | 8.4 | | Beech | 61 | .556 | .669 | 16.5 | 4.6 | 10.5 | | Birch, yellow | 72 | .545 | .661 | 17.0 | 7.9 | 9.0 | | Elm, rock | 46 | .578 | | | | | | slippery | 57 | .541 | .639 | 15.5 | 5.1 | 9.9 | | white | 66 | .430 | | | | | | Gum, red | 71 | .434 | | | | | | Hackberry | 50 | .504 | .576 | 14.0 | 4.2 | 8.9 | | Hickory, | | | | | | | | big shellbark | 64 | .601 | | 17.6 | 7.4 | 11.2 | | ” ” | 55 | .666 | | 20.9 | 7.9 | 14.2 | | bitternut | 65 | .624 | | | | | | mockernut | 64 | .606 | | 16.5 | 6.9 | 10.4 | | ” | 57 | .662 | | 18.9 | 8.4 | 11.4 | | ” | 48 | .666 | | | | | | nutmeg | 76 | .558 | | | | | | pignut | 59 | .627 | | 15.0 | 5.6 | 9.8 | | ” | 54 | .667 | | 15.3 | 6.3 | 9.5 | | ” | 55 | .667 | | 16.9 | 6.8 | 10.9 | | ” | 52 | .667 | | 21.2 | 8.5 | 13.8 | | shagbark | 65 | .608 | | 16.0 | 6.5 | 10.2 | | ” | 58 | .646 | | 18.4 | 7.9 | 11.4 | | ” | 64 | .617 | | | | | | ” | 60 | .653 | | 15.5 | 6.5 | 9.7 | | water | 74 | .630 | | | | | | Locust, honey | 53 | .695 | .759 | 8.6 | | | | Maple, red | 69 | .512 | | | | | | sugar | 57 | .546 | .643 | 14.3 | 4.9 | 9.1 | | ” | 56 | .577 | | | | | | Oak, post | 64 | .590 | .732 | 16.0 | 5.7 | 10.6 | | red | 80 | .568 | .660 | 13.1 | 3.7 | 8.3 | | swamp white | 74 | .637 | .792 | 17.7 | 5.5 | 10.6 | | tanbark | 88 | .585 | | | | | | white | 58 | .594 | .704 | 15.8 | 6.2 | 8.3 | | ” | 62 | .603 | .696 | 14.3 | 4.9 | 9.0 | | ” | 78 | .600 | .708 | 16.0 | 4.8 | 9.2 | | yellow | 77 | .573 | .669 | 14.2 | 4.5 | 9.7 | | ” | 80 | .550 | | | | | | Osage orange | 31 | .761 | .838 | 8.9 | | | | Sycamore | 81 | .454 | .526 | 13.5 | 5.0 | 7.3 | | Tupelo | 121 | .475 | .545 | 12.4 | 4.4 | 7.9 | |—————————————————————————-|
|—————————————————————————-| | TABLE XIV (CONT.) | |—————————————————————————-| | SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN WOODS | | (Forest Service Cir. 213) | |—————————————————————————-| | | | Specific gravity | Shrinkage from green to | | | | oven-dry, based on | oven-dry condition | | COMMON NAME | |——————–+—————————| | OF SPECIES | Mois- | Volume | Volume | In | | Tangen- | | | ture | when | when | volume | Radial | tial | | | content | green | oven-dry | | | | |—————–+———+———+———-+——–+——–+———| | | Per | | | Per | Per | Per | | | cent | | | cent | cent | cent | | | | | | | | | | Conifers | | | | | | | | | | | | | | | | Arborvitae | 55 | .293 | .315 | 7.0 | 2.1 | 4.9 | | Cedar, incense | 80 | .363 | | | | | | Cypress, bald | 79 | .452 | .513 | 11.5 | 3.8 | 6.0 | | Fir, alpine | 47 | .306 | .321 | 9.0 | 2.5 | 7.1 | | amabilis | 117 | .383 | | | | | | Douglas | 32 | .418 | .458 | 10.9 | 3.7 | 6.6 | | white | 156 | .350 | .437 | 10.2 | 3.4 | 7.0 | | Hemlock (east.) | 129 | .340 | .394 | 9.2 | 2.3 | 5.0 | | Pine, lodgepole | 44 | .370 | .415 | 11.3 | 4.2 | 7.1 | | ” | 58 | .371 | .407 | 10.1 | 3.6 | 5.9 | | longleaf | 63 | .528 | .599 | 12.8 | 6.0 | 7.6 | | red or Nor | 54 | .440 | .507 | 11.5 | 4.5 | 7.2 | | shortleaf | 52 | .447 | | | | | | sugar | 123 | .360 | .386 | 8.4 | 2.9 | 5.6 | | west yellow | 98 | .353 | .395 | 9.2 | 4.1 | 6.4 | | ” ” | 125 | .377 | .433 | 11.5 | 4.3 | 7.3 | | ” ” | 93 | .391 | .435 | 9.9 | 3.8 | 5.8 | | white | 74 | .363 | .391 | 7.8 | 2.2 | 5.9 | | Redwood | 81 | .334 | | | | | | ” | 69 | .366 | | | | | | Spruce, | | | | | | | | Engelmann | 45 | .325 | .359 | 10.5 | 3.7 | 6.9 | | ” | 156 | .299 | .335 | 10.3 | 3.0 | 6.2 | | red | 31 | .396 | | | | | | white | 41 | .318 | | | | | | Tamarack | 52 | .491 | .558 | 13.6 | 3.7 | 7.4 | |—————————————————————————-|
This weight divided by 62.43 gives the specific gravity per green volume. It is purely a fictitious quantity. To convert this figure into actual density or specific gravity of the dry wood, it is necessary to know the amount of shrinkage in volume. If S is the percentage of shrinkage from the green to the oven-dry condition, based on the green volume; D, the density of the dry wood per cubic foot while green; and d the actual D
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 v
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 deg.F. or less is considered necessary to produce frost splits.
A still more recent theory is that of Busse[36] who considers the mechanical action of the wind a very important factor. He observed: (_a_) Frost splits sometimes occur at higher temperatures than 14 deg.F. (_b_) Most splits take place shortly before sunrise, _i.e._, at the time of lowest air and soil temperature; they are never heard to take place at noon, afternoon, or evening. (_c_) They always occur between two roots or between the collars of two roots, (_d_) They are most frequent in old, stout-rooted, broad-crowned trees; in younger stands it is always the stoutest members that are found with frost splits, while in quite young stands they are altogether absent, (_e_) Trees on wet sites are most liable to splits, due to difference in wood structure, just as difference in wood structure makes different species vary in this regard. (_f_) Frost splits are most numerous less than three feet above the 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 larvae of many insects are destructive to wood. Some attack the wood of living trees, others only that of felled or converted material. Every hole breaks the continuity of the fibres and impairs the strength, and if there are very many of them the material may be ruined for all purposes where strength is required.
Some of the most common insects attacking the wood of living trees are the oak timber worm, the chestnut timber worm, carpenter worms, ambrosia beetles, the locust borer, turpentine beetles and turpentine borers, and the white pine weevil.
The insect injuries to forest products may be classed according to the stage of manufacture of the material. Thus round timber with the bark on, such as poles, posts, mine props, and sawlogs, is subject to serious damage by the same class of insects as those mentioned above, particularly by the round-headed borers, timber worms, and ambrosia beetles. Manufactured unseasoned products are subject to damage from ambrosia beetles and other wood borers. Seasoned hardwood lumber of all kinds, rough handles, wagon stock, etc., made partially or entirely of sapwood, are often reduced in value from 10 to 90 per cent by a class of insects known as powder-post beetles. Finished hardwood products such as handles, wagon, carriage and machinery stock, especially if ash or hickory, are often destroyed by the powder-post beetles. Construction timbers in buildings, bridges and trestles, cross-ties, poles, mine props, fence posts, etc., are sometimes seriously injured by wood-boring larvae, termites, black ants, carpenter bees, and powder-post beetles, and sometimes reduced in value from 10 to 100 per cent. In tropical countries termites are a very serious pest in this respect.
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 deg.F. and is said to be killed by a temperature of 110 deg.F.[41] Fully 70 per cent humidity is necessary in the air in which a timber is surrounded for the growth of this fungus, and probably the wood must be quite near its fibre saturation condition. Nevertheless _Merulius lacrymans_ (one of the most important species) has been found to live four years and eight months in a dry condition.[42] Thorough kiln-drying will kill this fungus, but will not prevent its redevelopment. Antiseptic treatment, such as creosoting, is the best prevention.
[Footnote 41: Falck, Richard: Die Meruliusfauele des Bauholzes, Hausschwammforschungen, 6. Heft., Jena, 1912.]
[Footnote 42: Mez, Carl: Der Hausschwamm. Dresden, 1908, p. 63.]
All fungi require moisture and air[43] for their growth. Deprived of either of these the fungus dies or ceases to develop. Just what degree of moisture in wood is necessary for the “dry-rot” fungus has not been determined, but it is evidently considerably above that of thoroughly air-dry timber, probably more than 15 per cent moisture. Hence the importance of free circulation of air about all timbers in a building.
[Footnote 43: A culture of fungus placed in a glass jar and the air pumped out ceases to grow, but will start again as soon as oxygen is admitted.]
Warmth is also conducive to the growth of fungi, the most favorable temperature being about 90 deg.F. They cannot grow in extreme cold, although no degree of cold such as occurs naturally will kill them. On the other hand, high temperature will kill them, but the spores may survive even the boiling temperature. Mould fungus has been observed to develop rapidly at 130 deg.F. in a dry kiln in moist air, a condition under which an animal cannot live more than a few minutes. This fungus was killed, however, at about 140 deg. or 145 deg.F.[44]
[Footnote 44: Experiments in kiln-drying _Eucalyptus_ in Berkeley, U.S. Forest Service.]
The fungus (_Endothia parasitica_ And.) which causes the chestnut blight kills the trees by girdling them and has no direct effect upon the wood save possibly the four or five growth rings of the sapwood.[45]
[Footnote 45: See Anderson, Paul J.: The morphology and life history of the chestnut blight fungus. Bul. No. 7, Penna. Chestnut Tree Blight Com., Harrisburg, 1914, p. 17.]
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) | |———————————————————————-| | KIND OF STRENGTH | Longleaf | Spruce | Chestnut | | | pine | | | |—————————-+————-+————-+————-| | | (1) (2) | (1) (2) | (1) (2) | | | | | | | Crushing strength parallel | | | | | to grain | 2.89 2.60 | 3.71 3.41 | 2.83 2.55 | | Elastic limit in | | | | | compression | | | | | parallel to grain | 2.60 2.34 | 3.80 3.49 | 2.40 2.26 | | Modulus of rupture in | | | | | bending | 2.50 2.20 | 2.81 2.50 | 2.09 1.82 | | Stress at elastic limit in | | | | | bending | 2.90 2.55 | 2.90 2.58 | 2.30 2.00 | | Crushing strength at right | | | | | angles to grain | | 2.58 2.48 | | | Shearing strength parallel | | | | | to grain | 2.01 1.91 | 2.03 1.95 | 1.55 1.47 | | Modulus of elasticity in | | | | | compression parallel to | | | | | grain | 1.63 1.47 | 2.26 2.08 | 1.43 1.29 | | Modulus of elasticity in | | | | | bending | 1.59 1.35 | 1.43 1.23 | 1.44 1.21 | |———————————————————————-| | NOTE.–The figures in the first column show the relative increase in | | strength between a green specimen and a kiln-dry specimen of equal | | size. The figures in the second column show the relative increase of | | strength of the same block after being dried from a green condition | | to 3.5 per cent moisture, correction having been made for shrinkage. | | That is, in the first column the strength values per actual unit of | | area are used; in the second the values per unit of area of green | | wood which shrinks to smaller size when dried. | | |
| See also Cir. 108, Fig. 1, p. 8. | |———————————————————————-|
The moisture content has a decided bearing also upon the manner in which wood fails. In compression tests on very dry specimens the entire piece splits suddenly into pieces before any buckling takes place (see Fig. 9.), while with wet material the block gives way gradually, due to the buckling or bending of the walls of the fibres along one or more shearing planes. (See Fig. 14.) In bending tests on wet beams, first failure occurs by compression on top of the beam, gradually extending downward toward the neutral axis. Finally the beam ruptures at the bottom. In the case of very dry beams the failure is usually by splitting or tension on the under side (see Fig. 17.), without compression on the upper, and is often sudden and without warning, and even while the load is still increasing. The effect varies somewhat with different species, chestnut, for example, becoming more brittle upon drying than do ash, hemlock, and longleaf pine. The tensile strength of wood is least affected by drying, as a rule.
In drying wood no increase in strength results until the free water is evaporated and the cell walls begin to dry[49]. This critical point has been called the _fibre-saturation point_. (See Fig. 24.) Conversely, after the cell walls are saturated with water, any increase in the amount of water absorbed merely fills the cavities and intercellular spaces, and has no effect on the mechanical properties. Hence, soaking green wood does not lessen its strength unless the water is heated, whereupon a decided weakening results.
[Footnote 49: The wood of _Eucalyptus globulus_ (blue gum) appears to be an exception to this rule. Tiemann says: “The wood of blue gum begins to shrink immediately from the green condition, even at 70 to 90 per cent moisture content, instead of from 30 or 25 per cent as in other species of hardwoods.” Proc. Soc. Am. For., Washington, Vol. VIII, No. 3, Oct., 1913, p. 313.]
[Illustration: FIG. 24.–Relation of the moisture content to the various strength values of spruce. FSP = fibre-saturation 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 deg. F.; in the case of spruce the decrease is 0.32 per cent.
The effects of high temperature on wet wood are very marked. Boiling produces a condition of great pliability, especially in the case of hardwoods. If wood in this condition is bent and allowed to dry, it rigidly retains the shape of the bend, though its strength may be somewhat reduced. Except in the case of very dry wood the effect of cold is to increase the strength and stiffness of wood. The freezing of any free water in the pores of the wood will augment these conditions.
The effect of steaming upon the strength of cross-ties was investigated by the U.S. Forest Service in 1904. The conclusions were summarized as follows:
“(1) The steam at pressure up to 40 pounds applied for 4 hours, or at a pressure of 20 pounds up to 20 hours, increases the weight of ties. At 40 pounds’ pressure applied for 4 hours and at 20 pounds for 5 hours the wood began to be scorched.
“(2) The steamed and saturated wood, when tested immediately after treatment, exhibited weaknesses in proportion to the pressure and duration of steaming. (See Table XVI.) If allowed to air-dry subsequently the specimens regained the greater part of their strength, provided the pressure and duration had not exceeded those cited under (1). Subsequent immersion in water of the steamed wood and dried specimens showed that they were weaker than natural wood similarly dried and resoaked.”[51]
[Footnote 51: Cir. 39. Experiments on the strength of treated timber, p. 18.]
|——————————————————————————————| | TABLE XVI | |——————————————————————————————| | EFFECT OF STEAMING ON THE STRENGTH OF GREEN LOBLOLLY PINE | | (Forest Service, Cir. 39) | |——————————————————————————————| | | Cylinder conditions | Strength | | |———————————+——————————————–| | | Steaming | Static | Impact | |