CHAPTER II.

Chapter 24,692 wordsPublic domain

PROPERTIES OF WOOD.

There are many properties of wood,--some predominant in one species, some in another,--that make it suitable for a great variety of uses. Sometimes it is a combination of properties that gives value to a wood. Among these properties are hygroscopicity, shrinkage, weight, strength, cleavability, elasticity, hardness, and toughness.

THE HYGROSCOPICITY[1] OF WOOD.

It is evident that water plays a large part in the economy of the tree. It occurs in wood in three different ways: In the sap which fills or partly fills the cavities of the wood cells, in the cell walls which it saturates, and in the live protoplasm, of which it constitutes 90 per cent. The younger the wood, the more water it contains, hence the sap-wood contains much more than the heart-wood, at times even twice as much.

In fresh sap-wood, 60 per cent. of the water is in the cell cavities, 35 per cent. in the cell walls, and only 5 per cent. in the protoplasm. There is so much water in green wood that a sappy pole will soon sink when set afloat. The reason why there is much less water in heart-wood is because its cells are dead and inactive, and hence without sap and without protoplasm. There is only what saturates the cell walls. Even so, there is considerable water in heart-wood.[2]

The lighter kinds have the most water in the sap-wood, thus sycamore has more than hickory.

Curiously enough, a tree contains about as much water in winter as in summer. The water is held there, it is supposed, by capillary attraction, since the cells are inactive, so that at all times the water in wood keeps the cell walls distended.

THE SHRINKAGE OF WOOD.

When a tree is cut down, its water at once begins to evaporate. This process is called "seasoning."[A] In drying, the free water within the cells keeps the cell walls saturated; but when all the free water has been removed, the cell walls begin to yield up their moisture. Water will not flow out of wood unless it is forced out by heat, as when green wood is put on a fire. Ordinarily it evaporates slowly.

[Footnote A: See _Handwork in Wood_, Chapter III.]

The water evaporates faster from some kinds of wood than from other kinds, _e.g._, from white pine than from oak, from small pieces than from large, and from end grain than from a longitudinal section; and it also evaporates faster in high than in low temperatures.

Evaporation affects wood in three respects, weight, strength, and size. The weight is reduced, the strength is increased, and shrinkage takes place. The reduction in weight and increase in strength, important as they are, are of less importance than the shrinkage, which often involves warping and other distortions. The water in wood affects its size by keeping the cell walls distended.

If all the cells of a piece of wood were the same size, and had walls the same thickness, and all ran in the same direction, then the shrinkage would be uniform. But, as we have seen, the structure of wood is not homogeneous. Some cellular elements are large, some small, some have thick walls, some thin walls, some run longitudinally and some (the pith rays) run radially. The effects will be various in differently shaped pieces of wood but they can easily be accounted for if one bears in mind these three facts: (1) that the shrinkage is in the cell wall, and therefore (2) that the thick-walled cells shrink more than thin-walled cells and (3) that the cells do not shrink much, if any, lengthwise.

(1) The shrinkage of wood takes place in the walls of the cells that compose it, that is, the cell walls become thinner, as indicated by the dotted lines in Fig. 35, which is a cross-section of a single cell. The diameter of the whole cell becomes less, and the opening, or lumen, of the cell becomes larger.

(2) Thick-walled cells shrink more than thin-walled cells, that is, summer cells more than spring cells. This is due to the fact that they contain more shrinkable substance. The thicker the wall, the more the shrinkage.

Consider the effects of these changes; ordinarily a log when drying begins to "check" at the end. This is to be explained thus: Inasmuch as evaporation takes place faster from a cross than from a longitudinal section, because at the cross-section all the cells are cut open, it is to be expected that the end of a piece of timber, Fig. 36, A, will shrink first. This would tend to make the end fibers bend toward the center of the piece as in B, Fig. 36. But the fibers are stiff and resist this bending with the result that the end splits or "checks" as in C, Fig. 36. But later, as the rest of the timber dries out and shrinks, it becomes of equal thickness again and the "checks" tend to close.

(3) For some reason, which has not been discovered, the cells or fibers of wood do not shrink in length to any appreciable extent. This is as true of the cells of pith rays, which run radially in the log, as of the ordinary cells, which run longitudinally in it.

In addition to "checking" at the end, logs ordinarily show the effect of shrinkage by splitting open radially, as in Fig. 37. This is to be explained by two factors, (1) the disposition of the pith (or medullary) rays, and (2) the arrangement of the wood in annual rings.

(1) The cells of the pith rays, as we have seen in Chapter I, run at right angles to the direction of the mass of wood fibers, and since they shrink according to the same laws that other cells do, viz., by the cell wall becoming thinner but not shorter, the strain of their shrinkage is contrary to that of the main cells. The pith rays, which consist of a number of cells one above the other, tend to shrink parallel to the length of the wood, and whatever little longitudinal shrinkage there is in a board is probably due mostly to the shrinkage of the pith rays. But because the cells of pith rays do not appreciably shrink in their length, this fact tends to prevent the main body of wood from shrinking radially, and the result is that wood shrinks less radially than tangentially. Tangentially is the only way left for it to shrink. The pith rays may be compared to the ribs of a folding fan, which keep the radius of unaltered length while permitting comparative freedom for circumferential contraction.

(2) It is evident that since summer wood shrinks more than spring wood, this fact will interfere with the even shrinkage of the log. Consider first the tangential shrinkage. If a section of a single annual ring of green wood of the shape A B C D, in Fig. 38, is dried and the mass shrinks according to the thickness of the cell walls, it will assume the shape A' B' C' D'. When a number of rings together shrink, the tangential shrinkage of the summer wood tends to contract the adjoining rings of spring wood more than they would naturally shrink of themselves. Since there is more of the summer-wood substance, the spring-wood must yield, and the log shrinks circumferentially. The radial shrinkage of the summer-wood, however, is constantly interrupted by the alternate rows of spring-wood, so that there would not be so much radial as circumferential shrinkage. As a matter of fact, the tangential or circumferential shrinkage is twice as great as the radial shrinkage.

Putting these two factors together, namely, the lengthwise resistance of the pith rays to the radial shrinkage of the mass of other fibers, and second, the continuous bands of summer wood, comparatively free to shrink circumferentially, and the inevitable happens; the log splits. If the bark is left on and evaporation hindered, the splits will not open so wide.

There is still another effect of shrinkage. If, immediately after felling, a log is sawn in two lengthwise, the radial splitting may be largely avoided, but the flat sides will tend to become convex, as in Fig. 39. This is explained by the fact that circumferential shrinkage is greater than radial shrinkage.

If a log is "quartered,"[A] the quarters split still less, as the inevitable shrinkage takes place more easily. The quarters then tend to assume the shape shown in Fig. 40, C. If a log is sawed into timber, it checks from the center of the faces toward the pith, Fig. 40, D. Sometimes the whole amount of shrinkage may be collected in one large split. When a log is slash-sawed, Fig. 40, I, each board tends to warp so that the concave side is away from the center of the tree. If one plank includes the pith, Fig. 40, E and H, that board will become thinner at its edges than at its center, _i.e._, convex on both faces. Other forms assumed by wood in shrinking are shown in Fig. 40. In the cases A-F the explanation is the same; the circumferential shrinkage is more than the radial. In J and K the shapes are accounted for by the fact that wood shrinks very little longitudinally.

[Footnote A: See _Handwork in Wood_, p. 42.]

Warping is uneven shrinkage, one side of the board contracting more than the other. Whenever a slash board warps under ordinary conditions, the convex side is the one which was toward the center of the tree. However, a board may be made to warp artificially the other way by applying heat to the side of the board toward the center of the tree, and by keeping the other side moist. The board will warp only sidewise; lengthwise it remains straight unless the treatment is very severe. This shows again that water distends the cells laterally but not longitudinally.

The thinning of the cell walls due to evaporation, is thus seen to have three results, all included in the term "working," viz.: _shrinkage_, a diminution in size, _splitting_, due to the inability of parts to cohere under the strains to which they are subjected, and _warping_, or uneven shrinkage.

In order to neutralize warping as much as possible in broad board structures, it is common to joint the board with the annual rings of each alternate board curving in opposite directions, as shown in _Handwork in Wood_, Fig. 280, _a_, p. 188.

Under warping is included bowing. Bowing, that is, bending in the form of a bow, is, so to speak, longitudinal warping. It is largely due to crookedness or irregularity of grain, and is likely to occur in boards with large pith rays, as oak and sycamore. But even a straight-grained piece of wood, left standing on end or subjected to heat on one side and dampness on the other, will bow, as, for instance a board lying on the damp ground and in the sun.

Splitting takes various names, according to its form in the tree. "Check" is a term used for all sorts of cracks, and more particularly for a longitudinal crack in timber. "Shakes" are splits of various forms as: _star shakes_, Fig. 41, _a_, splits which radiate from the pith along the pith rays and widen outward; _heart shakes_, Fig. 41, _b_, splits crossing the central rings and widening toward the center; and _cup_ or _ring shakes_, Fig. 41, _c_, splits between the annual rings. _Honeycombing_, Fig. 41, _d_, is splitting along the pith rays and is due largely to case hardening.

These are not all due to shrinkage in drying, but may occur in the growing tree from various harmful causes. See p. 232.

Wood that has once been dried may again be swelled to nearly if not fully its original size, by being soaked in water or subjected to wet steam. This fact is taken advantage of in wetting wooden wedges to split some kinds of soft stone. The processes of shrinking and swelling can be repeated indefinitely, and no temperature short of burning, completely prevents wood from shrinking and swelling.

Rapid drying of wood tends to "case harden" it, _i.e._, to dry and shrink the outer part before the inside has had a chance to do the same. This results in checking separately both the outside and the inside, hence special precautions need to be taken in the seasoning of wood to prevent this. When wood is once thoroly bent out of shape in shrinking, it is very difficult to straighten it again.

Woods vary considerably in the amounts of their shrinkage. The conifers with their regular structure shrink less and shrink more evenly than the broad-leaved woods.[3] Wood, even after it has been well seasoned, is subject to frequent changes in volume due to the varying amount of moisture in the atmosphere. This involves constant care in handling it and wisdom in its use. These matters are considered in _Handwork in Wood_, Chapter III, on the Seasoning of Wood.

THE WEIGHT OF WOOD.

Wood substance itself is heavier than water, as can readily be proved by immersing a very thin cross-section of pine in water. Since the cells are cut across, the water readily enters the cavities, and the wood being heavier than the water, sinks. In fact, it is the air enclosed in the cell cavities that ordinarily keeps wood afloat, just as it does a corked empty bottle, altho glass is heavier than water. A longitudinal shaving of pine will float longer than a cross shaving for the simple reason that it takes longer for the water to penetrate the cells, and a good sized white pine log would be years in getting water-soaked enough to sink. As long as a majority of the cells are filled with air it would float.

In any given piece of wood, then, the weight is determined by two factors, the amount of wood substance and the amount of water contained therein. The amount of wood substance is constant, but the amount of water contained is variable, and hence the weight varies accordingly. Moreover, considering the wood substance alone, the weight of wood substance of different kinds of wood is about the same; namely, 1.6 times as heavy as water, whether it is oak or pine, ebony or poplar. The reason why a given bulk of some woods is lighter than an equal bulk of others, is because there are more thin-walled and air-filled cells in the light woods. Many hard woods, as lignum vitae, are so heavy that they will not float at all. This is because the wall of the wood cells is very thick, and the lumina are small.

In order, then, to find out the comparative weights of different woods, that is, to see how much wood substance there is in a given volume of any wood, it is necessary to test absolutely dry specimens.

The weight of wood is indicated either as the weight per cubic foot or as specific gravity.

It is an interesting fact that different parts of the same tree have different weights, the wood at the base of the tree weighing more than that higher up, and the wood midway between the pith and bark weighing more than either the center or the outside.[4]

The weight of wood has a very important bearing upon its use. A mallet-head, for example, needs weight in a small volume, but it must also be tough to resist shocks, and elastic so as to impart its momentum gradually and not all at once, as an iron head does.

Weight is important, too, in objects of wood that are movable. The lighter the wood the better, if it is strong enough. That is why spruce is valuable for ladders; it is both light and strong. Chestnut would be a valuable wood for furniture if it were not weak, especially in the spring wood.

The weight of wood is one measure of its strength. Heavy wood is stronger than light wood of the same kind, for the simple reason that weight and strength are dependent upon the number and compactness of the fibers.[5]

THE STRENGTH OF WOOD.

Strength is a factor of prime importance in wood. By strength is meant the ability to resist stresses, either of tension (pulling), or of compression (pushing), or both together, cross stresses. When a horizontal timber is subjected to a downward cross stress, the lower half is under tension, the upper half is under compression and the line between is called the neutral axis, Fig. 42.

Wood is much stronger than is commonly supposed. A hickory bar will stand more strain under tension than a wrought iron bar of the same length and weight, and a block of long-leaf pine a greater compression endwise than a block of wrought iron of the same height and weight. It approaches the strength of cast iron under the same conditions.

Strength depends on two factors: the strength of the individual fibers, and the adhesive power of the fibers to each other. So, when a piece of wood is pulled apart, some of the fibers break and some are pulled out from among their neighbors. Under compression, however, the fibers seem to act quite independently of each other, each bending over like the strands of a rope when the ends are pushed together. As a consequence, we find that wood is far stronger under tension than under compression, varying from two to four times.

Woods do not vary nearly so much under compression as under tension, the straight-grained conifers, like larch and longleaf pine, being nearly as strong under compression as the hard woods, like hickory and elm, which have entangled fibers, whereas the hard woods are nearly twice as strong as the conifers under tension.

Moisture has more effect on the strength of wood than any other extrinsic condition. In sound wood under ordinary conditions, it outweighs all other causes which affect strength. When thoroly seasoned, wood is two or three times stronger, both under compression and in bending, than when green or water soaked.[6]

The tension or pulling strength of wood is much affected by the direction of the grain, a cross-grained piece being only 1/10th to 1/20th as strong as a straight-grained piece. But under compression there is not much difference; so that if a timber is to be subjected to cross strain, that is the lower half under tension and the upper half under compression, a knot or other cross-grained portion should be in the upper half.

Strength also includes the ability to resist shear. This is called "_shearing strength_." It is a measure of the adhesion of one part of the wood to an adjoining part. Shearing is what takes place when the portion of wood beyond a mortise near the end of a timber, A B C D, Fig. 43, is forced out by the tenon. In this case it would be shearing along the grain, sometimes called detrusion. The resistance of the portion A B C D, _i.e._, its power of adhesion to the wood adjacent to it on both sides, is its shearing strength. If the mortised piece were forced downward until it broke off the tenon at the shoulder, that would be shearing across the grain. The shearing resistance either with or across the grain is small compared with tension and compression. Green wood shears much more easily than dry, because moisture softens the wood and this reduces the adhesion of the fibers to each other.[7]

CLEAVABILITY OF WOOD.

Closely connected with shearing strength is cohesion, a property usually considered under the name of its opposite, cleavability, _i.e._, the ease of splitting.

When an ax is stuck into the end of a piece of wood, the wood splits in advance of the ax edge. See _Handwork in Wood_, Fig. 59, p. 52. The wood is not cut but pulled across the grain just as truly as if one edge were held and a weight were attached to the other edge and it were torn apart by tension. The length of the cleft ahead of the blade is determined by the elasticity of the wood. The longer the cleft, the easier to split. Elasticity helps splitting, and shearing strength and hardness hinder it.

A normal piece of wood splits easily along two surfaces, (1) along any radial plane, principally because of the presence of the pith rays, and, in regular grained wood like pine, because the cells are radially regular; and (2) along the annual rings, because the spring-wood separates easily from the next ring of summer-wood. Of the two, radial cleavage is 50 to 100 per cent. easier. Straight-grained wood is much easier to split than cross-grained wood in which the fibers are interlaced, and soft wood, provided it is elastic, splits easier than hard. Woods with sharp contrast between spring and summer wood, like yellow pine and chestnut, split very easily tangentially.

All these facts are important in relation to the use of nails. For instance, the reason why yellow pine is hard to nail and bass easy is because of their difference in cleavability.

ELASTICITY OF WOOD.

Elasticity is the ability of a substance when forced out of shape,--bent, twisted, compressed or stretched, to regain its former shape. When the elasticity of wood is spoken of, its ability to spring back from bending is usually meant. The opposite of elasticity is brittleness. Hickory is elastic, white pine is brittle.

Stiffness is the ability to resist bending, and hence is the opposite of pliability or flexibility. A wood may be both stiff and elastic; it may be even stiff and pliable, as ash, which may be made into splints for baskets and may also be used for oars. Willow sprouts are flexible when green, but quite brittle when dry.

Elasticity is of great importance in some uses of wood, as in long tool handles used in agricultural implements, such as rakes, hoes, scythes, and in axes, in archery bows, in golf sticks, etc., in all of which, hickory, our most elastic wood, is used.[8]

HARDNESS OF WOOD.

Hardness is the ability of wood to resist indentations, and depends primarily upon the thickness of the cell walls and the smallness of the cell cavities, or, in general, upon the density of the wood structure. Summer wood, as we have seen, is much harder than spring wood, hence it is important in using such wood as yellow pine on floors to use comb-grain boards, so as to present the softer spring wood in as narrow surfaces as possible. See _Handwork in Wood_, p. 41, and Fig. 55. In slash-grain boards, broad surfaces of both spring and summer wood appear. Maple which is uniformly hard makes the best floors, even better than oak, parts of which are comparatively soft.

The hardness of wood is of much consequence in gluing pieces together. Soft woods, like pine, can be glued easily, because the fibers can be forced close together. As a matter of fact, the joint when dry is stronger than the rest of the board. In gluing hard woods, however, it is necessary to scratch the surfaces to be glued in order to insure a strong joint. It is for the same reason that a joint made with liquid glue is safe on soft wood when it would be weak on hard wood.[9]

TOUGHNESS OF WOOD.

Toughness may be defined as the ability to resist sudden shocks and blows. This requires a combination of various qualities, strength, hardness, elasticity and pliability. The tough woods, _par excellence_, are hickory, rock elm and ash. They can be pounded, pulled, compressed and sheared. It is because of this quality that hickory is used for wheel spokes and for handles, elm for hubs, etc.

In the selection of wood for particular purposes, it is sometimes one, sometimes another, and more often still, a combination of qualities that makes it fit for use.[10]

It will be remembered that it was knowledge of the special values of different woods that made "the one horse shay," "The Deacon's Masterpiece."

"So the Deacon inquired of the village folk Where he could find the strongest oak, That couldn't be split nor bent nor broke,-- That was for spokes and floor and sills; He sent for lancewood to make the thills; The cross bars were ash, from the straightest trees, The panels of whitewood, that cuts like cheese, But lasts like iron for things like these. The hubs of logs from the "Settler's Ellum,"-- Last of its timber,--they couldn't sell 'em. Never an ax had seen their chips, And the wedges flew from between their lips, Their blunt ends frizzled like celery tips; Step and prop-iron, bolt and screw, Spring, tire, axle and linch pin too, Steel of the finest, bright and blue; Thorough brace, bison skin, thick and wide; Boot, top dasher from tough old hide, Found in the pit when the tanner died. That was the way to "put her through." 'There!' said the Deacon, 'naow she'll dew!'"

[Footnote 1: Hygroscopicity, "the property possessed by vegetable tissues of absorbing or discharging moisture and expanding or shrinking accordingly."--_Century Dictionary._]

[Footnote 2: This is shown by the following table, from Forestry Bulletin No. 10, p. 31, _Timber_, by Filibert Roth:

POUNDS OF WATER LOST IN DRYING 100 POUNDS OF GREEN WOOD IN THE KILN.

Sap-wood or Heart-wood outer part. or interior.

1. Pines, cedars, spruces, and firs 45-65 16-25 2. Cypress, extremely variable 50-65 18-60 3. Poplar, cottonwood, basswood 60-65 40-60 4. Oak, beech, ash, elm, maple, birch, hickory, chestnut, walnut, and sycamore 40-50 30-40 ]

[Footnote 3: The following table from Roth, p. 37, gives the approximate shrinkage of a board, or set of boards, 100 inches wide, drying in the open air:

Shrinkage Inches. 1. All light conifers (soft pine, spruce, cedar, cypress) 3

2. Heavy conifers (hard pine, tamarack, yew, honey locust, box elder, wood of old oaks) 4

3. Ash, elm, walnut, poplar, maple, beech, sycamore, cherry, black locust 5

4. Basswood, birch, chestnut, horse chestnut, blue beech, young locust 6

5. Hickory, young oak, especially red oak Up to 10

The figures are the average of radial and tangential shrinkages.]

[Footnote 4: How much different woods vary may be seen by the following table, taken from Filibert Roth, _Timber_, Forest Service Bulletin No. 10, p. 28:

WEIGHT OF KILN-DRIED WOOD OF DIFFERENT SPECIES.

]

[Footnote 5: For table of weights of different woods see Sargent, _Jesup Collection,_ pp. 153-157.]

[Footnote 6: See Forestry Bulletin No. 70, pp. 11, 12, and Forestry Circular No. 108.]

[Footnote 7: For table of strengths of different woods, see Sargent, _Jesup Collection_, pp. 166 ff.]

[Footnote 8: For table of elasticity of different woods, see Sargent, _Jesup Collection_, pp. 163 ff.]

[Footnote 9: For table of hardnesses of different woods, see Sargent, _Jesup Collection_, pp. 173 ff.]

[Footnote 10: For detailed characteristics of different woods see Chapter III.]

THE PROPERTIES OF WOOD.

REFERENCES[A]

Moisture and Shrinkage.

Roth, _For. Bull._, No. 10, pp. 25-37. Busbridge, _Sci. Am. Sup._ No. 1500. Oct. 1, '04.

Weight, Strength, Cleavability, Elasticity and Toughness.

Roth, _For. Bull._, 10, p. 37-50. Boulger, pp. 89-108, 129-140. Roth, _First Book_, pp. 229-233. Sargent, _Jesup Collection_, pp. 153-176.

Forest Circulars Nos. 108 and 139.

[Footnote A: For general bibliography, see p. 4.]