Wood and Forest Part 7

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 ma.s.s shrinks according to the thickness of the cell walls, it will a.s.sume 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 circ.u.mferentially. 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 circ.u.mferential shrinkage.

As a matter of fact, the tangential or circ.u.mferential shrinkage is twice as great as the radial shrinkage.

[Ill.u.s.tration: Fig. 38. Diagram to Show the Greater Shrinkage of Summer Cells, A, B, than of Spring Cells, C, D.]

Putting these two factors together, namely, the lengthwise resistance of the pith rays to the radial shrinkage of the ma.s.s of other fibers, and second, the continuous bands of summer wood, comparatively free to shrink circ.u.mferentially, 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 circ.u.mferential shrinkage is greater than radial shrinkage.

[Ill.u.s.tration: Fig. 39. Shrinkage of a Halved Log.]

If a log is "quartered,"[A] the quarters split still less, as the inevitable shrinkage takes place more easily. The quarters then tend to a.s.sume 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 a.s.sumed by wood in shrinking are shown in Fig.

40. In the cases A-F the explanation is the same; the circ.u.mferential 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.]

[Ill.u.s.tration: Fig. 40. Shapes a.s.sumed by Wood in Shrinking.]

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.

[Ill.u.s.tration: Fig. 41. _a_, Star Shakes; _b_, Heart Shakes; _c_, Cup Shakes or Ring Shakes; _d_, Honeycombing.]

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 gla.s.s 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.

[Ill.u.s.tration: Fig. 42. A Timber Under Cross Stress, Showing Neutral Axis, and the Lines of Tension and Compression. A knot occurring in such a timber should be in the upper half, as at A.]

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.

[Ill.u.s.tration: Fig. 43. Shearing Strength is Measured by the Adhesion of the Portion A, B, C, D or to the Wood on both sides of it.]

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, princ.i.p.ally 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 ba.s.s 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.