Part 7 (1/2)

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.