THE SHRINKAGE OF WOOD.
When a tree is cut down, its water at once begins to evaporate. This process is called "seasoning."* 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.
* 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.
Fig. 35. How Cell Walls Shrink.
(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.
Fig. 36. The Shrinkage and Checking at the End of a Beam.
(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.
Fig. 37. The Shrinkage and Splitting of a Log.
(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.
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 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.
Fig. 39. Shrinkage of a Halved Log.
If a log is "quartered,"* 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.
* See Handwork in Wood, p. 42.
Fig. 40. Shapes Assumed 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.
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.