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 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.