* 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.