Where but one side of a board is seen or used and where the full strength is not needed, warping and twisting can be largely prevented by lengthways saw-cuts on the back or under surface, as in a drawing-board, the crossways strength required being secured by the cleats. Doors and most forms of panelled work also illustrate these matters of swelling and shrinking (see Doors and Panels).
Fig. 702.
Shakes.—Heart-shakes are cracks radiating from the centre in the line of the medullary rays, widest at the pith and narrowing toward the outside, and supposed to be chiefly caused by the shrinkage of the older wood due to the beginning of decay while the tree is standing (Fig. 702). Slight heart-shakes are common, but if large and numerous or twisting in the length of the log, they injure the timber seriously for cutting up.
Fig. 703.
Star-shakes are also radiating cracks, but, unlike the heart-shakes, the cracks are widest at the outside, narrowing toward the centre (Fig. 703), and are often caused by the shrinkage of the outer part due to the outside of the tree drying faster than the inside, as it naturally does from being more exposed after being felled; but they are sometimes owing to the beginning of decay and other causes.
Fig. 704.
Cup-shakes are cracks between some of the annual rings, separating the layers more or less (Fig. 704), sometimes reaching entirely around, separating the centre from the outer portion, and are supposed to be caused by the swaying of the tree in the wind (hence sometimes known as wind-shakes), or to some shock or extreme changes of temperature, or other causes.