17. When large trees are converted into timber it sometimes occurs that parts of a board or plank separate from each other and become two pieces, and occasionally the wood is so "shaky" as to render it utterly useless as timber. This separation of the wood is due to " shakes " which are formed in the living tree.
These shakes are of two kinds: A. Heart or star shakes, which are splits or clefts occurring in the centre of the tree, as shown in Fig. 13, They are common in nearly every kind of tree, but unless the cracks are very large they do no great harm. B. Cup shakes or cracks separating one layer from another, as shown in Figs. 14 and 15, It has been commonly supposed that they are produced by the wrenching of the tree during heavy wind storms, but a recent English writer believes that they, and also the heart shakes, are produced by the expanding of the sapwood.
Cup shakes often injure oak, hard pine, mahogany, walnut and elm, and are the chief defect in hemlock timber. Trees less than 10 inches in diameter are not subject to shakes.