Shrinkage is a term applied to the decrease in diameter of the tree, due to sap evaporation.
Fig. 240. Effect of shrinkage on lumber
In the case of the board it means a decrease in width, and it varies greatly in different trees and their woods. As shrinkage is always across the grain, its effect on a common joint may be illustrated in [Fig. 240]. At a is shown a middle lap joint just put together. If the wood is not well seasoned, shrinkage will in time change it to the form shown at b, which is exaggerated to make the meaning clear.
A square piece of timber, one corner of which is the centre of the tree, will change from c to d. Shrinkage as well as warping must be taken into consideration in construction.
Fig. 241. The development of the panelled door
The development of the panelled door is a good illustration. Suppose we wish to close a space with a door, knowing little about shrinkage. Let us construct it by the simplest method, say four vertical boards. If the width of these boards equals the opening when the door is built ([Fig. 241]) there will soon be an opening wide enough for the fingers to enter and lift up a latch on the inside. The door is very much of a failure. We notice, however, that there is no opening at top or bottom. An idea! We construct a door with planks placed horizontally. Now although we find after a while no opening at the sides we do find openings at top and bottom. The panelled door is not constructed solely for beauty but to overcome shrinkage as far as possible. [Fig. 241] shows the various parts. The rails maintain the width, the only shrinkage being in the cross grain of the stiles, and they preserve the height except for the small amount in the rails. The remaining spaces are panelled, the construction being shown at a. Both stiles and rails have a groove plowed out to receive the edges of the panel. This should be free to shrink in the grooves, where it is invisible, but if the mistake is made of fastening the panel edges rigidly in these grooves the panel will shrink anyway and frequently split from top to bottom.