Why is green wood heavier and softer than dry wood, and the sapwood of green timber softer than the heart? Because of the sap or water contained. The amount of water is sometimes even as much as fifty per cent. of the weight of the wood, but the quantity depends upon the kind of tree, the season, etc. Now the more water the green log contains, the more it will shrink. It begins to dry and shrink as soon as the tree has been cut down. The sapwood shrinks more than the heart because it contains more water, and faster because, being on the outside, it is more exposed. The log shrinks most in the line of the annual rings, that is, around the tree. It shrinks much less in the line of the medullary rays, that is, across the tree. Shrinkage lengthways is too slight to be considered[9] (Fig. 11).
The result of all this unequal shrinking is that the log tends to split, or crack open, at the circumference (Fig. 12), the cracks running in toward the centre, in the line of the medullary rays. If the log is halved or quartered, so that the inner parts are exposed, the drying goes on more uniformly throughout, the cracking is not so bad, and the parts of the log will shrink somewhat as shown in Figs. 13 and 14.
Fig. 12.
Fig. 13.
Fig. 14.
The beams, joists, planks, or boards cut from a log have the same tendency to shrink unevenly that is found in the log itself. This causes them to be irregular in shape and to curl or warp more or less, according to the part of the log from which they are taken. A piece cut from the centre of a log will thus hold its shape better than if cut from one side (Fig. 15).
