Carpentry and Woodwork

by Edwin W. Foster

The new or sap wood, then, is further from the centre each year, and while the old cells may not be dead, they contain less and less sap, are therefore drier, and after a few years change colour, becoming darker.

There is often a very great contrast between the colour of the heart wood and that of the sap wood, although the latter may be represented by several years of growth.

These annual rings are not actually circular, but very irregular, and often wider in some parts than in others. The study of these rings is very interesting, and it shows that the tree usually increases in diameter more rapidly during the first few years than later. Very often, after growing slowly for several years, the tree will apparently grow rapidly again. The cause of this cannot be determined without a knowledge of the tree's history.

It has been proved by experiment that thinning the forest increases the growth of the remaining trees 18 per cent., and these peculiarities in the rings may have been due to some like cause. The bearing of this fact on the peculiarities of warping and shrinkage is that when cut down the log is drier at the heart and more sappy at the outside, so that evaporation occurs near the surface.

Fig. 239. Warping, wind, and shrinkage.

The effect of it is shown in [Fig. 239]. The outside drawing together has opened the wood, or "checked" it, most at the outside, diminishing toward the centre. The evaporation would have occurred just the same had the log been cut into planks, causing them to curl as shown at a. This is known as warping, and it is one of the troubles of the woodworker. In construction it must be constantly guarded against, and overcome as far as possible. It cannot be entirely prevented, but if the wood has been well seasoned before it is used a large part of the warp will be taken out in the planing mill, or in the squaring up.

Twisting, winding, and warping are also caused by the two sides of a board having been subjected to different degrees of heat, moisture, etc. If a plank is laid on the floor, the upper part is more exposed to the air and to changes of temperature and humidity; therefore it curls.

If a board is stood on end or placed in a rack where there is a free circulation of air, the curling will be much less. Even in a rack, if several boards are piled one on another, the top one will have different conditions from the others and be apt to curl or wind.