You will see if you look at the ends of logs and stumps that the heart is frequently not in the centre, in some cases taking such a devious course throughout the stem as to make the grain so crooked that no method of sawing will remove the tendency to warp or twist, just shown. Such trees may show a beautiful grain. Even in straight trees the pith is not usually quite straight, and is apt to take a somewhat zigzag course, due to the crooked way the tree grew when young (Fig. 699).
Fig. 699.
Imagine, for an exaggerated illustration, that you could see with X-rays the pith as crooked as Fig. 699. that shown in Fig. 700. Imagine that from this tree you could saw out the board indicated, keeping with it the whole pith or heart as if it were a wire rope woven in and out of the board, so that the appearance would be somewhat like that shown in Fig. 701. Bear in mind that the annual rings are layers of wood, so to speak, which may vary in thickness, growing around the heart. You will see that these layers, or rings, as they dip below or rise above the surface of the board, will cause the grain to form various patterns, perhaps somewhat as shown in Fig. 701, which makes no claim to accurately showing the grain in this case. In fact, all such variations of grain in lumber are due to the surface of the piece being at an angle with the layers.
Fig. 700.
Fig. 701.
In addition, the knots caused by branches, the twisting of the stems screw-fashion (as is seen in cedar), wounds, and other causes, often produce very crooked and tangled grain, and the wood of many broad-leaved trees is sometimes extremely complicated in texture, especially when all these irregularities occur in the same piece. It is the nature of some kinds of mahogany, from whatever cause, to have the fibres strangely interlaced or running in very different directions in layers which are quite near each other.
The warping, twisting, and cracking is obviated in many cases where it is objectionable (as in the wooden frames of machines, the tops of benches) by building up with a number of smaller pieces, of which you will often see illustrations. To do this to the best advantage, the pieces should be selected and put together so that, though the grain will run in the same direction lengthways, the annual rings at the ends will not run together as in a whole beam, but will be reversed or arranged in various combinations, so that the tendencies of the different parts to warp or twist will counteract each other. Instead of a single board, which would naturally become warped in one large curve, a number of strips can be glued up with the grain of the strips arranged in alternate fashion (Fig. 559), so that in place of one large curve the warping will merely result in a slightly wavy line.