Elements of woodwork

by Charles A. King

4. The medullary rays.—Every tree has medullary rays (Fig. 2, g), usually spoken of by wood-workers as the “silver streak,” or “silver grain.” These rays connect the center of the tree with the outside, and are more prominent in such woods as oak, beech, and sycamore than in other woods. It is to take advantage of the beauty which these medullary rays impart that so much quarter-sawed lumber is used, though lumber sawed in this way is preferred for other reasons, which will be discussed later.

In many woods these rays are so small as to be invisible to the naked eye, as in pine, for instance, which has fifteen thousand to the square inch. Aside from adding much to the beauty of the lumber, they also give strength; if lumber is dried out too rapidly by artificial heat, it is apt to check, or crack, upon the line of the medullary rays.

Fig. 3.—Defects in Lumber.

a, wind shakes or cup shakes; b, heart shakes; c, star shakes; d, branch broken off, showing the method by which the annual layers gradually cover broken branches; e, hard knot.

5. The grain in trees.—In open or exposed situations trees usually grow more or less gnarled or crooked, which tends to improve the grain. The strength of lumber cut from trees grown as described is impaired, however, as the wood is more cross-grained than that from trees which grow in the heart of a forest, with practically the same conditions on all sides. As forest trees are continually reaching up for the sunlight, they grow taller and straighter, which makes it possible for straighter-grained lumber to be cut from them, and as the branches are at the top, where they receive the sunlight and air, there are few cross-grained places.

When the annual rings are large, the grain is said to be coarse, and if the rings are fine, the term fine-grained is used to describe it. When the direction of the fibers is nearly parallel with the sides and the edges of the board, it is said to be straight-grained; when the lumber is taken from a crooked tree, it is said to be cross-grained, as the grain follows the shape of the log, while the board is sawed straight. Cross-grained lumber is the handsomer; in this the fibers, being at different angles with the surface of the board, form a variety of figures, which add much to the beauty of the wood.

6. Defects found in lumber.—(A.) Some of the most common defects found in lumber are wind shakes, or cup shakes (Fig. 3, a), which are cracks following the line of the porous part of the annual rings. These are caused by the action of severe winds. (B.) Heart shakes (Fig. 3, b) are cracks radiating from the center of the tree, and may be found in any kind of wood, as they are the result of deficient nutrition, or loss of vitality. (C.) Star shakes (Fig. 3, c) are caused by the shrinkage of the tree upon the outside, which is the result of a long dry spell of intense cold, or of the deficient action of the sap. Star shakes differ from heart shakes in being larger upon the outside of the tree; the heart shakes are larger at the center.

Shakes of all kinds are common defects and sometimes are so numerous as to make the log worthless.

(D.) Discolorations are caused by decay which has at some time gained a foothold, but which the tree was sufficiently vigorous to overcome; or they may be due to imperfect or insufficient nutrition, which generally results in the entire tree being affected instead of small places upon the tree.