Читать книгу Wood and Forest - William Noyes - Страница 8

Table of Contents

The term "grain" is used in a variety of meanings which is likely to cause confusion. This confusion may be avoided, at least in part, by distinguishing between grain and texture, using the word grain to refer to the arrangement or direction of the wood elements, and the word texture to refer to their size or quality, so far as these affect the structural character of the wood. Hence such qualifying adjectives as coarse and fine, even and uneven, straight and cross, including spiral, twisted, wavy, curly, mottled, bird's-eye, gnarly, etc., may all be applied to grain to give it definite meaning, while to texture the proper modifying adjectives are coarse and fine, even and uneven.

Usually the word grain means the pattern or "figure" formed by the distinction between the spring wood and the summer wood. If the annual rings are wide, the wood is, in common usage, called "coarse grained," if narrow, "fine grained," so that of two trees of the same species, one may be coarse grained and the other fine grained, depending solely on the accident of fast or slow growth.

The terms coarse grain and fine grain are also frequently used to distinguish such ring-porous woods as have large prominent pores, like chestnut and ash, from those having small or no pores, as cherry and lignum vitae. A better expression in this case would be coarse and fine textured. When such coarse textured woods are stained, the large pores in the spring wood absorb more stain than the smaller elements in the summer wood, and hence the former part appears darker. In the "fine grained" (or better, fine textured,) woods the pores are absent or are small and scattered, and the wood is hard, so that they are capable of taking a high polish. This indicates the meaning of the words coarse and fine in the mind of the cabinet-maker, the reference being primarily to texture.

If the elements of which a wood are composed are of approximately uniform size, it would be said to have a uniform texture, as in white pine, while uniform grain would mean, that the elements, tho of varying sizes, were evenly distributed, as in the diffuse-porous woods.

The term "grain" also refers to the regularity of the wood structure. An ideal tree would be composed of a succession of regular cones, but few trees are truly circular in cross-section and even in those that are circular, the pith is rarely in the center, showing that one side of the tree, usually the south side, is better nourished than the other, Fig. 14, p. 23.

The normal direction of the fibers of wood is parallel to the axis of the stem in which they grow. Such wood is called "straight-grained," Fig. 22, but there are many deviations from this rule. Whenever the grain of the wood in a board is, in whole or in part, oblique to the sides of the board, it is called "cross-grained." An illustration of this is a bend in the fibers, due to a bend in the whole tree or to the presence of a neighboring knot. This bend makes the board more difficult to plane. In many cases, probably in more cases than not, the wood fibers twist around the tree. (See some of the logs in Fig. 107, p. 253.) This produces "spiral" or "twisted" grain.

Fig. 22.

Straight Grained Long-leaf Pine (full size).

Fig. 23.

Mahogany, Showing Alternately Twisted Grain (full size).

Often, as in mahogany and sweet gum, the fibers of several layers twist first in one direction and then those of the next few layers twist the other way, Fig. 24. Such wood is peculiarly cross-grained, and is of course hard to plane smooth. But when a piece is smoothly finished the changing reflection of light from the surface gives a beautiful appearance, which can be enhanced by staining and polishing. It constitutes the characteristic "grain" of striped mahogany, Fig. 23. It is rarely found in the inner part of the tree.

Fig. 24. Spiral Grain in Cypress.

After Roth.

Sometimes the grain of wood is "cross," because it is "wavy" either in a radial or a tangential section, as in maple, Fig. 25, and Fig. 26.

Fig. 25.

Planed Surface of Wavy-Grained Maple (full size).

Fig. 26.

Split Surface of Wavy-Grained Maple (full size).

"Curly grain" refers to the figure of circlets and islets and contours, often of great beauty, caused by cutting a flat surface in crooked-grained wood. See Fig. 27, curly long-leaf pine, and Fig. 28, yellow poplar. When such crookedness is fine and the fibers are contorted and, as it were, crowded out of place, as is common in and near the roots of trees, the effect is called "burl," Fig. 29. The term burl is also used to designate knots and knobs on tree trunks, Fig. 31. Burl is used chiefly in veneers.

Fig. 27.

Curly Grained Long-leaf Pine (full size).

Fig. 28.

Curly Yellow Poplar (full size).

Irregularity of grain is often caused by the presence of adventitious and dormant buds, which may be plainly seen as little knobs on the surface of some trees under the bark. In most trees, these irregularities are soon buried and smoothed over by the successive annual layers of wood, but in some woods there is a tendency to preserve the irregularities. On slash (tangent) boards of such wood, a great number of little circlets appear, giving a beautiful grain, as in "Bird's-eye maple," Fig. 30. These markings are found to predominate in the inner part of the tree. This is not at all a distinct variety of maple, as is sometimes supposed, but the common variety, in which the phenomenon frequently appears. Logs of great value, having bird's-eyes, have often unsuspectingly been chopped up for fire wood.

Fig. 29.

Redwood Burl (full size).

Fig. 30.

Bird's-eye Maple (full size.)

Fig. 31. Burl on White Oak.

The term "grain" may also mean the "figure" formed by the presence of pith rays, as in oak, Fig. 32, or beech, or the word "grain" may refer simply to the uneven deposit of coloring matter as is common in sweet gum, Fig. 33, black ash, or Circassian walnut.

Fig. 32.

Figure Formed by Pith Rays in Oak (full size).

Fig. 33.

Sweet Gum, Showing Uneven Deposit of Coloring Matter (full size.)

The presence of a limb constitutes a knot and makes great irregularity in the grain of wood, Fig. 34. In the first place, the fibers on the upper and lower sides of the limb behave differently, those on the lower side running uninterruptedly from the stem into the limb, while on the upper side the fibers bend aside making an imperfect connection. Consequently to split a knot it is always necessary to start the split from the lower side. On the other hand it is easier to split around a knot than thru it. The texture as well as the grain of wood is modified by the presence of a branch. The wood in and around a knot is much harder than the main body of the trunk on account of the crowding together of the elements. Knots are the remnants of branches left in the trunk. These once had all the parts of the trunk itself, namely bark, cambium, wood, and pith. Normally, branches grow from the pith, tho some trees, as Jack pine and redwood, among the conifers, and most of the broad-leaf trees have the power of putting out at any time adventitious buds which may develop into branches. When a branch dies, the annual layer of wood no longer grows upon it, but the successive layers of wood on the trunk itself close tighter and tighter around it, until it is broken off. Then, unless it has begun to decay, it is successively overgrown by annual layers, so that no sign of it appears until the trunk is cut open. A large trunk perfectly clean of branches on the outside may have many knots around its center, remnants of branches which grew there in its youth, as in Fig. 34, and Fig. 8, p. 19. The general effect of the presence of a knot is, that the fibers that grow around and over it are bent, and this, of course, produces crooked grain.

Following are the designations given to different knots by lumbermen: A sound knot is one which is solid across its face and is as hard as the wood surrounding it and fixed in position. A pin knot is sound, but not over ¼" in diameter. A standard knot is sound, but not over 1½" in diameter. A large knot is sound, and over 1½" in diameter. A spike knot is one sawn in a lengthwise position. A dead, or, loose knot is one not firmly held in place by growth or position.

(4) Pith. At the center or axis of the tree is the pith or medulla, Fig. 34. In every bud, that is, at the apex of every stem and branch, the pith is the growing part; but as the stem lengthens and becomes overgrown by successive layers of wood the pith loses its vital function. It does not grow with the plant except at the buds. It varies in thickness, being very small,—hardly more than 1/16", in cedar and larch,—and so small in oak as to be hardly discernible; and what there is of it turns hard and dark. In herbs and shoots it is relatively large, Fig. 5, p. 15, in a three-year old shoot of elder, for example, being as wide as the wood. In elder, moreover, it dies early and pulverizes, leaving the stem hollow. Its function is one of only temporary value to the plant.

Fig. 34.

Section Thru the Trunk of a Seven Year Old Tree, Showing Relation of Branches to Main Stem. A, B, two branches which were killed after a few years' growth by shading, and which have been overgrown by the annual rings of wood; C, a limb which lived four years, then died and broke off near the stem, leaving the part to the left of XY a "sound" knot, and the part to the right a "dead" knot, which unless rotting sets in, would in time be entirely covered by the growing trunk; D, a branch that has remained alive and has increased in size like the main stem; P, P, pith of both stem and limb.