Wood

I

Introduction

Wood, hard, tough substance that forms the trunks of trees, and that has been used for thousands of years as a fuel and as a material of construction (see Building Construction). Technically, the term wood includes similar materials in other parts of the plant, including even the so-called veins in leaves, but only those portions of wood which have commercial importance are discussed in this article.

For the botanical aspects of wood, including its structure and growth, see Tree; Xylem. For growth and distribution, see Forest. For the cultivation of trees for wood, see Forestry. For the cutting of trees and the manufacture of lumber, see Lumber Industry.

II

Grain and Structure

The typical markings, called grain, that are found on all types of natural wood are due to the structure of the wood. Wood consists essentially of fine cellular ducts or tubes, which carry water and dissolved minerals from the roots to the leaves, and which are thus arranged more or less vertically within the trunk. When the wood is cut parallel to the axis of the trunk, straight-grained lumber is usually produced. In some trees, however, the ducts are helical; that is, they twist around the trunk as they ascend. Such trees produce cross-grained lumber, which is also obtained from ordinary trees when the cut is not parallel to the axis of the trunk.

Many woods have prominent annual rings. The trunk of a tree does not grow in length, except at its tip, but does grow in width. The only portion of the trunk that is engaged in active growth is the cambium, a thin layer entirely surrounding the trunk. In trees of the Temperate Zone, the cambium lays down new wood during the spring and summer, and in most trees the early wood is more porous and therefore lighter in color than the wood produced later in the season. The trunk of a tree is thus surrounded each year by a new pair of concentric sheaths, one darker than the other.

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Although the thin layer of cambium is the only part of the trunk that is alive in the sense that it is engaged in active growth, living cells are also interspersed among the xylem cells of the sapwood. As the tree grows older, however, the central portion of the trunk dies completely; the ducts become plugged with gums or resins, or merely air (see Gum). This central part of the trunk is called heartwood. The internal changes are accompanied by changes in color typical of the species of trees, so that the heartwood is usually darker than the sapwood.

III

Classification

Woods are classified as softwood or hardwood, depending on the tree from which they come. Woods from broad-leaved trees are called hardwoods, and woods from coniferous trees are called softwoods, regardless of their actual hardness. Thus many softwoods are actually harder than some of the so-called hardwoods. The hardwoods have long, continuous ducts leading through the trunk; the softwoods do not have such ducts, and the fluids are transported from cell to cell. Many softwoods have resin ducts running parallel to the grain, and softwoods in general contain considerable resin, whereas few hardwoods have any such material in the wood. Most lumber in the United States is softwood; the hardwood is generally employed for furniture and high-grade flooring.

Knots are areas of the trunk in which the base of a branch has become embedded in the body of the wood. When the wood is sawed into planks, the knot becomes evident as a roughly circular discontinuity or irregularity in the grain structure. Where the branch begins within the tree, the rings of the knot are continuous with the grain of the wood, producing an intergrown knot; farther out toward the surface, the grain of the trunk has grown around the branch, producing an encased knot.

During seasoning, when the shrinkage in a board varies with the direction of the grain, the knots shrink at a faster rate than the remainder of the wood. An encased knot may thus fall completely out of the plank, forming a knothole. An intergrown knot cannot fall out, but the wood around the knot is distorted by the uneven shrinking, and the board may be weakened even more than by the presence of a knothole. Knots are generally undesirable in lumber from the standpoint of appearance, apart from consideration of their effect on the strength of the wood. In some cases, however, knotty pine and similar types of lumber are desired for paneling interiors, because of the patterns in the grain formed by the knots.

The wood’s appearance is one of the most important properties when the wood is being considered for some interior use such as furniture or paneling. Certain woods, such as walnut, have straight, parallel graining, which, when combined with dark, attractive color and great hardness, makes them highly desirable for veneer (see also Plywood below). Irregularities of grain may make attractive patterns, or lumber may purposely be cut from a crotch to yield wavy, interlocking patterns. Many veneers are made by peeling a thin layer circumferentially from around the trunk, so that the knife cuts across the annual ring marks only at considerable distances, producing the large, irregular patterns typical of many plywoods.

IV

Physical Properties

The principal physical properties of wood are strength, hardness, stiffness, and density. Density is generally an indication of the mechanical properties, inasmuch as dense woods are usually hard and strong (see Ironwood). The term strength covers a number of essentially different properties; a wood that is high in one kind of strength is not necessarily high in others. Moreover, the strength varies greatly with the state of seasoning, or dryness, of the wood, and with the direction of the grain; wood is always much stronger when cut along the grain rather than across it, and for this reason planks and such articles as poles and handles are always cut with the grain running the long way. Wood has high compression strength, in some cases higher in proportion to its weight than steel; it has low tensile strength and moderate shear strength. See Materials Science and Technology: Mechanical Properties of Materials.

High compression strength is required for foundations, and for the main supports of buildings. Bending strength is essential for most structural wooden members, including joists, studding, and beams of all sorts. Many woods that are commonly used for high bending strength have high compression strength, and vice versa; but oak, for example, is very strong in bending and comparatively weak in compression, whereas redwood is strong in compression and comparatively weak in bending.

Toughness is a measure of strength against sudden and repeated stress. Hickory and ash are outstanding for their toughness and are used in wagon spokes, baseball bats, and ax handles; because hickory is stiffer than ash, it is preferred for thin handles, such as those of golf clubs.

Other less important mechanical properties may be critical for a particular use; for example, the elasticity and resonance of spruce render it the only material suitable for the sounding board of a fine piano.

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