ENCYCLOPEDIA OF WOOD

PROPERTIES OF WOOD

n experienced woodworker pays close attention to the selection of wood for a project. Every species has unique qualities that can make it ideal for one application but unsuitable for another. Among the key properties that distinguish woods are color, grain, tex­ture, figure, weight and odor.

Many species are prized for their dis­tinctive colors. Padauk is a fiery orange - red; black walnut often exhibits deep purples and chocolate tones. Color in
wood is the result of extractives such as tannins, gums and resins in the wood. When cut lumber is exposed to air, these substances gradually oxidize, deepening the wood’s color. In some cases, howev­er, the color may fade.

Grain and texture are two distinct properties that are often confused. Grain describes the direction and regularity of the wood fibers relative to the axis of the tree trunk. As illustrated on page 28, the grain displayed by a piece of lumber
depends on the growth pattern of the tree from which it was cut.

A wood’s texture depends on the size and distribution of its cells. Ring-porous hardwoods with large vessels have a coarse texture, while diffuse-porous hardwoods with fine vessels have a fin­er texture. In some softwoods, abrupt transitions from earlywood to latewood produce an uneven texture. Where there is little or no transition, as in white pine, the wood has an even texture.

Figure—an important quality in veneers—is the pattern displayed on the surface of a board. This is the expression of a board’s “character”—the sum of its grain, contrast between earlywood and latewood, eccentricity of growth rings, mineral streaks, disease and the method used to saw the log. Some of the more stunning figures in different species are illustrated below. For example, plain-sawn white birch reveals a so-called landscape figure. Interlocked grain produces the rib­
bon figure common in African mahogany. Wavy grain in maples results in a fiddle - back figure, so named because of its use in the backs of violins. And irregular growths on the outer surfaces of trees, such as elm, yield an intricate burl figure.

The weight of different wood species is expressed as specific gravity, or its den­sity compared to an equal volume of water. The specific gravity of an oven - dried sample of American elm, for example, is 0.50, making it half as heavy
as a tropical hardwood like ekki, which has the same specific gravity as water— 1.00. Lignum vitae, the heaviest wood, has a specific gravity of 1.23. The high­er a wood’s specific gravity, the less porous it is and the more impervious it will be to a finish.

A wood’s odor—usually caused by oils in the heartwood—may also deter­mine its use. An aromatic species like cedar, for example, is often used for clothes chests and cigar boxes.