Inspecting a collection of various kinds of wood would show considerable variability in physical properties such as color, density, and hardness. Densities range over an order of magnitude, from 110 kg/m3 for balsa to 1330 kg/m3 for lignum vitae. Colors vary from the nearly white of some varieties of maple to ebony wood, which lends its name as a synonym for black. A measure of hardness, the Janka test [A], shows again the extremes between lignum vitae, with a hardness of 20 000 N, and balsa, at 440 N. Some woods display almost no obvious grain structure, while others have structures so remarkable that they are prized for applications such as making fine furniture. Yet, despite the great range of diversity among woods, all varieties, regardless of botanical or geographical origin, also have many properties in common.

All woods have a cellular structure in which the cell walls are composed of a matrix of biopolymers, discussed later in this chapter. Wood is anisotropic, displaying different physical properties along the three major axes. Anisotropy results from the structure of cellulose in the cell walls, along with the shapes of wood cells and their orientation vis-à-vis the trunk. Wood gains and loses moisture (i.e. it is hygroscopic) as a result of changes in humidity and temperature. Because of the anisotropy, gain or loss of moisture results in unequal swelling or shrinkage along the three axes. Fungi, bacteria, and insects such as termites attack wood. The fact that wood is, as a result, biodegradable has both good and bad points. Obviously it is undesirable when, for example, a wooden structure rots. But, this behavior can also be turned to good use in chemical processing, when fungi or bacteria can be used deliberately to degrade the biopolymers in wood to produce useful chemical or fuel products.