WOOD PROPERTIES

Wood properties Density and specific gravity Density is the weight or mass of a unit volume of wood, and specific gravity is the ratio of the density of wood to that of water. In the metric system, density and specific gravity are numerically identical; e.g., the average density of the wood of Douglas fir is 0.45 grams per cubic centimetre (28 pounds per cubic foot) and its specific gravity 0.45, because one cubic centimetre of water weighs one gram. The density of wood varies from about 0.1 to 1.2 grams per cubic centimetre (specific gravity 0.1 to 1.2). Differences between species and among samples of the same species are attributable to different proportions of wood substance, void volume (volume of cell lumens and wall spaces), and the content of extractives. The amount of extractives in wood varies from less than 3 percent to more than 30 percent of the oven-dry weight. It is obvious that the presence of these materials, located to a large extent within the cell wall, can have a major effect upon the density. Determination of the density of wood in relation to that of other materials is difficult because wood is hygroscopic, and both its weight and volume are greatly influenced by moisture content. In order to obtain comparable figures, weight and volume are determined at specified moisture contents. The standards are oven-dry weight (practically zero moisture content) and either oven-dry or green volume (moisture content above fibre saturation point, which averages about 30 percent). Most mechanical properties of wood are closely correlated to density and specific gravity. It is possible to learn more about the nature of a wood sample by determining its specific gravity than by any other simple measurement. Hygroscopicity Wood is hygroscopic (i.e., exhibits an affinity for water) and can absorb water as a liquid, if in contact with it, or in the form of vapour from the surrounding atmosphere. Though wood may absorb other liquids and gases, water is the most important. Because of its hygroscopicity, wood, either as a part of the living tree or as a material, always contains moisture. (Water and moisture are used here without distinction.) This moisture affects all wood properties, but it should be noted that only moisture contained in cell walls is important; moisture in the cavities merely adds weight. The amount of moisture held in cell walls varies from about 20 to 35 percent (on the basis of oven-dry weight of wood). The theoretical point at which cell walls are completely saturated and cell cavities empty is known as the fibre saturation point. Beyond this point, moisture goes into the cavities, and when these are completely filled, the maximum moisture content of the wood is reached. Moisture content of some woods can be high. Very light woods, such as balsa, can hold up to about 800 percent, pine 250 percent, beech 120 percent, and so on. When green wood is exposed to the atmosphere, its moisture content gradually decreases. Moisture in the cell cavities is lost first. In time the moisture content of wood falls to levels ranging (for localities in the temperate zones) from about 6 to 25 percent (average 1215 percent). Local conditions of air temperature and relative humidity dictate the final moisture level. Hygroscopicity is of primary importance because moisture in wood affects all wood properties. It has a direct relation to weight of logs and green lumber, with consequent influence on transportation costs. Dimensions change, as explained below (see Shrinkage and swelling). Resistance to decay and insects is greatly affected. Also influenced is processing, such as drying, preservative treatment, and pulping. Gluing and finishing and the mechanical, thermal, and acoustical properties of wood are all affected by its moisture content.