The Composition and Physical Properties of Wood

Wood is a composite material that is made, up basically of a mixture of three main constituents, cellulose, hemicellulose, and lignin (see Textbox 54), all of them biopolymers synthesized by the plants, which differ from one another in composition and structure (see Textbox 58). The physical properties of any type of wood are determined by the nature of the tree in which the wood grows, as well as on the environmental conditions in which the tree grows. Some of fhe properties, such as the density of wood from differenf fypes of frees, are exfremely variable, as can be appreciated from the values listed in Table 71. No distinctions as to the nature of a wood, whether it is a hardwood or a softwood, for example, can be drawn from the value of its specific gravify. 

Most wood can be classified into two broad categories, hardwood and softwood, terms that distinguish between the nature of the trees rather than between the properties in wood. The term hardwood, for example, refers to the wood of deciduous, broad-leaved trees such as birch, oak, and poplar, from the temperate and tropical regions of the world. Softwood, on the other hand, is used to refer to the wood of coniferous trees, needleleaved, evergreen trees such as fir, hemlock, and pine, which grow mainly in temperate and cold regions (see Table 71). The terms hardwood and 

The two types of wood differ, however, in their nature and structure. The main structural characteristic of the hardwoods (which are botanically known as angiosperms, plants that flower to pollinate for seed reproduction) is that in their trunks or branches, the volume of wood taken up by dead cells, varies greatly, although it makes up an average of about 50% of the total volume. In softwoods (from the botanical group gymnosperms, which do not have flowers but use cones for seed reproduction) the dead cells are much more elongated and fibrous than in hardwoods, and the volume taken up by dead cells may represent over 90% of the total volume of the wood. 

The study of the nature of wood growth rings revealed, during the first half of the twentieth century, that there is a direct relationship between the 

FIGURE 68 Wooden bowl. A wooden bowl made of boxwood, first century c.e., from Qumran, Israel the wood used to make the bowl seems to have been imported from Turkey. The excellent preservation of the bowl is due to extremely hot and diy environmental conditions in the region. Three conditions are necessary for wood to decay (1) a favorable temperature (0-32°C), (2) moisture in excess of the fiber saturation point (above 25-30%) and (3) an adequate supply of oxygen. If any one of these is eliminated wood remains well preserved for long periods of time. 

---