Introduction to wood as material

Wood has always been indispensable to human needs, and it is therefore not surprising that we find wood at the heart of our cultural heritage. Because of its unique physical properties, wood holds honoured status as an engineering material and functional commodity. But the beauty of the material itself, when considered with tactile properties and working characteristics, assures that wood has prevailed as a medium in the decorative arts. Although the manner in which wood has been used often takes advantage of its aesthetic values, historical use is most closely related to its material properties. Only by studying wood as an engineering material, while remembering its biological origin, can we fully appreciate the craft and arts which developed around it.

The virtues and properties of wood are so well known to everyone that it is sometimes difficult to step back and consider them objectively - and scientifically. Given that there are tens of thousands of species that yield usable timber, an obviously wide array of characteristics can be expected. However, certain basic features are common to all woody plants and it is appropriate to begin by considering those generalities which are most important yet generally understated.

One idea is paramount: wood comes from trees. While such a statement seems foolishly elementary, it is fundamental to understanding the complex nature of wood. Remembering this basic reality will help to prevent or solve many problems associated with wood. The structure of wood is the outcome of a complex series of chemical reactions. It begins with photosynthesis, which takes place in the living tree. Photosynthesis is the process whereby carbon dioxide and water are converted, using sunlight energy captured by chlorophyll in the leaves, into simple sugars. These simple sugars ultimately form both the food and structural materials of the tree. The stem provides mechanical support for the crown, serves as an avenue of conduction between the crown and the roots, and, on occasion, stores appreciable amounts of reserve food material. Wood is therefore strong yet, once seasoned, is relatively light in weight, since its cells are then largely full of air. Being of plant origin, it is soft, in comparison with iron or stone (other materials of equivalent strength), and is therefore relatively easily worked, yet it is surprisingly durable. These properties, together with the rich variation in decorative characteristics arising from grain patterns and colour markings on the longitudinal surfaces, make wood unique among building materials.

Although its fabrication and many of its applications are comparatively simple, wood itself is a substance of great complexity. To make the best use of this material a degree of scientific and technical understanding is necessary. Technically defined, wood is the xylem from the stem (trunk or bole) of plants which are vascular, perennial and persistent and capable, by virtue of the activity of the cambium or growing layer, of secondary thickening. Wood may also usefully be described as a cellular polymeric composite. This short phrase encapsulates many important ideas about wood. First, wood is cellular tissue. The appearance, identification and working properties of wood can best be understood and interpreted through the nature, arrangement and distribution of the different types of its cells. In the second place, it is the polymeric composite nature of wood that best explains the mechanical properties and wood-water relationships.

The study of wood routinely begins at the cellular level. It is both appropriate and important to think of wood as a mass of cells. Woody cells evolved in satisfying the needs of the tree, on the one hand to be good structural beams and columns, on the other hand to provide systems for conduction of sap and for storage of food materials. The cells specialized for these mechanical and physiological functions are primarily elongated and fibre-like, and parallel to the tree stem axis. The alignment of these longitudinal cells in wood determines its 'grain direction'. The stem of a tree 'grows' in diameter by adding cylindrical layers of cells, which we recognize as growth rings. The combination of the axial direction of longitudinal cells, and their arrangement in growth rings, gives wood tissue a three-dimensional orientation, and the properties of wood are significantly different along its three structural directions.

The principal chemical components of cell wall substance - namely, cellulose, hemicellu-loses, and lignin - are strikingly similar across the array of different woods. However, as living sapwood (an outer, functionally active portion of the stem) transforms to non-living heartwood, the formation of chemical substances known as extractives even in rather trace amounts, may impart significant changes to certain properties.

While many generalities can be applied to all woods, it is important to appreciate the wide range of differences that exist. For example, overall, density is probably the one physical characteristic of a wood that best predicts many other properties and determines its potential uses. Density is the mass of a unit volume of a substance, that is, mass divided by volume. In SI units, density may be expressed in kilograms per cubic metre (kg/m3), in grams per cubic centimetre (g/cc) or in pounds per cubic foot (lb/ft3). The term specific gravity was the former term for the ratio of the density of a substance to that of water. The term relative density is now used instead. The realization that the range of relative density of less than 0.1 for the lightest woods to greater than 1.3 for the heaviest illustrates an obvious diversity among woods.

In the traditional approach to classifying wood, botanical taxonomy serves as the logical framework, in which timber trees are categorized in one of two broad groups, called softwoods and hardwoods. The words softwoods and hardwoods, however, are unfortunate terminology as they do not accurately apply to the relative hardness or density of woods they represent. Rather, woods of the two groups differ in the types and arrangement of cells comprising them. The softwoods belong to a group of trees called the Gymno-sperms, primitive, conifers or cone-bearing trees with naked seeds and mostly needle-like leaves. Correspondingly, hardwoods belong to a group of trees more accurately called Angiosperms. In fact, on the basis of cellular differences, woods of the two groups can be readily distinguished visually at relatively low magnification. Further separation of woods within each group for identification involves examination of additional cellular detail, commonly with microscopic magnification. The systematic study of anatomy goes hand in hand with wood identification, although familiarity with anatomy is fundamental to the understanding of many other aspects of wood properties as well.

Wood-moisture relationships are probably at the source of more problems in using wood and in the conservation of objects than any other aspect of wood properties. Although such problems can be complex, the underlying principles can be easily summarized. First, trees are wet, containing large amounts of moisture in the form of sap. Second, as the timber taken from trees is dried to a condition appropriate for use, it loses most of its moisture. Third, the loss of this moisture affects many properties, such as increasing strength, but decreasing dimensions (shrinkage). Fourth, after initial drying, wood remains hygroscopic, and will continue to adsorb or desorb moisture, and consequently change dimensions and other properties, in response to changes in relative humidity of its environment.

The following account will highlight in more detail the various aspects of relationships between the appearance, structure and function of wood that are needed properly to understand many of the problems encountered by furniture conservators.

terns of longitudinal surface appearance or 'figure' by which we recognize wood are most commonly the result of this early-wood-late-wood variation. Distinct early-wood-late-wood contrast usually indicates variation in cell characteristics resulting in late-wood having greater density than early-wood, but in some woods, there may be no significant difference in properties within growth rings.

Individual wood cells usually have an elongated shape, although they vary in proportions from short and barrel-shaped to long and needle-like (see Figure 2.4). Most cells are longi-

Wood Working for Amateur Craftsman

Wood Working for Amateur Craftsman

THIS book is one of the series of Handbooks on industrial subjects being published by the Popular Mechanics Company. Like Popular Mechanics Magazine, and like the other books in this series, it is written so you can understand it. The purpose of Popular Mechanics Handbooks is to supply a growing demand for high-class, up-to-date and accurate text-books, suitable for home study as well as for class use, on all mechanical subjects. The textand illustrations, in each instance, have been prepared expressly for this series by well known experts, and revised by the editor of Popular Mechanics.

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