Timber Structure, Properties, Conversion and Use

Timber has been used by man since the early days of recorded history, first as a means of constructing a shelter for himself, later as a hunting tool in the form of spear or bow, and later still as a widely used artefact of an industrialised society with uses ranging from truck sides and bottoms, electrical sockets and plug tops, tool- handles, sports equipment, boats, railway sleepers and musical instruments, to name but a few.

Almost all woody plants with which we are familiar have three principal parts: roots, stems and leaves. The characteristic that separates trees from other woody plants is their single main stem, commonly referred to as the trunk or bole (Figure 2.1).

In Chapter 2, attention was drawn to the presence between the bark and the wood of living cells capable of subdividing to form new wood or bark cells. Attention was also drawn to the three functions that the trunk of a tree has to perform. Following their formation, the new cells on the woody side of the cambium undergo, over a period of up to three weeks, a series of changes which is known as cell differentiation. The cells tend to change shape and a secondary wall is formed, the structure of which is presented in Chapter 4. The cell dies and the degenerated cell contents are frequently found lining the cell wall; the cell is now ready to assume one or more of the three basic functions of conduction, support and storage.

Previous chapters have dealt with the gross structure of wood as seen by the naked eye, and the cellular structure as can be observed with the aid of a microscope or hand lens. This chapter looks at the structure of wood at an even finer level by enlisting techniques such as the electron microscope, chemical analysis and X- ray diffraction analysis. These have identified the basic building blocks in wood and have revealed how these fit together to produce the high level of physical and mechanical performance that wood possesses.

In the utilisation of timber, perhaps the single most important factor detracting from its outstanding performance as a material is its variability. In all applications of timber, whether it is in furniture manufacture or housing construction, large quantities are rejected on the grounds that they are different in appearance or behave differently in machining. It is most unlikely that two pieces of timber are identical in both appearance and performance: within a batch of timber pieces, there will be a wide spectrum in what we could loosely call quality. At some arbitrary point, a line is drawn above which the pieces though variable are acceptable, and below which the pieces are unacceptable for a particular use.

The identification of timbers may, at first sight, appear to be a comparatively simple matter; however, when it is realised that there are tens of thousands of woody species in the world, it will be appreciated that in some cases correct identification may be exceedingly difficult. Actually it is not always possible to arrive at the correct specific name from the examination of a single sample of wood, although it is usually possible to narrow down the identification to a group of related species, and this may be sufficient for most practical purposes. Moreover, although there are so many species that produce woody stems, only a small proportion grow to timber size. Even so, the number of species producing commercial timber runs into several hundreds. The characters available for distinguishing woods are not numerous, and identifications should be based on an examination of features that are known to be reliable, rather than on the more obvious characters, such as colour and weight, that tend to be far from consistent.

Although vast quantities of timber are used in construction, and much of this is softwood, there are still large quantities of wood used primarily on the grounds of its aesthetic appeal, for example, for furniture, door skins, wall panelling and certain sports goods. The decorative appearance of many timbers is due to the texture, or figure, or colour of the wood, and often to a combination of two, if not all three of these characteristics.

Perhaps the single most important property controlling the mechanical performance of a piece of wood is its density. Density is the ratio of mass to volume.

One of the most important variables influencing the performance of wood is its moisture content. The amount of water present not only influences its strength, stiffness and mode of failure, but it also affects its dimensions, its susceptibility to fungal attack, its workability as well as its ability to accept adhesives and finishes. For wood to perform well, its moisture content must be reduced to a level corresponding to at least 12 percent of its oven-dry mass.

The principal physical properties affecting the general performance of wood, namely density and moisture, have been discussed in detail in Chapters 8 and 9. There are however a number of other physical properties, as distinct from mechanical properties (see Chapter 11), which are of secondary importance in many general timber applications, and of particular importance in a few specialised areas. This collection of properties is covered in this chapter.

The strength of a material such as wood refers to its ability to resist applied forces that could lead to its failure, while its elasticity determines the amount of deformation that would occur under the same applied forces. These forces may be applied slowly at constant rate whereby we refer to the inherent resistance of the material as its static strength, or they may be applied exceptionally quickly, when we refer to the resistance of the material as its dynamic strength.

It is not the intention in this chapter to give a detailed account of sawmilling practice, and readers desirous of obtaining a comprehensive account of the subject should read one of the specialist texts on the subject. Rather it is the intention here to provide only the basic principles in an attempt to make a bridge between the properties of the tree previously described, and the utilisation of timber to be discussed in future chapters.

The primary aim in seasoning is to render timber as dimensionally stable as possible, thereby ensuring that once it is made up into flooring, furniture, fittings, etc., movement will be negligible or for practical purposes non-existent; simultaneously, other advantages accrue. Most wood- rotting and all sap-stain fungi can grow in timber only if the moisture content of the wood is above 22 per cent: hence, seasoning arrests the development of incipient decay in wood and removes the risk of infection of sound timber. Seasoning, however, does not confer immunity from subsequent infection should the moisture content of previously dry wood be raised above the critical minimum, as a result, for example, of prolonged exposure to damp conditions. Reduction in weight of wood accompanies loss of moisture; this is of practical importance as it reduces handling costs, and may effect economies in freight charges. Seasoning also prepares timber for various ‘finishing processes, such as painting and polishing, and it is an essential prerequisite if good penetration of wood preservatives is sought. Finally, most strength properties increase as timber dries and, although the increases may not in themselves justify the expense of seasoning, they are of more than academic significance.

Following conversion from the log and subsequent seasoning, timber to be used in furniture, joinery and toy manufacture has to be further processed; this will involve some additional sawing followed by planing, spindle moulding and usually sanding.

The quality of sawn timber varies widely depending on the species, where and how it was grown and the age at which the tree was felled. Thus, quality of wood is determined primarily by the density of wood, and strongly influenced by the size and distribution of knots and other defects which have been described in detail in Chapters 2, 5 and 11.

The competent and efficient use of timber either in the production of manufactured items or in the wide area of timber construction depends on two important criteria: (1) the selection of the most suitable timber for the task, and (2) the choice of the most appropriate method of joining together the different pieces of timber.

With the passage of time and the influence of the environment, all materials exhibit a loss in performance, a process which the materials scientist refers to under the all-embracing title of degradation. Undesirable changes occur to the material which, at best, are restricted to its surface layers, but which usually permeate the whole of the material and have a most pronounced effect on its mechanical performance, especially in terms of strength and toughness. The rate of the degradation is usually specific to a particular material and to specific environmental conditions.

Most forms of decay and sap-stain in timber are caused by fungi that feed either on the wall tissue or cell contents of woody plants. It is important to distinguish between wood-rotting fungi, responsible for decay in timber, and those that feed on the cell contents, causing stains. The former consume constituents of the cell wall, and lead to the disintegration of woody tissue, whereas the latter remove only stored plant food material in the cell cavities, leaving the cellular structure intact. Wood-rotting fungi seriously weaken timber, ultimately rendering it valueless, whereas sap-stain fungi spoil the appearance of wood, but do not affect most strength properties. Sap-stain is not a preliminary stage of decay, but such stained timber, exposed to suitable conditions, may later be attacked by wood-rotting fungi.

The damage referred to as worm in timber is the result of insect activity but, in salt water, teredo or ship worm and a wood-louse-like animal belonging to the crustacean family are responsible for damage of this type. Insects tunnel in timber, spoiling the appearance of exposed faces and, if the tunnels are numerous, they may so reduce strength properties as to make the wood valueless. Some insects only attack living trees or newly felled logs, some only seasoned wood, and others only the sapwood of certain species. In consequence, the presence of insect damage is not in itself necessarily a cause for alarm: the damage may be of the first type and therefore of no consequence in seasoned timber, beyond the disfigurement caused. Moreover, some insects and crustaceans commonly associated with timber are of no importance because they do not attack it.

Timber used as railway sleepers, fence posts, power-line poles and the like, is inevitably exposed to conditions favouring attack throughout its service life. By contrast, certain conditions of service ensure that timber never becomes attacked: piling timbers in deep water or timbers in the abnormally dry atmosphere of the Egyptian tombs will remain free from attack indefinitely. Much more timber is used in circumstances between these extremes of certain attack and complete immunity, giving rise to the need for prophylactic measures or problems of eradication.

The principal causes of deterioration of wood in service, as distinct from deterioration during seasoning, are attack by wood-destroying fungi, insects and marine borers, fire, and mechanical abrasion and weathering. Mechanical abrasion and weathering will not be considered here, but the resistance of timber to the other agents of destruction may frequently be enhanced by the application of suitable chemical formulations. Various substances were used for this purpose by the ancient Egyptians and the Romans, but the extensive use of wood preservatives is essentially a development of the last hundred years or so. This chapter will be devoted mainly to the preservation of timber against attack by biological agencies; methods used to decrease the risk of fire damage are dealt with briefly at the end of the chapter.

Unless very durable timbers are employed, the service life of wooden components in contact with the external climate will be determined primarily by the efficacy of the applied finish. This in turn is determined not only by its chemical formulation but also by the nature of the substrate.