Appearance of Timber in Relation to Structure
Most of you will agree that many timbers are aesthetically pleasing and the various and continuing attempts to simulate the appearance of timber in the surface of synthetic materials bear testament to the very attractive appearance of many timbers.
Although a very large proportion of the timber consumed in the UK is used within the construction industry, where the natural appearance of timber is of little consequence, excepting the use of hardwoods for flush doors, internal panelling and wood-block floors, a considerable quantity of timber is still utilised purely on account of its attractive appearance, particularly for furniture and various sports goods.
The decorative appearance of many timbers is due to the texture, or to the figure, or to the colour of the material and, in many instances, to combinations of these.
The texture of timber depends on the size of the cells and on their arrangement. A timber such as boxwood in which the cells have a very small diameter is said to be fine‑textured, while a coarsetextured timber such as keruing has a considerable percentage of large-diameter cells.
Where the distribution of the cell-types or sizes across the growth ring is uniform, as in beech, or where the thickness of the cell wall remains fairly constant across the ring, as in some of the softwoods, e.g. yellow pine, the timber is described as being even-textured; conversely, where variation occurs across the growth ring, either in distribution of cells as in teak or in thickness of the cell walls as in larch or Douglas fir, the timber is said to have an uneven texture.
Figure is defined as the ‘ornamental markings seen on the cut surface of timber, formed by the structural features of the wood’, but the term is also frequently applied to the effect of marked variations in colour. The four most important structural features inducing figure are grain, growth rings, rays and knots.
Grain: The cells of wood, though often described as vertically orientated, frequently deviate from this convenient arrangement. In the majority of cases this deviation takes the form of a spiral, the magnitude of the angle varying with distance from the pith.
Although of considerable technical importance because of loss in strength and induced machining problems, the common form of spiral grain has no effect on the figure presented on the finished timber. However, two other forms of grain deviation do have a very marked influence on the resulting figure of the wood.
Thus, in certain hardwood timbers, and the mahoganies are perhaps the best example, the direction of the spiral in the longitudinal–tangential plane alternates from left to right hand at very frequent intervals along the radial direction; grain of this type is said to be interlocked.
Tangential faces of machined timber will be normal, but the radial face will be characterised by the presence of alternating light and dark longitudinal bands produced by the reflection of light from the tapered cuts of fibres inclined in different directions (Fig. 1).
This type of figure is referred to as ribbon or stripe and is desirous in timber for furniture manufacture. If instead of the grain direction alternating from left to right within successive layers along the radial direction as above, the grain direction alternates at right angles to this pattern, i.e. in the longitudinal–radial plane, a wavy type of grain is produced.
This is very conspicuous in machined tangential faces where it shows up clearly as alternating light and dark horizontal bands (Fig. 2); this type of figure is described as fiddleback, since timber with this distinctive type of figure has been used traditionally for the manufacture of the backs of violins. It is also found on the panels and sides of expensive wardrobes and bookcases.
Growth rings: Where variability occurs across the growth ring, either in the distribution of the various cell types or in the thickness of the cell walls, distinct patterns will appear on the machined faces of the timber. Such patterns, however, will not be regular like many of the man-made imitations, but will vary according to changes in width of the growth ring and in the relative proportions of early and latewood.
On the radial face the growth rings will be vertical and parallel to one another, but on the tangential face a most pleasing series of concentric arcs is produced as successive growth layers are intersected. In the centre part of the plank of timber illustrated in Fig. 3, the growth rings are cut tangentially forming these attractive arcs, while the edge of the board with parallel and vertical growth rings reflects timber cut radially.
In the case of ring-porous timbers, it is the presence of the large earlywood vessels that makes the growth ring so conspicuous, while in timbers like Douglas fir or pitch pine, the striking effect of the growth ring can be ascribed to the very thick walls of the latewood cells.
Rays: Another structural feature that may add to the attractive appearance of timber is the ray, especially where, as in the case of oak, the rays are both deep and wide. When the surface of the plank coincides with the longitudinal–radial plane, these rays can be seen as sinuous light-coloured ribbons running across the grain.
Knots: Knots, though troublesome from the mechanical aspects of timber utilisation, can be regarded as a decorative feature; the fashion for knotty-pine furniture and wall panelling in the early seventies is a very good example of this. However, as a decorative feature, knots do not possess the subtlety of variation in grain and colour that arises from the other structural features described above.
Exceptionally, trees produce a cluster of small shoots at some point on the trunk and the timber subsequently formed in this region contains a multitude of small knots. Timber from these burrs is highly prized for decorative work, especially if walnut or yew.
In the absence of extractives, timber tends to be a rather pale straw colour, which is characteristic of the sapwood of almost all timbers. The onset of heartwood formation in many timbers is associated with the deposition of extractives, most of which are coloured, thereby imparting coloration to the heartwood zone. In passing, it should be recalled that although a physiological heartwood is always formed in older trees, extractives are not always produced; thus, the heartwood of timbers such as ash and spruce is colourless.
Where coloration of the heartwood occurs, a whole spectrum of colours exists among the different species. The heartwood may be yellow, e.g. boxwood; orange, e.g. opepe; red, e.g. mahogany; purple, e.g. purpleheart; brown, e.g. African walnut; green, e.g. greenheart; or black, e.g. ebony. In some timbers the colour is fairly evenly distributed throughout the heartwood, while in other species considerable variation in the intensity of the colour occurs.
In zebrano, distinct dark brown and white stripes occur, while in olive wood patches of yellow merge into zones of brown. Dark gum-veins, as present in African walnut, contribute to the pleasing alternations in colour. Variations in colour such as these are regarded as contributing to the ‘figure’ of the timber.
It is interesting to note in passing that the noncoloured sapwood is frequently coloured artificially to match the heartwood, thereby adding to the amount of timber converted from the log. In a few rare cases, the presence of certain fungi in timber in the growing tree can result in the formation of very dark coloured heartwood; the activity of the fungus is terminated when the timber is dried. Both brown oak and green oak, produced by different fungi, have always been prized for decorative work.