Figure in wood
Words: Jugo Ilic
Figure in timber is associated with irregularities of grain, texture, prominence of cells, variations in colour and combinations of these attributes. All these features contribute to the beauty of timber which gives it its greatest advantage over other materials. Beautifully figured timber has been used from time immemorial for decorative purposes and it has few if any serious rivals. The great variety of natural patterns, the blending of colours, delicate variations of light and shade, lustre and warmth of wood are all attributes which make it a unique visually beautiful material.
Figure also depends on the way the wood has been cut. Knowledge of the different sawing methods and of the way various wood elements (cells) have contributed to the different effects is essential when buying or cutting timber if a particular kind of figure is desired. Terminology is also important. Terms such as grain and texture – structural features which give rise to figure – tend to be used loosely, leading to confusion.
Many timbers may have ‘plain figure’ and don’t normally show a distinctive or unusual figured pattern in any way. Others are always highly figured. For the most part, figure boils down to one or more of the following:
1. Sawing pattern
2. Distribution of wood cells
3. Grain variation and irregularities
4. Irregularities in the tree
5. Variation in colour
1. Figure arising from sawing pattern
Wood can be cut along the grain in two distinct ways. One, tangentially to the growth rings and at right angles to the rays1 which run from the centre of the tree to the outside, like the spokes of a wheel. This produces backsawn (also called plain-sawn, backcut or flat-sawn) timber. Two, across the growth rings and parallel to the rays, producing radially cut or quartersawn2 timber.
Depending on the species, in the south-eastern states of Australia eucalypts are mainly quartersawn because the wood moves less3 and it is easier to dry, producing less degrade. On the other hand, backsawn timber can be cut with less waste and in many wood species, including eucalypts, true ash, elm, and almost all softwoods, it shows a nicer figure.
In timbers in which growth rings are distinct because of the presence of definite bands of latewood, backsawn material will show a distinctive figure due to the cutting through of a number of these bands. When the timber is ring-porous, i.e. a timber in which the early wood vessels (pores) are much larger in size and more numerous than in the latewood, backsawn surfaces have a very distinctive appearance. Examples are red cedar (Toona ciliata) and alpine ash (Eucalyptus delegatensis), and imported timbers such as hickory, elm (Ulmus spp.), oak, ash (Fraxinus spp.) and teak (Tectona grandis).
In softwoods such as spruce (Picea spp.), Baltic pine (Pinus sylvestris), Douglas fir, radiata pine (Pinus radiata), celery top pine (Phyllocladus asplenifolius) and King William pine (Athrotaxis selaginoides), backsawn material shows distinctive patterns of irregular appearance similar to those of hardwoods, except that the light-coloured areas are earlywood and the darker areas latewood.
In many timbers, little or no distinctive figure is revealed on the quartersawn surfaces, although beautiful figure variations are sometimes possible with certain grain irregularities to be discussed later. How- ever, distinctive figure occurs in those woods with wide (large) rays. For example, true oak (Quercus spp.) when quartersawn shows a pronounced flaky or silver figure due to large rays. Such figure is also com- mon in some of our well-known Australian timbers known as oaks, notably the silky oaks (Cardwellia sublimis and Grevillea robusta), and the sheoaks (Casuarina spp.). A somewhat subdued version of this figure is evident in red tulip oak (Argyrodendron peralatum) from North Queensland.
Veneers are highly versatile; they can be either cut from squared timber or flitches, or peeled in continuous strips or layers from a rotating log to form rotary peeled veneer.
Straight pieces known as sliced veneer can be cut with a saw or sliced with a knife. The advantage of sawn or sliced veneer is that it can be cut in any direction, radially or tangentially or along any intermediate plane. Usually it is done radially so as to produce the types of figure similar to quartersawing, e.g. silver grain or flaky grain as in tulip or silky or true oak.
On the other hand, rotary veneer has the advantage that it can be produced in large sheets at low cost. The figure of rotary cut veneer is continuous whereas that from sawn or sliced veneers merges towards both edges to a form intermediate between fully backsawn or quartersawn figure.
2. Arrangement of wood cells
Figure of pleasing appearance on backsawn surfaces of some hardwoods may be due to the presence of numerous bands of parenchyma (soft tissue). These bands run parallel to the growth rings and give a similar type of pattern to that obtained from distinct earlywood-late-wood bands. They form delicate (fine) markings, as seen on backsawn surfaces of rose mahogany (Dysoxylum fraseranum), coachwood (Ceratopetalum apetalum), and Crow’s ash (Flindersia australis).
Texture refers to the size and disposition of the wood pores (vessels). It can be coarse or fine, even or uneven. In merbau (Intsia bijuga) and blackbean (Castanospermum australe) the large pores accentuated by soft tissue give rise to a coarse texture and are readily visible to the unaided eye. On the other hand, in myrtle beech (Nothofagus cunninghamii) and musk (Olearia argophylla) the small pores create a fine texture invisible to the unaided eye. Even or uniform texture results from little variation in pore size within the growth rings, as in taun (Pometia pinnata) and turpentine (Syncarpia glomulifera). Uneven texture is seen in woods in which the pores vary in size and number across the growth ring, as in elm (Ulmus spp.), teak (Tectona grandis) and hickory (Carya spp.).
The presence of wide sheaths of parenchyma surrounding the pores shows distinctive markings on longitudinal surfaces in some hardwoods. Thus, the vessel lines appear distinct as white or pale against the surrounding wood tissue, particularly in dark timbers. The effect is prominent in black bean.
The inset photomicrograph in fig.2 above is a cross section of blackbean showing the numerous thin-walled parenchyma cells which form a sheath around the pores. Monkey pod (Pithecellobium saman), a south-west Pacific timber often used for turned bowls and carvings, shows the same effect.
These are very fine markings seen on backsawn surfaces of certain timbers, and in some cases they are sufficiently distinct to give a fine regular figure. Such markings are often discernable in cedar mangrove (Xylocarpus granatum) and in several other Australian timbers, notably black bean and red tulip oak. These fine ripples arise from the regularly aligned or storied arrangement of the rays and other wood cells. They should not be confused with fiddleback due to wavy grain.
3. Figure due to grain irregularities
Grain deviation away from along the tree axis, in or out of the tangential or radial plane of the wood, gives rise to wavy or interlocked grain. These can all have marked influence on the figure of timber. Some of these irregularities of grain direction give life or lustre to the timber.
The beauty of high lustre is hard to describe, but it may be referred to as the manner in which light reflects from the wood elements. It is responsible for grain irregularities appearing different when the wood surface is viewed from different angles. Thus grain direction appears to move when viewed from different angles. The two photos shown left of quartersawn sapele mahogany (Entandrophragma cylindricum) illuminated from a different angle illustrate this.
Figure from wavy or crossgrain
In wavy-grain logs, quartersawing or slicing results in the best figure. Light reflects at varying angles from the surfaces because the individual elements are cut across at varying angles. Fiddleback is used to describe the fine and regular markings or waves on the quartercut surfaces which appear as lustrous bars across the piece. Fiddleback is found in maple, blackwood, mountain ash and alpine ash, jarrah and other species. The attractiveness of this figure is due almost entirely to the variation in lustre when the surface is viewed from different angles or moved in the light.
When the waves from the changing direction of the fibres occur singly, or in groups, appearing as short intermittent lustrous streaks, the figure is referred to as ‘raindrop’ because it is reminiscent of the streaks made by drops striking a surface on a slant. This type of figure is common in Qld maple (Flindersia brayleyana).
Another figure produced by local irregu- lar waves extending for short distances across a quartersawn face is often referred to as ‘mottle’. Mottle may also be applied to the type of figure shown on the backsawn surfaces of silky oak due to the appearance of the wide rays.
The surface of quartersawn timber possessing regularly interlocked grain appears as alternate longitudinal lustrous stripes. Such stripes are due to the changing inclination of the elements in successive growth layers. In some cases, the elements are not always in continuous spirals around the tree, but are in large waves up and down the trunk. The best ribbon stripe appears when the interlocking grain spirals alternately in successive growth layers around the tree. When the interlocking is not in successive growth zones, the waves up and down the trunk produce the effect known as ‘broken stripe’.
‘Roe’ figure arises from short broken stripe. The beauty of all these stripe types of figure is due to the ever-changing lustrous effects obtained when the surface is viewed at varying angles. Good examples are found in many Australian timbers. Qld maple and Qld walnut are the main commercial species showing this type of figure.
In addition to the various type of ribbon stripe and the variations of cross-figure, the two types may occur in combination to add further to the beauty and variety of patterns resulting from interlocked and wavy grain.
4. Figure due to tree irregularities
Large knots in timber generally result from cutting through the base of a branch. Knots slope upwards to some extent, and the wood elements pass from below them up into the lower part of what was the branch. Small knots result from branches which have dropped off or been broken off while the tree was comparatively young. As a result, the knot is enclosed in the trunk of the tree and revealed during sawing. There is often wrinkling and alteration of the direc- tion of the wood elements around knots. In a board this gives rise to a lustrous effect arising from the variations in the light reflected from these areas.
Knots are most common in softwood timber species and aside from lustre confer some degree of attractiveness on the timber. When the timber is backsawn, the small knots are cut across the grain and hence have a characteristic appearance in the resulting board. In those species where knots are sound and firm, backsawn boards have a certain figure. For example, the knots in cypress pine (Callitris spp.) enhance its general appearance in flooring and furniture.
Burls are abnormal growths or excres- cence, common to many different species; they are formed by the local development of numerous dormant buds. These result from some injury or irritation to the trunk of the tree. Burls are of usable size and quality in comparatively few species, of which redwood (Sequoia sempervirens), walnut, maple, amboyna (Pterocarpus spp.) and black ash (Fraxinus nigra) are among the best known. They occur fairly com- monly in a number of eucalypts such as jarrah (Eucalyptus marginata), but as they are the results of injury to the cambium, numerous kino veins are usually present. They are highly prized for veneers and articles of turnery, since the interwoven mass of wood elements gives an attrac- tive and unusual figure no matter which way they are cut. Also, since the wood elements are so gnarled, the variations in the angle of reflected light give added life to the surface.
This particular figure or mottling is very common in backsawn pieces of northern hemisphere maple (Acer spp.). It is also found and highly prized in musk (Olearia argophylla). In maple, the surface of the wood has dimples or pits and there are spikes in the inner bark which enter into the pits. After these depressions are formed, the succeeding growth rings follow the same contour for years. Thus, on the backsawn faces, the timber appears to have a number of oval or circular eyes. These depressions are thought to result from local injuries to the cambium layer brought about by a parasitic fungus.
Crotches and buttresses
In the crotch of a forked tree or at the origin of a root at the buttressed base of a tree, the folding or wrinkling of the wood elements is quite marked, and of- ten there are local deposits of extraneous materials in the cells. Such areas of the tree are much in demand for veneers and special articles. Much of the highly figured veneer from Qld walnut is obtained from stumps. Veneer from the crotch of a mahogany tree often exhibits feather-like curls or a flame-like appearance.
5. Variation in colour
The wood colour can have a profound effect on the normal figure. For example, while two timbers may have an identical appearance due to structural or grain irregularities, one will appear more beautiful because of colour variations. Such variations may arise from irregular deposits of coloured matter (wood extractives). These are generally independent of the growth ring bands.
The pleasing appearance of Qld walnut (Endiandra palmerstoni) and blackwood (Acacia melanoxylon) arises from the marked variations in the depth of colour which highlights the growth rings. When quartersawn, plain stripe figure is evident.
The bizarre brown and black striping of zebrano (Brachystegia spp.) and wenge (Millettia laurentii) against a creamy pale or greyish-brown background also arises from the dark coloured deposits. The staining action of certain fungi (not wood destroying forms) also influences the colour of wood, and many distinctive variations result, as in southern sassafras (Atherosperma moschatum).
Figure is one of the most highly valued attributes of timber and derives from complex relationships between the factors discussed.
1. Medullary rays is an archaic term that is no longer used for ray tissue in wood that we deal with. It refers to the ray cells that are formed in he wood that is laid down during first stages of growth.
2. Quartersawn is derived from the early practice of cutting a log into quarters and sawing the boards parallel to the faces.
3. See The science: why wood moves
Dr Jugo Ilic worked as a wood scientist at CSIRO for 36 years. As part of his research activities he authored the CSIRO Atlas of Hardwoods and curated the Australian Wood Collection. He has written several articles for Australian Wood Review, some of which are shown above right. This article was first published in issue 57.