Colour in Wood
The infinite variations of colour, figure and lustre in wood make it a unique and visually beautiful material to work with.
The colour of wood, like that of other materials, results entirely from the reflection of light. What we see is dependent on the nature and the content of the tissues on the surface. For example, ray flecks in silky oak (Grevillea) in comparison with the fibres (axial tissue-grain) reflect light differently. Appearance also depends on the texture (fineness) and smoothness of the surface finish upon which the light impinges, and on surface dryness.
Wood like any other material absorbs a certain part of the visible spectrum and reflects the remainder. Our eyes can see light over the spectrum of wavelengths from approximately 800nm (1 nano metre = 10–9m, approximately 100,000 thinner than a human hair) which appears red, to approximately 380nm which appears violet.
Above and below the visible wavelengths are infrared and ultraviolet, both unseen by the human eye. Infrared light manifests as heat. White light is a combination of the three primary colours: red, blue and yellow. All other colours result from combinations of the primaries. (Note that combinations of tints and dyes don’t always produce the same colours as similar combinations of light because these are made from tints which absorb some colours from the spectrum and reflect others.)
When you look at a fresh leaf it appears green, because the surface cells absorb all the spectral colours except green and small amounts of colour on either side of green which is then reflected back as light giving the leaf a green appearance. When this reflected portion of light enters the eye it produces an impression perceived as colour. The chemical compounds in wood (mainly the wood extractives) act as absorbers that only allow certain parts of the colour spectrum to reflect back.
Lustre or sheen is highly underrated as an important factor influencing the appearance of wood. Woods that exhibit a high degree of reflected light (for example gombe, shown below) are usually referred to as lustrous, golden, silvery, or satiny.
The lustre or sheen of a wood species may provide a distinctive feature. This is particularly beautiful when variations of grain direction are evident on the surface as in fiddleback or wavy grain, or other well known figured effects. In teak the appearance is dull and greasy; in lignum vitae the wood appears to be coated with a fine wax film, due to the gum guaiacum, which may become oxidised locally and appear as small, bluish patches. The lustre of brown ash (Fraxinus nigra) is dull, a useful distinguishing feature, since other species of true ash are generally somewhat lustrous. Spruce, especially Sitka spruce (Picea sitchensis) has a faint, almost satiny lustre, while in some of the ebonies the lustre resembles that of bone.
Above: Gombe (Didelotia sp) is a species which clearly shows lustre.
African satinwood (Distemonanthus benthamianus) is another truly remarkable timber that is sulphur yellow, sometimes with a greenish tinge, and tends to oxidize to yellow brown. It is highly lustrous. The yellow colour comes from a water soluble compound which can be used as a textile dye.
Describing wood colour
Wood colour can be difficult to describe—particularly if it is intermediate between the common wood colours and also because of the variability encountered within a species. Sometimes even within the one sample, some woods can be placed in more than one
The five categories illustrated throughout this article are used in formal wood identification and timber descriptions to describe the colour of freshly cut wood. Some examples of each are shown.
The wood colour spectrum
Most woods are some shade of brown, ranging from the rich reddish brown of the mahoganies, to the pale browns of the willows and alders, and the grey browns of some walnuts. Among the red coloured woods may be noted the rich purple red of Andaman padauk (Pterocarpus dalbergioides), and the brighter red of some of the African species of this genus.
Yellows range from the pale lemon yellows of the boxwoods (Buxus spp.) and haldu (Adina cordifolia), to the orange of opepe (Nauclea diderrichii), while the hollies (Ilex and Pittosporum) are among the relatively few timbers which may be white. It is the unusual colours, of course, which are most useful as diagnostic characters.
On the other hand, few woods are black, although this occurs in the heartwood of a some ebonies. Pink is also an unusual colour, but it is seen in the pink ivory (Rhamnus zeyheri) and in species of Sickingia, while the heartwood of the blue mahoe (Hibiscus tiliaceus) may be of a bluish colour. The rich purple of purpleheart (Peltogyne) again is an unusual heartwoodcolour.
Heartwood and sapwood
Timber colour varies vary considerably between sapwood and heartwood, among and within species and even within a single tree. Some timbers may show little difference between the pale colour of sapwood and often the darker coloured heartwood, for example in white cheesewood (Alstonia), quandong (Elaeocarpus) and poplar (Populus). In other woods, the difference may be quite pronounced as in merbau (Intsia), jarrah (Eucalyptus marginata), blackwood (Acacia melanoxylon) and New Guinea walnut (Dracontomelon dao).
A timber from the heartwood zone often shows a considerable range of colour and some can be highly distinctive anddecorative. The pale greenish cream of American whitewood sometimes called yellow poplar (Liriodendron tulipifera) might be called the typical colour of this species, but the wood may be black and even a small piece sometimes shows a startling range of browns, purples and greens.
Irregular distribution of colour in the form of streaks and stripes may, on occasion, be distinctive, as for example, in the dark, irregular streaks which occur in the dark reddish-brown snakewood or letterwood (Piratinera); the black and white patches of marblewood (Diospyros marmorata); or the pale and dark brown stripes of the zebrawood, also a species of Diospyros, zebrano (Brachystegia fleuryana) and wenge (Millettia laurentii).
There are a few species which produce only a very small (narrow) heartwood and, because of this and other reasons, practically all the stock used commercially tends to come from the sapwood. Persimmon (Diospyros virginiana), hickory (Carya) and yellow walnut (Beilschmiedia) are examples.
Unusual heartwood colour
There are trees that do not form a coloured heartwood as such, but through injury and the entry of micro-organisms can produce characteristic colouration which tends to be distributed in patches across the stem. Mostly through storm or other mechanical damage, broken branches enable rainwater to seep into the stem which can often find its way right into the tree centre.
A typical example of this process occurs in southern sassafras (Atherosperma moschatum) from Tasmania. The tree grows as an under-storey species and can attain a height of up to 45 metres and is often partly covered in lichen. It gets its name from the fragrance of the bark, and the associated oils which have a smell similar to cinnamon, whilst its leaves have a strong sarsaparilla scent (interestingly the timber is not scented at all). The characteristic blackheart colouring is found in only a small proportion of older trees. The extent of the dark colour is variable but the effect produced is one of striking black flashes or streaks—no two pieces of timber are ever identical!
Colour can change
Time: The length of time wood surfaces have been exposed to air or sunlight is an important factor—cutting off the surface layers from the tangential face or sometimes through a cross-section can reveal changes. Decay and sapstain change wood colour, as obviously will surface coatings such as paints, stains or varnishes. Tree age can have an effect on colour as wood from young or fastergrown trees generally tends to have a paler heartwood.
Exposure: The rich heartwood colours of the padauk and purpleheart turn to a dull brown on exposure when machined. My first experience with purpleheart was some 30 years ago when presented with a board with an inquiry about the type of stain used to obtain such a distinct and uniform purple with a view to reproducing it. Ripping a sample from the board longitudinally the underlying colour was distinctly pale pink and my first impression was that the sample had indeed been stained. However, coming back to it an hour later the surface had regained the purple colour.
Similarly, lignum vitae tends to develop a greenish-blue waxy colouration after the fresh surface has oxidised. In the standing tree, alder (Alnus) wood is white, but after felling the colour changes rapidly to reddish or orange brown and equally quickly fades to a pale warm brown.
Steaming: Another amazing transformation is how some pale creamy coloured woods can become a striking pink colour. The pale yellow-brown heartwood of European beech is normally indistinct from its sapwood but turns pink when steamed. A number of pale coloured eucalypts from the ash group can undergo a similar colour change when green wood is steamed or if the steam reconditioning process is carried out for too long, particularly if the moisture content of the wood is still relatively high after insufficient pre-drying.
Ebonising: It’s curious how the wood of most eucalypts darkens when it comes into contact with iron in the presence of water either as green wood or rewetted dry wood. Everyone is familiar with the unsightly black stain trails emanating from the heads of nails on a hardwood paling fence. Many eucalypts, particularly those from the pale coloured ash group, contain soluble ellagi-tannins which react with iron in the presence of water to produce an intense black or bluish stain. I have seen many examples of this effect over the years in buried eucalypt logs in which this process had been going on for years and the result was a through-through black, or dark brown wood often with black or paler streaks. The impression is that hey presto we have a black coloured ebony-eucalypt! Superficially yes, but the wood lacks that almost ‘bone’ look which tends to buff up with true ebony.
Another very easy way to obtain an absolutely black surface is to apply a solution of ferric chloride, ferric oxide or simply rust to timber containing tannins. Both of the chemicals are highly water soluble. And once the reaction has occurred the result is relatively permanent. Some interesting effects can be obtained by using different concentrations, or by moderating the intensely matt black surface with a weak acid which has the effect of rupturing the iron-tannin combination. The stronger the acid, the greater will be the ability to undo the reaction. In fact that is one way to remove the dreaded unsightly stains from both wood and concrete. Steel wool in vinegar can have the desired effect of dissolving the iron to produce soluble iron (ions) which can react with the tannin, but it may also tend to undo the reaction that produces the black colour depending on the concentration.
Fuming: Another pair of startling colour changes can be obtained by exposing the heartwood of brushbox to fuming ammonia (this needs to be done with great care). A distinct green colouration is produced on the surface from a reaction with the tannins therein. A different process that enables a pale coloured wood (such as pine, hoop pine or poplar) to be made into a darker brown colour without using stains is to apply a solution of potassium permanganate. The latter is a strong oxidising agent called permanganate of potash or Condy’s crystals, which is a beautiful deep purple-violet colour when dissolved in water. Once applied to the surface the timber surface initially retains the same colour but within minutes begins to change as it oxidises to a light then a darker brown colour depending on the concentration of the as the reacting compounds. This can be a nice way of darkening lighter shades of reddish-brown woods.
Finally, certain woods when viewed under ‘black light’ emit a mysterious glow or fluorescence. This has to be seen to be believed and is certain to inspire creative ideas for woodworkers, carvers and turners. Fluorescence, unlike reflection, is produced when a fluorescing material absorbs invisible light and emits visible light.
Ultraviolet (long-wave UV) light is invisible and often called ‘black light’. This is the same light that banks use for checking passbook signatures and that is popularly seen in CSI shows for spotting blood traces. Other sources are black light globes, fluoros and black light torches. Higher powered lamps are also available and tend to be reasonably pricey but they will enable observation in daylight or subdued light.
Compounds in fluorescing materials absorb this invisible black light and transform the energy, emitting it back as visible light within a part of the visible spectrum. Some old timers may remember lights in discotheques of the late 60s; dim violet/lilac and when you walked in your shirt glowed brightly so much so that the light reflected from it could be seen to light up other objects. The same effect is produced from certain woods that fluoresce although less dramatically but it is spectacular particularly when the wood is freshly cut or machined.
The fluorescing materials from timber originate exclusively from the heartwood extractives. The effect is seen best when the wood is exposed to UV in slightly darkened surroundings. Fluorescing woods can produce a range of different colours including yellow, yellow green, green, bluish and orange. Water and alcohol extracts from the wood also show a similar effect with weird swirls of light.
Most common woods (including eucalypts and the pines, and sapwood from all species) do not fluoresce, and appear to glow with non-descript dim lavender-violet colour mainly from the reflected black light. Fluorescence is barely recognisable in most species under normal (undecayed) conditions, but it can be strikingly brilliant in some. Timber families including Leguminosae, the family of blackwood and blackbean, blacklocust, purpleheart and merbau (Intsia); Anarcadiaceae, the family of New Guinea walnut (Dracontomelon), sumac (Rhus), to name a few, furnish the majority of fluorescing woods while the others are more rare.
The most prevalent is yellow and it appears as a brilliant glow from blacklocust (Robinia pseudoacacia), while padauk (Pterocarpus indicus) is a greenish yellow. Reportedly, growth rings from the heartwood of sumac show yellow-green and lavender blue sequences. Purpleheart shows a coppery, and blackwood has a yellow-greenish, mango (Mangifera) has an orangey-yellow and wild cashew (Anacardium excelsum) has a greenish glow.
A range of environmental, structural, chemical and biological factors contribute towards the infinitely varying colour and figure of wood and ultimately why it is such a beautiful material to work with.
Jugo Ilic is a wood scientist and author of numerous publications on wood anatomy and identification, as well as a database of botanical and common names of world woods.