Spalted wood – Gone to rot!

Spalting is a by-product of the rotting process that affects all wood to a greater or lesser extent. It’s caused by moulds and fungi, and can create many different and beautiful colours and patterns in the wood. This makes spalted wood highly prized for its looks, especially by turners

While there are many examples of wooden artefacts that are a thousand years old but still in perfect condition, the inherent tendency of wood to decompose and return back to nature is always there. It’s merely held at bay by Man, but no matter how long the timber has survived, it takes only a minor change in environmental conditions to trigger off the decay. Left in the wild, trees age and die, and then rot away and return nutrients to the soil to begin the growth process over again. However, we can delay this process, primarily by getting the wood properly dry during seasoning.

Cause and effect

Decay can be brought about through a number of agencies such as bacteria, insects and most importantly, fungi. There’s often an interaction between them, making it difficult to judge which is cause and which is effect. Whatever the cause, some of this degradation can work to our advantage as woodworkers, producing some amazingly decorative effects. Spalted timber is a prime example of this relationship, the wonderful patterns being the result of a fungal infection.

The three degrees

Fungi are very low forms of life and are unable to produce food for themselves like a living plant, so to survive they take nourishment from the timber they have infected. The degree to which this affects the timber gives us a broad classification of the huge fungal group into moulds, stains and rots.

Moulds are infections of the wood surface, photo 1, whereas stains penetrate more deeply into the actual cell structure. Both feed off the carbohydrate stored in cell cavities, but don’t actually affect the cell structure. The most obvious symptom is discoloration – usually a bluey grey colour that is disfiguring rather than attractive.

Rots, on the other hand, are far more destructive. They feed by producing enzymes that break down the cell structure itself. In the early stages of attack the first symptom is again some form of staining, photo 2, which is often referred to as ‘dote’ and only moderately affects the strength properties of the timber.

Wasting away

The process of decay is progressive, however, and advanced decay results in a softening of the wood and eventually total loss of strength. There are many totally different forms of attack. Some fungi thrive only on the heartwood of standing timber, leaving behind the characteristic hollowed-out shell. Others colonise logs only after they’ve been felled, photo 3, or sawn timber while it is being seasoned, photo 4.

Perhaps the most important group from an economic viewpoint are the fungi that attack timber after it has been put into service, including the highly damaging forms such as dry rot, photo 5.

Dispensing spores

Wood-rotting and sapstain fungi belong to a huge group of plants that includes mushrooms and toadstools. These large visible growths are the fruiting bodies of the fungus, photo 6. They produce the single-celled spores which disperse to continue the colonisation. The damage is caused by their vegetative feeding system, which is often not visible on the surface, but consists of hundreds of fine tube-like structures called hyphae which grow rapidly through the wood structure, devouring both the cell walls and their contents.

Four ways to flourish

Like all living structures, fungi need some basic conditions in their favour to survive and thrive. Knowing what these conditions are and then regulating them gives us a real means of controlling them. There are four requirements.

• – Temperature The optimum temperature for fungal development is in the range of 20 to 30°C. There is little activity beyond these extremes, which explains the sudden burst of summer activity in temperate regions with cold winters. Unfortunately this is also the temperature range within which we like to live.
• – Oxygen There must be an air supply for rot to occur; fully waterlogged timber rarely decays because of the lack of oxygen. This is why in some countries logs are often stored in water until they are ready for conversion, and why timber piles for harbours and piers will last forever when fully immersed.
• – Moisture The ideal moisture state for fungal attack is at (or just above) the fibre saturation point. It is not until wood is dried below about 20 per cent moisture content that you can be sure it’s safe from attack. Interestingly, drying infected timber doesn’t necessarily kill the infection. It may just lapse into a dormant state, ready to return if the moisture content rises again. Dry rot is an exception, in that it usually dies off in prolonged dry conditions.
• – Food The wood on which fungi live provides the necessary food source. The fungi actively break down the cell walls and their contents, particularly the stored carbohydrate in the sapwood.

The heartwood of some trees contains deposited chemicals that are poisonous to fungi, rendering these species resistant to decay. This explains why some timbers such as oak are naturally durable; fence posts exposed to the weather for years quickly lose all their sapwood, but the heartwood core remains untouched. Jarrah, often used to make railway sleepers before concrete took over, still looks perfect after years of service exposed to the weather, photo 7, showing no sign of staining, fungal infection or decay.

All of these conditions must be in place for rotting to occur. Lack of one of them usually halts the process, even if all the others are present. As an example consider bog oak, often buried for thousands of years but with no available oxygen. It can be dug up perfectly intact, but very soon after being exposed to the air it starts to fall apart.

Identifying fungi

The two main constituents of wood are lignin and cellulose, and identifying which of these is attacked helps to finger the particular fungus involved. The brown rots feed mainly on cellulose, while the white rots feed on both cellulose and lignin. The degree to which either of these substances is affected results in a different form to the decay – cubic rot, pocket rot and so on.

As lignin is the main constituent holding all the wood cells together, anything that attacks that is particularly destructive. A lot of the pigmentation of wood is formed in the lignin, so white rots have a pronounced effect on timber colour. If white rot becomes really advanced the timber becomes pale, soft and spongy, photo 8.

Standing tree fungi

Fungi that attack standing trees are responsible for losses to the forest owner, but they will rarely affect us as timber users because once seasoned, such wood is safe from further decay from this fungus group. An exception here is brown oak, which is normal oak that has been infected by the Beefsteak fungus (Fistulina hepatica). This gains entry to the tree through a wound of some sort, but causes no damage to the living tree apart from extracting nourishment. However, it causes chemical changes which result in a highly-prized brown coloration that highlights the wonderful figure in the oak, photo 9.

Log and plank fungi

Sometimes trees are felled and left lying for some time before being converted into planks. This delay before conversion and drying is the prime cause of infection of logs by decaying fungi. The potential problem is far worse in tropical countries, where warmer temperatures mean that fungal development is much more rapid. These climates also tend to harbour more variety in the way of bark-boring insects and ambrosia beetles.

Converted timber kept in adverse drying conditions is just as likely as logs to be infected with fungi of some sort, photo 10, particularly those timber species which don’t have naturally resistant heartwood.

Finished wood fungi

The only sure way to prevent wood-rotting fungi from attacking timber in its finished situation, such as in a building, is to use sound, kiln-dried material which is free from fungal infection in the first place, and then to provide sufficient ventilation to prevent it from becoming damp. The very destructive ‘dry’ rot (Serpula lacrymans) is able to thrive in conditions of relatively low moisture content, and any infection it causes must be treated very seriously indeed if major structural damage is to be avoided.

Why spalted wood?
The first stage in the decay process is the invasion of the timber by a range of different fungi, which start to break down the wood material, often producing a variety of colour changes in the process. It is when these colour changes produce decorative effects, but without substantial loss of structure, that the wood is said to be ‘spalted’.

It is only in recent years that the merits of partly rotted wood as a decorative woodworking material have come to the fore. There is nothing more spectacular than a highly spalted piece of wood. The rich intermingling of colours occurring in a stunning kaleidoscope-like pattern can be almost breathtaking.

How spalting occurs

What actually happens here is an extremely complicated biological process, the outcome of which depends on a number of interrelated factors. Fungal spores are ever-present in the atmosphere, floating round us all the time. If one happens to land on a piece of wood which is in a suitable state for growth, then the infection begins.

Once the fungus is established, it starts spreading out via a mat-like mass of tiny roots called the mycelium. As the individual strands of the mycelium, the hyphae, grow out, they digest the wood material ahead of them.

Usually this first infection by the pioneer fungus has little effect on the strength of the timber, merely discolouring it, photo 11. There is not just one species involved here, but dozens of different types, some specific to a particular wood species or group.

Multiple invasion

It is only when another species of fungus joins in that spalting, as opposed to rotting, starts. Each fungus – and there may be many in a single infection – makes its own way through the wood, leaving a different colour behind as it goes. Whenever one species comes up against another, then a black zone line is formed. This is what characterises spalted wood, with the pockets of different colours each surrounded by a black line, photo 12.

Sometimes there may be only a couple of different fungal species involved, so the spalting is quite bland, but if there are lots of them all intermingling then the zonal lines become much more numerous and decorative. They also vary in width, from the thickness of a human hair up to several millimetres across, photo 13.

There’s a lot more to this, though, as one fungus may eventually overcome another, usually by producing what are effectively antibiotics, so spalting is not inevitable. It all depends on the infecting species.

The arrangement of the spalting appears to follow no specific evolutionary pattern either. Sometimes it seems to follow the direction of the rays, while at other times there may be a distinct boundary between spalted and not, photo 14.

Using spalted wood

The trick is to use the wood when it is still infected by these pioneer fungi, but while the composition is still reasonably sound. The next stage is infection by secondary fungi that are usually much more specific in action and will totally destroy the timber. The yellow patches of secondarily decayed timber can sometimes be quite decorative, but you have to strike a balance between this looking attractive yet being unusable as a material, photo 15.

Light-coloured woods are usually more susceptible to infection, having less natural resistance, a property usually imparted by the chemicals deposited in the heartwood of dark timbers. So beech, sycamore and maple will spalt readily, whereas walnut or elm is less likely to do so. These dark woods tend to need infection by a specific fungus, which is able to overcome the toxic effect of their deposited chemicals.

A final wordUsing spalted wood also has its own problems. In a well-figured piece the different colours produced by the different fungal species all leave wood with different densities. Consequently, when you come to work it some areas will cut cleanly, some will rip a bit and some will just pull out in chunks, making it a very frustrating experience, photo 16. There are also the health aspects of inhaling the infected dust to be considered as well.

I’ll take a look at all these pitfalls and their solutions next month, and illustrate them by turning a piece of spalted beech for you.