Timbers vary widely in their natural durability. The outer part of the tree, the sapwood that is often lighter in colour, is susceptible to rot and insect attack. The heartwood is often darker in colour and may have more durability.
Ultimately timber is tested for natural durablity by placing stakes in the ground - so called 'graveyard' tests - and seeing how many years elapse before they fail.
Soft and light coloured timbers are often perishable (Balsa, Poplar) whereas many higher density and dark coloured timbers can last for 40 years or more (Greenheart, Teak). Of course there are exceptions to this rule. Western Red Cedar for example, widely used for cladding and garden sheds, is low density but full of natural compounds that give it protection.
In many applications, naturally durable timbers are either not available in quantity or are too expensive. The wood protections industry seeks ways of giving enhanced durability to more available and cheaper materials. Building timbers and fencing are two large markets needing solutions.
Until a few years ago, the term wood preservation was applied. This meant impregnating timber with chemicals such as creosote, copper-chrome-arsenic, pentachlorphenol, etc. Whilst all these chemicals have given excellent service, many have been withdrawn or are restricted owning to environmental concerns. For many years now, we have been seeking other solutions, not all of them chemical. Strategies for wood protection may now include wood modification, design for durability and physical protection measures.
Having developed a suitable preservative, it then has to be applied. Most timbers are resistant to being treated, so the protection is limited to an envelope of treatment. Research continues into ways of improving penetration of these preservatives.
At the same time as developing preservatives, research continues into understanding the mechanisms by which organisms detect and digest wood and this has led to some novel solutions. One approach is to wrap fence-posts at the groung-line, altering moisture content and thus reducing attack of this critical region by decay organisms.
If timber is subjected to high temperature cycles in an oxygen-free atmosphere, chemical changes take place that limit its hygroscopicity, hence ability to be digested by insects and fungi. This process is known as thermal modification, Platowood and Thermowood are two examples, and many other thermally modified products are now widely available. Common uses include cladding and flooring.
Chemical modification can also increase the service life of wood, as well as providing dimensional stability. One product is Accoya, which is a trade name for acetylated Radiata pine. Other systems include Kebony, which is a furfuryl alcohol modification process.
The wood protection industry has interests in pre-treating wood before it is installed and also in the in-situ treatment of wood in service. Very often this is associated with other aspects of property care such as control of damp and has its own special challenges.