Certain characteristics of wood can be modified by chemicals, for instance vulnerability to insect and fungal attack, weathering and fire.
There are many chemicals, utilised singly or in combination, which are used to preserve wood against insect and fungal attack. There are three main groups of preservative: tar oils, water-borne preservatives and those borne by organic solvents. Preservative treatment does not affect the weathering of timber; for most species, some form of surface finish (eg paint, varnish or stain) is also needed, especially when used outdoors.
Creosote, a by-product of coal, is the most commonly used tar-oil. Because of its smell and dark colour, and tendency to bleed out of the treated wood, it is generally only suitable for wood used outside, eg as fence posts or transmission poles. The use of creosote preservatives in the EU will be restricted from 2003.
Formulations of copper, chromium and arsenic (CCA) are commonly used water-borne preservatives. Though toxic to insects and fungi, treated timber is non-toxic to humans and animals. Timber treated with water-borne preservatives needs to be re-dried after treatment. Once dried, treated timber can easily be painted, and the chemicals are odourless. The use of CCA preservatives is under debate in Europe and preservative treatment companies have developed a range of alternative treatments.
Organic solvent-based preservatives come in a wide range of formulations. They are toxic if ingested by humans and animals. They can be painted over or sealed. Organic solvents are widely used for prefabricated joinery since they do not wet the wood and cause swelling. Recent developments include delivering the active ingredients as water-borne micro-emulsions to avoid volatile emissions.
