Methods of preserving timber

Preservation is the treatment of timber with chemicals to protect it from decay by fungi and from attack by insects (termites, borers) and marine borers, so that its life increases. Timber should be seasoned first.

Methods

1. Brushing or spraying: a preservative (coal tar, creosote, copper sulphate solution) is applied on the surface with a brush or spray in 2-3 coats. It is cheap, but penetration is limited.
2. Dipping: timber is dipped in the preservative (for example, copper sulphate solution) for a few minutes or hours; used for small members.
3. Steeping (soaking): timber is soaked in a cold or hot preservative solution in a tank for several days; penetration is better.
4. Hot and cold open-tank treatment: timber is heated in hot preservative (about 90 C) for several hours and then moved to cold preservative, which is drawn into the wood as it cools.
5. Pressure treatment (full-cell/Bethell or empty-cell process): timber in an airtight cylinder is first evacuated, and the preservative is then forced in at 8-12 kg/cm², giving deep penetration; this is the best method but costly.
6. Charring: the surface of timber (such as posts) is burnt to a depth of about 15 mm; cheap but of limited use.
7. Diffusion and sap displacement (Boucherie) methods are also used for green timber.

Common preservatives: coal tar and creosote (oil type), copper-chrome-arsenic (CCA, water-soluble), and organic-solvent types (pentachlorophenol).