Classification and description of preservatives

Three major types of wood preservative are tar oil, organic solvent and water borne; their composition and use is comprehensively covered by British Standards and Codes of Practice.

Tar Oils (type TO). The principal type is creosote which is derived from coal or wood distillation. Heavy Creosote Oil are usually applied by vacuum pressure processes; lighter creosotes by dipping, spraying or brushing. The main problems are the smell, the risk of tainting, and the emission of toxic smoke if treated timbers catch fire. Timber treated with creosote is not suitable for painting, but may be stained after a period of weathering. This material is now only available for industrial and specialist professional use.

Waterborne Preservatives (Type WB). These are usually based on copper, with additions of other pesticides. They are applied by high pressure treatment, typically for ground contact uses, e.g. fence posts and decking.

Treated timber should not be used in direct contact with rubber or with light alloys, such as aluminium. Wood treated with copper salts has a characteristic green colour.

A different type of water-borne preservative is exemplified by disodium octaborate which is applied by a diffusion process to unseasoned wet wood before seasoning. The treated stock can then be converted and assembled without fear of distortion. These preservatives are not fixed and must be protected both on-site and after installation.

Also used are microemulsion preservatives based on organic biocides, applied by double vacuum treatment, principally for out of ground contact timbers, such as joinery, carcassing etc. Other examples of microemulsions are available for use in specialist curative treatment of buildings against decay and insect attack.

Organic Solvent Preservatives (Type OS). These are based on biocides soluble in hydrocarbon solvents. They have good penetration but also have a strong odour which may taint foodstuffs when fresh. They are often flammable and need careful handling until they are dry.

ICI Paints supplies such products under the brand name Cuprinol.

Treated wood must be allowed to dry before overpainting and this will require from two to seven days with normal ventilation. The solvent will take longer to evaporate in stacked timber.