Wood Preservation

Timber needs to be protected from:  Insects and fungi  Wear and tear  Effects of weathering Main types of preservatives:  Tar oils  Water-borne preservatives  Solvent based preservatives

Tar Oils Made from coal-tar mixed with chemicals Suitable for outdoor use Black / Brown colour Example: creosote (ESB poles) Applied by brushing, spraying, dipping or pressure treatment

Tar Oils Advantages  Toxic to fungi and insects  Easily absorbed by the wood  Long lasting  Inexpensive Disadvantages  Strong smell  Toxic to plants  Difficult to paint over

Water-Borne Preservatives Made from mixture of toxic salts(copper and zinc) and water When applied to wood, water evaporates leaving toxic salts to act as preservatives Usually pressure impregnated

Water-Borne Preservatives Advantages  Colourless  Odourless  Can be painted over Disadvantages  Timber needs to be dried out by a kiln after treatment  Can cause swelling  Salts can be washed out from the wood

Solvent Based Preservatives Made from toxic chemicals dissolved in a liquid other than water (usually white spirits) After treatment the solvent evaporates, leaving the preservative in the wood Can be applied with a brush or with a spray gun

Solvent Based Preservatives Advantages  Resistant to leaching  Can be painted over  Can be used indoors Disadvantages  Have a strong smell  Can be a fire hazard  Expensive

Methods of application Number of different methods:  Brushing  Spraying  Dipping  Pressure Treatment

Methods of application Brushing and Spraying  Easiest to use, not too costly  Does not penetrate wood deeply, only coats the outer surface Dipping  Wood is dipped into a tank of preservative  Wood soaks in the preservative quickly

Pressure Treatment Most effective and expensive method Preservative is forced into the wood by pressure Wood is placed in a chamber, air is sucked out creating a vacuum, preservative is pumped into the chamber and the vacuum is released. Pressure is applied to force preservative to enter wood cells Final vacuum is applied for a short period to remove any excess preservative Length of time the vacuum is released depends on the permeability of the wood