Preservative-treated wood is typically pressure-treated, where the chemicals are driven a short distance into the wood using a special vessel that combines pressure and vacuum. Although deeper penetration is highly desirable, the impermeable nature of dead wood cells makes it extremely difficult to achieve anything more than a thin shell of treated wood. Key results of the pressure-treating process are the amount of preservative impregnated into the wood (called retention), and the depth of penetration. These characteristics of treatment are specified in results-based standards.
Wood intended for use in construction has most commonly been treated in the past with the chemical chromated copper arsenate (CCA). The wood treatment industry in the US and Canada began a transition away from CCA for most residential uses in January 2004. Two alternatives are taking its place: amine copper quat (ACQ) and copper azole (CA). CCA is still available for shakes and shingles, permanent wood foundations, and most non-residential uses such as utility poles.
In CCA, the copper is the primary fungicide, the arsenic is a secondary fungicide and an insecticide, and the chromium is a fixative which also provides UV resistance. CCA is applied to wood in a water solution and chemically reacts with the wood to form a virtually insoluble precipitate. This reaction is called fixation. CSA standards require all wood to be tested to ensure fixation is complete before the wood leaves the treating plant. Most CCA-treated wood contains around 1.5% by weight of preservative, but the amount in the wood is normally expressed in kilograms per cubic metre.
Ammoniacal copper zinc arsenate (ACZA) is a CCA alternative with a limited range of applications, mostly in the marine environment. Amine copper quat (ACQ) is an alternative to CCA which was originally patented in Canada and has been commercially available in the USA, Europe and Japan for several years. It is now available in Canada. It contains copper as the primary fungicide and a quaternary ammonium compound (quat) as the secondary fungicide. Quats are surfactants and some types are used to clean pipes in breweries and dairies. Others are found in moist antiseptic wipes and eyewash. Copper azole (CA) is also available in Canada and is another alternative to CCA. This is a copper-based preservative with an organic secondary fungicide. Acid copper chromate and copper citrate are other CCA alternatives available in the U.S.
Other alternatives to CCA for certain applications are borate preservatives. Borates are seen as more environmentally benign than CCA, however they do not become fixed in the wood. This means they cannot be used in a continuously wet application, as the preservative would be at risk of washing out, leaving the wood unprotected. Like CCA, borate preservatives are also applied to the wood with pressure. Unlike CCA, borate will continue to diffuse into the wood after pressure treatment, yielding deeper penetration. In fact, it is possible to achieve penetration with borate throughout the entire piece. Borates are also commonly used in non-pressure methods of wood treatment.