Types of Preservatives
The most commonly used wood preservative in North America for decades has been chromated copper arsenate (CCA), although this is changing since CCA began phasing out of most residential applications in January 2004. Replacing it are amine copper quat (ACQ) and copper azole (CA). These preservatives are all applied with pressure. They all have a green tone, which fades over time.
Borates are another class of waterborne preservative gaining in popularity. Borates are colourless, non-toxic to humans, and can be applied to wood with or without pressure. Their use is currently limited to applications which are protected from rain and other persistent sources of water. These include framing in termite areas and repair of decayed framing in leaky buildings where the main moisture source has been eliminated.
Creosote is the well-known black oily wood preservative, the oldest type of preservative still in modern use. It’s now used in Canada almost exclusively for railroad ties, where its resistance to moisture movement is a key advantage. Pentachlorophenol in oil is mainly used for utility poles where the surface softening characteristics of the oil are useful in pole climbing.
Copper naphthenate and zinc naphthenate are two common preservatives used for treating field cuts.