Timber preservatives are used during manufacturing to ensure that wood used in construction lasts longer and resists decay. Each preservative offers slightly different features which lend certain protective properties depending on where the lumber will ultimately be used. Preservatives are predominantly oil- or water-based, and each type provides specific levels of protection.
Creosote is derived from the process of manufacturing coal tar and then further distilling it. Wood treated with creosote is dark brown to almost black and weathers to a lighter brown. Because creosote is oil-based, it is insoluble in water and leaves an oily residue. Creosote can stain fabrics and other surfaces with which it comes in contact. This preservative is used primarily on telegraph poles, marine timbers and pilings, heavy construction and railway ties.
Pentachlorophenol is another oil-based preservative made from chlorine and phenol which have been allowed to react with each other and then dissolved in petroleum. Wood treated with pentachlorophenol turns light to dark brown in colour and then weathers to a silvery grey. Because it is oil-based, pentachlorophenol is water repellent and helps wood resist mechanical wear and tear. Pentachlorophenol treated wood is used most often in telegraph poles, foundation piling, construction timbers and fence posts.
Chromated Copper Arsenate
Chromated Copper Arsenate, or CCA, is a water-based timber preservative comprised of arsenic, chromium and copper. The type of arsenic in CCA is a different variety of arsenic than that used in rat poisons and is not harmful to humans. Because CCA is water-based, it is not water repellent. A water repellent can be added, which slows the weathering process. Colour pigments are frequently added as well, which have no effect on the treated wood other than changing the colour. Once seasoned, wood treated with CCA is clean and ready for paint.
Ammoniacal Copper Arsenate
Ammoniacal Copper Arsenate is related to CCA; it is also water-based and consists of copper and arsenic mixed with ammonia. Like CCA, ammoniacal copper arsenate is used primarily in applications where human contact is expected to be high. Water repellents are usually added, as ammoniacal copper arsenate does not provide water resistance in addition to its preservative actions. Ammoniacal copper arsenate can also be pigmented if the application warrants.