Wood used for outdoor building is often pressure treated with a variety of chemicals to give it increased resistance to moisture, fungus and wood-eating insects. Some of these chemicals are water based, some are oil based; they almost all change the color of the grain, making pressure-treated wood easily identifiable. Some chemicals used in pressure treatment make wood unsuitable for residential use.
Chromated Copper Arsenate (CCA)
CCA was once the most common chemical used in pressure treating. It contains chromium, copper and arsenic and turns the wood dark green. CCA protects against decay, fungi and wood-boring insects but is also poisonous to humans and listed by the State of California as a carcinogen. Since 2004, the lumber industry, under pressure from the Environmental Protection Agency (EPA), discontinued its use for residential lumber. It is still used for industrial purposes such as pole, pilings and bridge timbers and is still the best preservative there is, according to the Natural Handyman.
Alkaline Copper Quat (ACQ)
ACQ contains copper and a quaternary ammonium compound, hence it is commonly called quat. It it less toxic than CCA and has replaced it as the most common pressure treatment for residential use. There are two kinds of quat commonly used, AQC-B and AQC-D. The former is formulated with ammoniacal copper and turns the wood a dark greenish-brown color. The latter uses amine copper and turns the wood lightish-brown. Both chemicals can penetrate difficult-to-treat species like Douglas Fir and are primarily used on the West Coast. ACQ protects wood against rot and insects, but unlike CCA, is unsuitable for underwater use.
Copper Azole (CBA)
CBA is another copper amine product that has been developed for residential use. It contains copper and tebuconazole and may also contain a little boric acid. It protects against rot and insects, but like ACQ, is unsuitable for marine use. It does not penetrate as readily as ACQ, and ammonia is sometimes added during the treatment to improve penetration. CBA turns the wood light brown.
Borate preservatives include salts such as sodium octabo-rate, sodium tetraborate, and sodium pentaborate, which are dissolved in water and provide protection against fungi and insects. These salts are highly penetrating, but because they are water-soluble, they leach out in rainwater or underground. Wood treated with borates is suitable for residential surface applications, primarily to protect against termites.
According to the U.S. Department of Agriculture, the most common oil-based preservatives are creosote, pentachlorophenol, and copper naphthenate. None of these are suitable for indoor residential use. Creosote is made from coal tar, turns the wood dark brown or black and gives the surface an oily feel. It is used for railroad ties and utility posts. Pentachlorophenol and copper naphthenate are crystals that must dissolve in oil. When either are used, the color of the wood depends on the oil in which they are dissolved.