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Timber treatment

Timber treatment or lumber treatment in forestry describes the intensive treatment of timber with chemical agents to increase its durability and ability to be used in an otherwise inappropriate situation like in ground use. Timber or lumber may be treated with a preservative that protects it from being destroyed by insects, fungus or exposure to moisture.

Hazards

Unfortunately, many of the timber treatments have proven to be extremely hazardous both to the workers and the environment; as a result many treatment centers have been forced to close and undertake massive environmental restoration and remediation. With many of the chemicals having a long period through which they are highly dangerous, the legacy of the chemical cocktails can probably never be totally eliminated.

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Preservatives

Timber or lumber that is treated with a preservative generally have it applied through combined vacuum and pressure treatment. The preservatives used to pressure-treat lumber are classified as pesticides. Treating lumber provides long-term resistance to organisms that cause deterioration. If it is applied correctly, it extends the productive life of lumber by five to ten times. If left untreated, wood that is exposed to moisture or soil for sustained periods of time will become weakened by various types of fungi, bacteria or insects.

Creosote

Creosote is one of the oldest wood preservatives, and was originally derived from a wood distillate. It often collects inside chimneys causing a fire hazard. Creosote is considered a carcinogen and a chronic poison and has been removed from general sale in Australia. It is still used for railway sleepers and utility pole.

Chromate copper arsenate (CCA)

An extremely common preservative developed in the 1930s. In CCA treatment, copper is the primary fungicide, arsenic is a secondary fungicide and an insecticide, and chromium is a fixative which also provides ultraviolet (UV) light resistance. Recognized for the greenish tint it imparts to lumber, CCA is a preservative that was extremely common for many decades, however it contained arsenic. The chemicals may leach from the wood into surrounding soil, resulting in concentrations higher than naturally occurring background levels. A study cited in Forest Products Journal found 12–13% of the chromate copper arsenate leached from treated wood buried in compost during a 12-month period. Once these chemicals have leached from the wood they are likely to bind to soil particles, especially in soils with clay or soils that are more alkaline than neutral. In the United States on 1 January 2004 the Environmental Protection Agency (EPA) began restricting the use of CCA in treated lumber in residential and commercial construction, with the exception of shakes and shingles, permanent wood foundations, and certain commercial applications. This was in an effort to reduce the use of arsenic and increase environmental safety. However some consumer outlets such as Home Depot were still reported to be selling CCA lumber long after the ban went into effect.

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Alkaline copper quaternary

Alkaline copper quaternary (ACQ) is a preservative made up of copper, a fungicide, and quaternary ammonium compound (quat), an insecticide which also augments the fungicidal treatment. Since it contains high levels of copper, ACQ-treated lumber is five times more corrosive to common steel, according to American Wood Preservers Association (AWPA) test results. It is necessary to use double-galvanized or stainless steel fasteners in ACQ lumber. The U.S. began mandating the use of ACQ in end-consumer lumber in 2004.

Other copper compounds

These include copper azole (CA), copper chromate, copper citrate, acid copper chromate and ammonia cal copper zinc arsenate (ACZA). The CA treatment is an alternative to CCA and ACQ in the United States and Canada. AZCA is generally used for marine applications.

Borate preservatives

Unlike most other preservatives, borate compounds do not become fixed in the wood and can be washed out. Therefore they cannot be used where they will be exposed to water.

Oil-based preservatives

These include pentachlorophenol, copper naphthenate, and creosote. All of them are toxic and are generally not used in consumer products. Creosote is a tar-based preservative that was commonly seen on telephone poles before it was withdrawn from production due to toxicity.

Tung oil

Tung oil has been known about for hundreds of years in China, where it was used as a preservative for wood ships. The oil penetrates the wood then hardens to form an impermeable hydrophobic layer up to 5 mm into the wood. As a preservative it is effective for exterior work above and below ground, but the thin layer makes it less useful in practice. It is not available as a pressure treatment. Some manufacturers recommend tung oil as a stabiliser for CCA.

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Naturally rot-resistant woods

This includes Western Red cedar, Huon Pine, Merbau, Ironbark, many cypresses and Coast Redwood. These species are resistant to decay in their natural state, due to high levels of organic chemicals called "extractives", mainly polyphenols. Extractives are chemicals that are deposited in the heartwood of certain tree species as they convert sapwood to heartwood. However, many of these species tend to be prohibitively expensive for general construction applications.

Ironbark is a good choice where available. It is harvested from both old-growth and plantation in Australia and is highly resistant to rot and termites. It is most commonly used for fence posts and house stumps.

Sodium silicate-based preservatives

The use of sodium silicate as a timber treatment began in 2005, after an environmental toxicologist's research on allergies and autism branched into her developing a method for rendering sodium silicate insoluble (rendering a pervasive barrier of amorphous glass) once the lumber has been treated with it. The toxicologist's then-14-year-old son conducted much of the early research through his 10th grade science fair project, which took first place in chemistry. This treatment preserves wood from moisture and insects and possesses some flame-resistant properties. Sodium silicate treated lumber is considered a safer alternative to both CCA and ACQ.

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