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Timber 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 inground 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 centres 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.
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 distilate. 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.
Chromated copper arsenate (CCA)
Main article: Chromated copper arsenate
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 percent of the chromated 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.
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 ammoniacal 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 5mm 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.
Naturally rot-resistant woods
This includes Western Redcedar, 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.
Huon pine was used for ship hulls in the 19th century, but overharvesting and
huon pine's tremendously slow grow rate makes this now a specialty timber.
Merbau is still a popular decking timber and has a long life in above ground
applications, but it is considered unsustainably logged (see Illegal logging)
and is too hard and brittle for general use. Huon pine is so rot resistant that
fallen trees from over 20000 years ago are still commercially valuable.
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 insoluable (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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