In general, the chemical preservatives can be classified into three broad categories: • water-borne preservatives • oil-borne preservatives • light
organic solvent preservatives (LOSPs) In recent years, another fourth category of chemical preservatives for wood, is based upon nanocompounds (i.e. nanometals of boron, copper, silver).
Micronized copper Particulate (
micronised or dispersed)
copper preservative technology has been introduced in the US and Europe. In these systems, copper is ground into micro sized particles and suspended in water rather than dissolved, as is the case with other copper products such as ACQ and copper azole. There are two particulate copper systems in production. One system uses a
quat biocide system (known as MCQ) and is a derivative of ACQ. The other uses an azole biocide (known as MCA or μCA-C) derived from copper azole. Two particulate copper systems, one marketed as MicroPro and the other as Wolmanized using μCA-C formulation, have achieved Environmentally Preferable Product (EPP) certification. The
EPP certification was issued by Scientific Certifications Systems (SCS) and is based on a comparative life-cycle impact assessments with an industry standard. The copper particle size used in the "micronized" copper beads ranges from 1 to 700 nm with an average under 300 nm. Larger particles (such as actual micron-scale particles) of copper do not adequately penetrate the wood cell walls. These micronized preservatives use nano particles of copper oxide or copper carbonate, for which there are alleged safety concerns. An environmental group petitioned EPA in 2011 to revoke the registration of the micronized copper products, citing safety issues.
Alkaline copper quaternary Alkaline copper quaternary (ACQ) is a preservative made of copper, a
fungicide, and a
quaternary ammonium compound (quat) like
didecyl dimethyl ammonium chloride, an
insecticide which also augments the fungicidal treatment. ACQ has come into wide use in the US, Europe, Japan and Australia following restrictions on
CCA. Its use is governed by national and international standards, which determine the volume of preservative uptake required for a specific timber end use. Since it contains high levels of copper, ACQ-treated timber is five times more corrosive to common
steel. It is necessary to use
fasteners meeting or exceeding requirements for ASTM A 153 Class D, such as ceramic-coated, as mere
galvanized and even common grades of
stainless steel corrode. The U.S. began mandating the use of non-arsenic containing wood preservatives for virtually all residential use timber in 2004. The American Wood Protection Association (AWPA) standards for ACQ require a retention of for above ground use and for ground contact. Chemical Specialties, Inc (CSI, now Viance) received
U.S. Environmental Protection Agency's
Presidential Green Chemistry Challenge Award in 2002 for commercial introduction of ACQ. Its widespread use has eliminated major quantities of arsenic and chromium previously contained in CCA.
Copper azole Copper
azole preservative (denoted as CA-B and CA-C under American Wood Protection Association/AWPA standards) is a major copper based wood preservative that has come into wide use in Canada, the US, Europe, Japan and Australia following restrictions on CCA. Its use is governed by national and international standards, which determine the volume of preservative uptake required for a specific timber end use. Copper azole is similar to ACQ with the difference being that the dissolved copper preservative is augmented by an azole co-
biocide like organic
triazoles such as
tebuconazole or
propiconazole, which are also used to protect food crops, instead of the quat biocide used in ACQ. The azole co-biocide yields a copper azole product that is effective at lower retentions than required for equivalent ACQ performance. The general appearance of wood treated with copper azole preservative is similar to CCA with a green colouration. Copper azole treated wood is marketed widely under the
Preserve CA and
Wolmanized brands in North America, and the
Tanalith brand across Europe and other international markets. The AWPA standard retention for CA-B is for above ground applications and for ground contact applications. Type C copper azole, denoted as CA-C, has been introduced under the Wolmanized and Preserve brands. The AWPA standard retention for CA-C is for above ground applications and for ground contact applications.
Copper naphthenate Copper naphthenate, invented in Denmark in 1911, has been used effectively for many applications including:
fencepost, canvas, nets, greenhouses, utility poles, railroad ties, beehives, and wooden structures in ground contact. Copper naphthenate is registered with the EPA as a non-restricted use pesticide, so there is no federal applicators licensing requirements for its use as a wood preservative. Copper Naphthenate can be applied by brush, dip, or pressure treatment. The University of Hawaii has found that copper naphthenate in wood at loadings of is resistant to Formosan termite attack. On February 19, 1981, the Federal Register outlined the EPA's position regarding the health risks associated with various wood preservatives. As a result, the National Park Service recommended the use of copper naphthenate in its facilities as an approved substitute for
pentachlorophenol,
creosote, and inorganic
arsenicals. A 50-year study presented to AWPA in 2005 by Mike Freeman and Douglas Crawford says, "This study reassessed the condition of the treated wood posts in southern Mississippi, and statistically calculated the new expected post life span. It was determined that commercial wood preservatives, like pentachlorophenol in oil, creosote, and copper naphthenate in oil, provided excellent protection for posts, with life spans now calculated to exceed 60 years. Surprisingly, creosote and penta treated posts at 75% of the recommended AWPA retention, and copper naphthenate at 50% of the required AWPA retention, gave excellent performance in this AWPA Hazard Zone 5 site. Untreated southern pine posts lasted 2 years in this test site." The AWPA M4 Standard for the care of preservative-treated wood products, reads, "The appropriateness of the preservation system for field treatment shall be determined by the type of preservative originally used to protect the product and the availability of a field treatment preservative. Because many preservative products are not packaged and labeled for use by the general public, a system different from the original treatment may need to be utilized for field treatment. Users shall carefully read and follow the instructions and precautions listed on the product label when using these materials. Copper naphthenate preservatives containing a minimum of 2.0% copper metal are recommended for material originally treated with copper naphthenate, pentachlorophenol, creosote, creosote solution or waterborne preservatives." The M4 Standard has been adopted by the International Code Council's (ICC) 2015 International Building Code (IBC) section 2303.1.9 Preservative-treated Wood, and 2015 International Residential Code (IRC) R317.1.1 Field Treatment. The American Association of State Highway and Transportation Officials AASHTO has also adopted the AWPA M4 Standard. A waterborne copper naphthenate is sold to consumers under the tradename QNAP 5W. Oilborne copper naphthenates with 1% copper as metal solutions are sold to consumers under the tradenames Copper Green, and Wolmanized Copper Coat, a 2% copper as metal solution is sold under the tradename Tenino.
Chromated copper arsenate (CCA) 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 timber, CCA is a preservative that was very common for many decades. In the
pressure treatment process, an aqueous solution of CCA is applied using a vacuum and pressure cycle, and the treated wood is then stacked to dry. During the process, the mixture of oxides reacts to form insoluble compounds, helping with leaching problems. The process can apply varying amounts of preservative at varying levels of pressure to protect the wood against increasing levels of attack. Increasing protection can be applied (in increasing order of attack and treatment) for: exposure to the atmosphere, implantation within soil, or insertion into a marine environment. In the last decade concerns were raised that 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 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 the US
Consumer Product Safety Commission issued a report in 2002 stating that exposure to arsenic from direct human contact with CCA treated wood may be higher than was previously thought. On 1 January 2004, the
Environmental Protection Agency (EPA) in a voluntary agreement with industry began restricting the use of CCA in treated timber 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 improve environmental safety, although the EPA were careful to point out that they had not concluded that CCA treated wood structures in service posed an unacceptable risk to the community. The EPA did not call for the removal or dismantling of existing CCA treated wood structures. In Australia, the
Australian Pesticides and Veterinary Medicines Authority (APVMA) restricted the use of CCA preservative for treatment of timber used in certain applications from March 2006. CCA may no longer be used to treat wood used in 'intimate human contact' applications such as children's play equipment, furniture, residential decking and handrailing. Use for low contact residential, commercial and industrial applications remains unrestricted, as does its use in all other situations. The APVMA decision to restrict the use of CCA in Australia was a precautionary measure, even though the report found no evidence that demonstrated CCA treated timber posed unreasonable risks to humans in normal use. Similarly to the US EPA, the APVMA did not recommend dismantling or removal of existing CCA treated wood structures. In Europe, Directive 2003/2/EC restricts the marketing and use of arsenic, including CCA wood treatment. CCA treated wood is not permitted to be used in residential or domestic constructions. It is permitted for use in various industrial and public works, such as bridges, highway safety fencing, electric power transmission and telecommunications poles. In the United Kingdom waste timber treated with CCA was classified in July 2012 as hazardous waste by the department for the Environment, Food and Rural Affairs.
Other copper compounds These include copper HDO (Bis-(N-cyclohexyldiazeniumdioxy)-copper or CuHDO),
copper chromate,
copper citrate, acid copper chromate, and ammoniacal copper zinc arsenate (ACZA). The CuHDO treatment is an alternative to CCA, ACQ and CA used in Europe and in approval stages for United States and Canada. ACZA is generally used for marine applications.
Borate Boric acid, oxides and salts (
borates) are effective wood preservatives and are supplied under numerous brand names throughout the world. One of the most common compounds used is
disodium octaborate tetrahydrate , commonly abbreviated DOT. Borate treated wood is of low toxicity to humans, and does not contain copper or other heavy metals. However, unlike most other preservatives,
borate compounds do not become fixed in the wood and can be partially leached out if exposed repeatedly to water that flows away rather than evaporating (evaporation leaves the borate behind so is not a problem). Even though leaching will not normally reduce boron concentrations below effective levels for preventing fungal growth, borates should not be used where they will be exposed to repeated rain, water or ground contact unless the exposed surfaces are treated to repel water. Zinc-borate compounds are less susceptible to leaching than sodium-borate compounds, but are still not recommended for below-ground use unless the timber is first sealed. Recent interest in low toxicity timber for residential use, along with new regulations restricting some wood preservation agents, has resulted in a resurgence of the use of borate treated wood for floor beams and internal structural members. Researchers at CSIRO in Australia have developed organoborates which are much more resistant to leaching, while still providing timber with good protection from termite and fungal attack. The cost of the production of these modified borates will limit their widespread take-up but they are likely to be suitable for certain niche applications, especially where low mammalian toxicity is of paramount importance.
PTI Recent concerns about the health and environmental effects of metallic wood preservatives have created a market interest in non-metallic wood preservatives such as
propiconazole-
tebuconazole-
imidacloprid better known as PTI. The American Wood Protection Association (AWPA) standards for PTI require a retention of for above ground use and when applied in combination with a wax stabilizer. The AWPA has not developed a standard for a PTI ground contact preservative, so PTI is currently limited to above ground applications such as decks. All three of the PTI components are also used in food crop applications. The very low required amounts of PTI in pressure treated wood further limits effects and substantially decreases the freight costs and associated
environmental impacts for shipping preservative components to the pressure treating plants. The PTI preservative imparts very little color to the wood. Producers generally add a color agent or a trace amount of copper solution so as to identify the wood as pressure treated and to better match the color of other pressure treated wood products. The PTI wood products are very well adapted for paint and stain applications with no bleed-through. The addition of the wax stabilizer allows a lower preservative retention plus substantially reduces the tendency of wood to warp and split as it dries. In combination with normal deck maintenance and sealer applications, the stabilizer helps maintain appearance and performance over time. PTI pressure treated wood products are no more corrosive than untreated wood and are approved for all types of metal contact, including aluminum. PTI pressure treated wood products are relatively new to the market place and are not yet widely available in building supply stores. However, there are some suppliers selling PTI products for delivery anywhere in the US on a job lot order basis.
Sodium silicate Sodium silicate is produced by fusing
sodium carbonate with sand or heating both ingredients under pressure. It has been in use since the 19th century. It can be a deterrent against insect attack and possesses minor
flame-resistant properties; however, it is easily washed out of wood by moisture, forming a flake-like layer on top of the wood. Timber Treatment Technology,
LLC, markets TimberSIL, a sodium silicate wood preservative. The TimberSIL proprietary process surrounds the wood fibers with a protective, non-toxic, amorphous glass matrix. The result is a product the company calls "Glass Wood," which they claim is Class A
fire-retardant, chemically inert, rot and decay resistant, and superior in strength to untreated wood. Timbersil is currently involved in litigation over its claims.
Potassium silicate There are a number of European natural paint fabricants that have developed
potassium silicate (potassium waterglass) based preservatives. They frequently include boron compounds, cellulose, lignin and other plant extracts. They are a surface application with a minimal impregnation for internal use.
Bifenthrin spray In Australia, a water-based
bifenthrin preservative has been developed to improve the insect resistance of timber. As this preservative is applied by spray, it only penetrates the outer 2 mm of the timber cross-section. Concerns have been raised as to whether this thin-envelope system will provide protection against insects in the longer term, particularly when exposed to sunlight for extended periods.
Fire retardant treated The fireproofing of wood utilizes a
fire retardant chemical that remains stable in high temperature environments. The fire retardant is applied under pressure at a wood treating plant like the preservatives described above, or applied as a surface coating. In both cases, treatment provides a physical barrier to flame spread. The treated wood chars but does not oxidize. Effectively this creates a convective layer that transfers flame heat to the wood in a uniform way which significantly slows the progress of fire to the material. There are several commercially available wood-based construction materials using pressure-treatment (such as those marketed in the United States and elsewhere under the trade names of 'FirePro', 'Burnblock' 'Wood-safe, 'Dricon', 'D-Blaze,' and 'Pyro-Guard'), as well as factory-applied coatings under the trade names of 'PinkWood' and 'NexGen'. Some site-applied coatings as well as brominated fire retardants have lost favor due to safety concerns as well as concerns surrounding the consistency of application. Specialized treatments also exist for wood used in weather-exposed applications. The only impregnation-applied fire retardant commercially available in Australia is 'NexGen'. 'Guardian', which used calcium formate as a 'powerful wood modifying agent', was removed from sale in early 2010 for unspecified reasons.
Oil-borne These include
pentachlorophenol ("penta") and
creosote. They emit a strong petrochemical odor and are generally not used in consumer products. Both of these pressure treatments routinely protect wood for 40 years in most applications.
Coal-tar creosote in
Albuquerque, New Mexico, in March 1943 Creosote was the first wood preservative to gain industrial importance more than 150 years ago and it is still widely used today for protection of industrial timber components where long service life is essential.
Creosote is a
tar-based preservative that is commonly used for
utility poles and
railroad ties or sleepers. Creosote is one of the oldest wood preservatives, and was originally derived from a
wood distillate, but now, virtually all creosote is manufactured from the distillation of
coal tar. Creosote is regulated as a
pesticide, and is not usually sold to the general public.
Linseed oil In recent years in Australia and New Zealand,
linseed oil has been incorporated in preservative formulations as a solvent and water repellent to "envelope treat" timber. This involves just treating the outer 5 mm of the cross-section of a timber member with preservative (e.g., permethrin 25:75), leaving the core untreated. While not as effective as CCA or LOSP methods, envelope treatments are significantly cheaper, as they use far less preservative. Major preservative manufacturers add a blue (or red) dye to envelope treatments. Blue colored timber is for use south of the Tropic of Capricorn and red for elsewhere. The colored dye also indicates that the timber is treated for resistance to termites/white ants. There is an ongoing promotional campaign in Australia for this type of treatment.
Other emulsions Light organic solvent preservatives (LOSP) This class of timber treatments use
white spirit, or light oils such as
kerosene, as the solvent carrier to deliver preservative compounds into timber. Synthetic pyrethroids are typically used as an insecticide, such as permethrin, bifenthrin or deltamethrin. In Australia and New Zealand, the most common formulations use permethrin as an insecticide, and propiconazole and tebuconazole as fungicides. While still using a chemical preservative, this formulation contains no heavy-metal compounds. With the introduction of strict
volatile organic compound (VOC) laws in the European Union, LOSPs have disadvantages due to the high cost and long process times associated with vapour-recovery systems. LOSPs have been emulsified into water-based solvents. While this does significantly reduce VOC emissions, the timber swells during treatment, removing many of the advantages of LOSP formulations.
Epoxy Various
epoxy resins usually thinned with a solvent like
acetone or
methyl ethyl ketone (MEK) can be used to both preserve and seal wood. The wood coatings market in general will exceed $12 billion by 2027.
New technologies Biological modified timber Biological modified timber is treated with biopolymers from agricultural waste. After drying and curing, the soft timber becomes durable and strong. With this process fast growing pinewood acquires properties similar to tropical hardwood. Production facilities for this process are in The Netherlands and is known under the trade name “NobelWood”. From agricultural waste, like sugarcane bagasse,
furfuryl alcohol is manufactured. Theoretically this alcohol can be from any fermented bio-mass waste and therefore can be called a green chemical. After condensation reactions pre-polymers are formed from furfuryl alcohol. Fast growing softwood is impregnated with the water-soluble bio-polymer. After impregnation the wood is dried and heated which initiates a polymerisation reaction between the bio-polymer and the wood cells. This process results in wood cells which are resistant to microorganisms. At the moment the only timber species which is being used for this process is
Pinus radiata. This is the fastest growing tree species on Earth that has a porous structure which is particularly suitable for impregnation processes. The technique is applied to timber mainly for the building industry as a cladding material. The technique is being further developed in order to reach similar physical and biological properties of other polyfurfuryl impregnated wood species. Besides the impregnation with the biopolymers the timber can also be impregnated with fire retardant resins. This combination creates a timber with durability class I and a fire safety certification of Euro class B.
Acetylation of wood near
Sneek, the
Netherlands, is designed to carry heavy traffic. Chemical modification of wood at the molecular level has been used to improve its performance properties. Many chemical reaction systems for the modification of wood, especially those using various types of
anhydrides, have been published; however, the reaction of wood with
acetic anhydride has been the most studied. The physical properties of any material are determined by its chemical structure. Wood contains an abundance of chemical groups called
free hydroxyls. Free
hydroxyl groups readily absorb and release water according to changes in the climatic conditions to which they are exposed. This is the main reason why wood's dimensional stability is impacted by swelling and shrinking. It is also believed that the digestion of wood by enzymes initiates at the free hydroxyl sites, which is one of the principal reasons why wood is prone to decay.
Acetylation effectively changes the compounds with free hydroxyls within wood into
acetate esters. This is done by reacting the wood with
acetic anhydride, which comes from
acetic acid. When free hydroxyl groups are transformed to
acetoxy groups, the ability of the wood to absorb water is greatly reduced, rendering the wood more dimensionally stable and, because it is no longer digestible, extremely durable. In general, softwoods naturally have an acetyl content from 0.5 to 1.5% and more durable hardwoods from 2 to 4.5%. Acetylation takes wood well beyond these levels with corresponding benefits. These include an extended coatings life due to acetylated wood acting as a more stable substrate for paints and translucent coatings.
acetylated wood is non-toxic and does not have the environmental issues associated with traditional preservation techniques. The acetylation of wood was first done in Germany in 1928 by Fuchs. In 1946, Tarkow, Stamm and Erickson first described the use of wood acetylation to stabilize wood from swelling in water. Since the 1940s, many laboratories around the world have looked at acetylation of many different types of woods and agricultural resources. In spite of the vast amount of research on chemical modification of wood, and, more specifically, on the acetylation of wood, commercialization did not come easily. The first patent on the acetylation of wood was filed by Suida in Austria in 1930. Later, in 1947, Stamm and Tarkow filed a patent on the acetylation of wood and boards using
pyridine as a catalyst. In 1961, the
Koppers Company published a technical bulletin on the acetylation of wood using no catalysis, but with an organic
cosolvent In 1977, in Russia, Otlesnov and Nikitina came close to commercialization, but the process was discontinued, presumably because cost-effectiveness could not be achieved. In 2007, Titan Wood, a London-based company, with production facilities in The Netherlands, achieved cost-effective commercialization and began large-scale production of acetylated wood under the trade name "Accoya".
Natural Copper plating Copper plating or
copper sheathing is the practice of covering wood, most commonly wooden hulls of ships, with copper metal. As metallic copper is both repellent and toxic to fungus, insects such as termites, and marine bi-valves this would preserve the wood and also act as an anti-fouling measure to prevent aquatic life from attaching to the ship's hull and reducing a ship's speed and maneuverability. Modern marine bottom paints often incorporate a significant amount of copper in their formulations for the same reason, although they are not recommended for aluminum hulls because of the possibilities for
galvanic corrosion.
Naturally rot-resistant woods These species are resistant to decay in their natural state, due to high levels of organic chemicals called
extractives, mainly
polyphenols, providing them antimicrobial properties. Extractives are chemicals that are deposited in the
heartwood of certain tree species as they convert
sapwood to
heartwood; they are present in both parts though.
Huon pine (
Lagarostrobos franklinii), merbau (
Intsia bijuga),
ironbark (
Eucalyptus spp.), totara (
Podocarpus totara),
puriri (
Vitex lucens), kauri (
Agathis australis), and many
cypresses, such as
coast redwood (
Sequoia sempervirens) and western red cedar (
Thuja plicata), fall in this category. 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 over-harvesting and Huon pine's extremely slow growth rate makes this now a specialty timber. Huon pine is so rot resistant that fallen trees from many years ago are still commercially valuable.
Merbau is still a popular decking timber and has a long life in above ground applications, but it is logged in an
unsustainable manner and is too hard and brittle for general use.
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. Eastern red cedar (
Juniperus virginiana) and black locust (
Robinia pseudoacacia) have long been used for rot-resistant fence posts and rails in
eastern United States, with the black locust also planted in modern times in Europe. Coast redwood is commonly used for similar applications in the
western United States.
Totara and
puriri were used extensively in
New Zealand during the
European colonial era when native forests were "mined", even as fence posts of which many are still operating. Totara was used by the
Māori to build large
waka (canoes). Today, they are specialty timbers as a result of their scarcity, although lower grade stocks are sold for landscaping use.
Kauri is a superb timber for building the hulls and decks of boats. It too is now a specialty timber and ancient logs (in excess of 3 000 years) that have been mined from swamps are used by wood turners and furniture makers. The natural durability or rot and insect resistance of wood species is always based on the heartwood (or "truewood"). The sapwood of all timber species should be considered to be non-durable without preservative treatment.
Natural extractives Natural substances, purified from naturally rot-resistant trees and responsible for natural durability, also known as natural
extractives, are another promising wood preservatives. Several compounds have been described to be responsible for natural durability, including different
polyphenols,
lignins,
lignans (such as
gmelinol,
plicatic acid),
hinokitiol,
α-cadinol and other
sesquiterpenoids,
flavonoids (such as
mesquitol), and other substances. These compounds are mostly identified in the
heartwood, although they are also present in minimal concentrations in the
sapwood.
Tannins, which have also shown to act as protectants, are present in the
bark of trees. Treatment of timber with natural extractives, such as
hinokitiol,
tannins, and different tree extracts, has been studied and proposed to be another environmentally-friendly wood preservation method.
Tung oil Tung oil has been used for hundreds of years in
China, where it was used as a preservative for wood ships. The oil penetrates the wood, and 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.
Heat treatments By going beyond
kiln drying wood, heat treatment may make timber more durable. By heating timber to a certain temperature, it may be possible to make the wood fibre less appetizing to insects. Heat treatment can also improve the properties of the wood with respect to water, with lower equilibrium moisture, less moisture deformation, and weather resistance. It is weather-resistant enough to be used unprotected, in facades or in kitchen tables, where wetting is expected. However, heating can reduce the amount of volatile organic compounds, There are four similar heat treatments — Westwood, developed in the United States; Retiwood, developed in France; Thermowood, developed in Finland by VTT; and Platowood, developed in The Netherlands. These processes autoclave the treated wood, subjecting it to pressure and heat, along with
nitrogen or water vapour to control drying in a staged treatment process ranging from 24 to 48 hours at temperatures of 180 °C to 230 °C depending on timber species. These processes increase the durability, dimensional stability and hardness of the treated wood by at least one class; however, the treated wood is darkened in colour, and there are changes in certain mechanical characteristics: Specifically, the modulus of elasticity is increased to 10%, and the modulus of rupture is diminished by 5% to 20%. Thus, the treated wood requires drilling for nailing to avoid splitting the wood. Certain of these processes cause less impact than others in their mechanical effects upon the treated wood. Wood treated with this process is often used for cladding or siding, flooring, furniture and windows. For the control of pests that may be harbored in wood packaging material (i.e.
crates and
pallets), the
ISPM 15 requires heat treatment of wood to 56 °C for 30 minutes to receive the
HT stamp. This is typically required to ensure the killing of the
pine wilt nematode and other kinds of wood pests that could be transported internationally.
Mud treatment Wood and
bamboo can be buried in
mud to help protect them from insects and decay. This practice is used widely in
Vietnam to build farm houses consisting of a wooden structural frame, a bamboo roof frame and bamboo with mud mixed with rice hay for the walls. While wood in contact with soil will generally decompose more quickly than wood not in contact with it, it is possible that the predominantly clay soils prevalent in Vietnam provide a degree of mechanical protection against insect attack, which compensates for the accelerated rate of decay. Also, since wood is subject to bacterial decay only under specific temperature and moisture content ranges, submerging it in water-saturated mud can retard decay, by saturating the wood's internal cells beyond their moisture decay range. ==Application processes==