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Wood Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin ...
easily degrades without sufficient preservation. Apart from structural wood preservation measures, there are a number of different chemical
preservative A preservative is a substance or a chemical that is added to products such as food products, beverages, pharmaceutical drugs, paints, biological samples, cosmetics, wood, and many other products to prevent decomposition by microbial growth or b ...
s and processes (also known as "timber treatment", "lumber treatment" or "pressure treatment") that can extend the life of wood,
timber Lumber is wood that has been processed into dimensional lumber, including beams and planks or boards, a stage in the process of wood production. Lumber is mainly used for construction framing, as well as finishing (floors, wall panels, w ...
, and their associated products, including engineered wood. These generally increase the durability and resistance from being destroyed by insects or fungi.


History

As proposed by Richardson, treatment of wood has been practiced for almost as long as the use of wood itself. There are records of wood preservation reaching back to
ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity ( AD 600), that comprised a loose collection of cu ...
during
Alexander the Great Alexander III of Macedon ( grc, Ἀλέξανδρος, Alexandros; 20/21 July 356 BC – 10/11 June 323 BC), commonly known as Alexander the Great, was a king of the ancient Greek kingdom of Macedon. He succeeded his father Philip II to ...
's rule, where bridge wood was soaked in
olive oil Olive oil is a liquid fat obtained from olives (the fruit of ''Olea europaea''; family Oleaceae), a traditional tree crop of the Mediterranean Basin, produced by pressing whole olives and extracting the oil. It is commonly used in cooking: ...
. The Romans protected their ship hulls by brushing the wood with tar. During the
Industrial Revolution The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe, and the United States, that occurred during the period from around 1760 to about 1820–1840. This transition included going f ...
, wood preservation became a cornerstone of the wood processing industry. Inventors and scientists such as Bethell, Boucherie, Burnett and Kyan made historic developments in wood preservation, with the preservative solutions and processes. Commercial pressure treatment began in the latter half of the 19th century with the protection of railroad cross-ties using creosote. Treated wood was used primarily for industrial, agricultural, and utility applications, where it is still used, until its use grew considerably (at least in the United States) in the 1970s, as homeowners began building decks and backyard projects. Innovation in treated timber products continues to this day, with consumers becoming more interested in less toxic materials.


Hazards

Wood that has been industrially pressure-treated with approved preservative products poses a limited risk to the public and should be disposed of properly. On December 31, 2003, the U.S. wood treatment industry stopped treating residential lumber with
arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, b ...
and chromium (
chromated copper arsenate Chromated copper arsenate (CCA) is a wood preservative containing compounds of chromium, copper, and arsenic, in various proportions. It is used to impregnate timber and other wood products, especially those intended for outdoor use, in order to p ...
, or CCA). This was a voluntary agreement with the
United States Environmental Protection Agency The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it ...
. CCA was replaced by copper-based pesticides, with exceptions for certain industrial uses. CCA may still be used for outdoor products like utility trailer beds and non-residential construction like piers, docks, and agricultural buildings. Industrial wood preservation chemicals are generally not available directly to the public and may require special approval to import or purchase, depending on the product and the jurisdiction where being used. In most countries, industrial wood preservation operations are notifiable industrial activities that require licensing from relevant regulatory authorities such as EPA or equivalent. Reporting and licensing conditions vary widely, depending on the particular chemicals used and the country of use. Although pesticides are used to treat lumber, preserving lumber protects natural resources (in the short term) by enabling wood products to last longer. Previous poor practices in industry have left legacies of contaminated ground and water around wood treatment sites in some cases. However, under currently approved industry practices and regulatory controls, such as implemented in Europe, North America, Australia, New Zealand, Japan and elsewhere, environmental impact of these operations should be minimal. Wood treated with modern preservatives is generally safe to handle, given appropriate handling precautions and personal protection measures. However, treated wood may present certain hazards in some circumstances, such as during combustion or where loose wood dust particles or other fine toxic residues are generated, or where treated wood comes into direct contact with food and agriculture. Preservatives containing copper in the form of microscopic particles have recently been introduced to the market, usually with "micronized" or "micro" trade names and designations such as MCQ or MCA. The manufacturers represent that these products are safe and EPA has registered these products. The American Wood Protection Association (AWPA) recommends that all treated wood be accompanied by a Consumer Information Sheet (CIS), to communicate safe handling and disposal instructions, as well as potential health and environmental hazards of treated wood. Many producers have opted to provide Material Safety Data Sheets (MSDS) instead. Although the practice of distributing MSDS instead of CIS is widespread, there is an ongoing debate regarding the practice and how to best communicate potential hazards and hazard mitigation to the end-user. Neither MSDS nor the newly adopted International Safety Data Sheets (SDS) are required for treated lumber under current U.S. Federal law.


Chemical

Chemical preservatives can be classified into three broad categories: water-borne preservatives, oil-borne preservatives, and light organic solvent preservatives (LOSPs).


Micronized copper

Particulate ( micronised or dispersed)
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
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. Proponents of the particulate copper systems argue that they perform as well or better than the dissolved copper systems as a wood preservative, but other industry researchers disagree. None of the particulate copper systems have been submitted to the American Wood Protection Association (AWPA) for evaluation; thus, the particulate systems should not be used in applications where AWPA standards are required. However, all of the particulate copper systems have been tested and approved for building code requirements by the International Code Council (ICC). The particulate copper systems provide a lighter color than dissolved copper systems such as ACQ or copper azole. Proponents of the micronized copper systems claim that the systems are subject to third party inspection under a quality monitor program. However, the monitoring program is not subject to oversight by the American Lumber Standard Committee (ALSC) as is required for the AWPA standard systems. 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 Fungicides are biocidal chemical compounds or biological organisms used to kill parasitic fungi or their spores. A fungistatic inhibits their growth. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality ...
, and a quaternary ammonium compound (quat) like didecyl dimethyl ammonium chloride, an
insecticide Insecticides are substances used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. Insecticides are used in agriculture, medicine, industry and by consumers. Insecticides are claimed t ...
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 Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistan ...
. It is necessary to use
fastener A fastener (US English) or fastening (UK English) is a hardware device that mechanically joins or affixes two or more objects together. In general, fasteners are used to create non-permanent joints; that is, joints that can be removed or disman ...
s meeting or exceeding requirements for ASTM A 153 Class D, such as ceramic-coated, as mere galvanized and even common grades of
stainless steel Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's r ...
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 A biocide is defined in the European legislation as a chemical substance or microorganism intended to destroy, deter, render harmless, or exert a controlling effect on any harmful organism. The US Environmental Protection Agency (EPA) uses a sli ...
like organic
triazole A triazole is a heterocyclic compound featuring a five-membered ring of two carbon atoms and three nitrogen atoms with molecular formula C2H3N3. Triazoles exhibit substantial isomerism, depending on the positioning of the nitrogen atoms within ...
s 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 Pentachlorophenol (PCP) is an organochlorine compound used as a pesticide and a disinfectant. First produced in the 1930s, it is marketed under many trade names. It can be found as pure PCP, or as the sodium salt of PCP, the latter of which dis ...
, creosote, and inorganic
arsenical Arsenicals are chemical compounds that contain arsenic. In a military context, the term arsenical refer to toxic arsenic compounds that are used as chemical warfare agents. This include blister agents, blood agents and vomiting agents. Examples Bli ...
s. 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 Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
is the primary
fungicide Fungicides are biocidal chemical compounds or biological organisms used to kill parasitic fungi or their spores. A fungistatic inhibits their growth. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality ...
,
arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, b ...
is a secondary fungicide and an
insecticide Insecticides are substances used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. Insecticides are used in agriculture, medicine, industry and by consumers. Insecticides are claimed t ...
, and
chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hard ...
is a fixative which also provides
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
(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 Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...
, 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 Compost is a mixture of ingredients used as plant fertilizer and to improve soil's physical, chemical and biological properties. It is commonly prepared by decomposing plant, food waste, recycling organic materials and manure. The resulting ...
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 Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay pa ...
or soils that are more
alkali In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...
ne than neutral. In the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...
the US
Consumer Product Safety Commission The United States Consumer Product Safety Commission (USCPSC, CPSC, or commission) is an independent agency of the United States government. The CPSC seeks to promote the safety of consumer products by addressing “unreasonable risks” of inj ...
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 A biophysical environment is a biotic and abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution. A biophysical environment can vary in scale ...
(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 The Australian Pesticides and Veterinary Medicines Authority (APVMA) is the Australian Government statutory agency responsible for the management and regulation of all agricultural and veterinary chemical products in Australia. History The APVMA ...
(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 Sodium silicate is a generic name for chemical compounds with the formula or ·, such as sodium metasilicate , sodium orthosilicate , and sodium pyrosilicate . The anions are often polymeric. These compounds are generally colorless transparent ...
is produced by fusing
sodium carbonate Sodium carbonate, , (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield moderately alkaline solutions ...
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 A limited liability company (LLC for short) is the US-specific form of a private limited company. It is a business structure that can combine the pass-through taxation of a partnership or sole proprietorship with the limited liability of ...
, 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

This treated wood utilizes a
fire retardant A fire retardant is a substance that is used to slow down or stop the spread of fire or reduce its intensity. This is commonly accomplished by chemical reactions that reduce the flammability of fuels or delay their combustion. Fire retardants ...
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 Pentachlorophenol (PCP) is an organochlorine compound used as a pesticide and a disinfectant. First produced in the 1930s, it is marketed under many trade names. It can be found as pure PCP, or as the sodium salt of PCP, the latter of which dis ...
("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

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 Tar is a dark brown or black viscous liquid of hydrocarbons and free carbon, obtained from a wide variety of organic materials through destructive distillation. Tar can be produced from coal, wood, petroleum, or peat. "a dark brown or black bi ...
-based preservative that is commonly used for
utility pole A utility pole is a column or post typically made out of wood used to support overhead power lines and various other public utilities, such as electrical cable, fiber optic cable, and related equipment such as transformers and street lights. I ...
s and
railroad tie A railroad tie, crosstie (American English), railway tie ( Canadian English) or railway sleeper ( Australian and British English) is a rectangular support for the rails in railroad tracks. Generally laid perpendicular to the rails, ties tran ...
s (UK: railway 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 Coal tar is a thick dark liquid which is a by-product of the production of coke and coal gas from coal. It is a type of creosote. It has both medical and industrial uses. Medicinally it is a topical medication applied to skin to treat pso ...
. Creosote is regulated as a
pesticide Pesticides are substances that are meant to control pests. This includes herbicide, insecticide, nematicide, molluscicide, piscicide, avicide, rodenticide, bactericide, insect repellent, animal repellent, microbicide, fungicide, and ...
, and is not usually sold to the general public.


Linseed oil

In recent years in Australia and New Zealand,
linseed oil Linseed oil, also known as flaxseed oil or flax oil (in its edible form), is a colourless to yellowish oil obtained from the dried, ripened seeds of the flax plant (''Linum usitatissimum''). The oil is obtained by pressing, sometimes followed by ...
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 White spirit (AU, UK & Ireland)Primarily in the United Kingdom and Australia. In New Zealand "white spirit" can also refer to Coleman fuel (white gas). or mineral spirits (US, Canada), also known as mineral turpentine (AU/NZ), turpentine substitu ...
, or light oils such as
kerosene Kerosene, paraffin, or lamp oil is a combustible hydrocarbon liquid which is derived from petroleum. It is widely used as a fuel in aviation as well as households. Its name derives from el, κηρός (''keros'') meaning " wax", and was re ...
, 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 Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature. High vapor pressure correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a ...
(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 Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also co ...
resins usually thinned with a solvent like
acetone Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour. Acetone is miscibl ...
or
methyl ethyl ketone Butanone, also known as methyl ethyl ketone (MEK), is an organic compound with the formula CH3C(O)CH2CH3. This colourless liquid ketone has a sharp, sweet odor reminiscent of acetone. It is produced industrially on a large scale, but occurs in n ...
(MEK) can be used to both preserve and seal wood.


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.


Wood acetylation

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 Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula (CH3CO)2O. Commonly abbreviated Ac2O, it is the simplest isolable anhydride of a carboxylic acid and is widely used as a reagent in organic synthesis. It is a co ...
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 In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydrox ...
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 : In organic chemistry, acetylation is an organic esterification reaction with acetic acid. It introduces an acetyl group into a chemical compound. Such compounds are termed ''acetate esters'' or simply '' acetates''. Deacetylation is the oppos ...
effectively changes the compounds with free hydroxyls within wood into
acetate An acetate is a salt formed by the combination of acetic acid with a base (e.g. alkaline, earthy, metallic, nonmetallic or radical base). "Acetate" also describes the conjugate base or ion (specifically, the negatively charged ion called ...
ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ...
s. This is done by reacting the wood with
acetic anhydride Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula (CH3CO)2O. Commonly abbreviated Ac2O, it is the simplest isolable anhydride of a carboxylic acid and is widely used as a reagent in organic synthesis. It is a co ...
, which comes from
acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...
. 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 Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid w ...
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 Copper sheathing is the practice of protecting the under-water hull of a ship or boat from the corrosive effects of salt water and biofouling through the use of copper plates affixed to the outside of the hull. It was pioneered and developed b ...
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 Galvanic corrosion (also called bimetallic corrosion or dissimilar metal corrosion) is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte. A sim ...
.


Naturally rot-resistant woods

These species are resistant to decay in their natural state, due to high levels of organic chemicals called ''extractives'', mainly
polyphenol Polyphenols () are a large family of naturally occurring organic compounds characterized by multiples of phenol units. They are abundant in plants and structurally diverse. Polyphenols include flavonoids, tannic acid, and ellagitannin, some ...
s, 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'' is a species of conifer native to the wet southwestern corner of Tasmania, Australia. It is often known as the Huon pine or Macquarie pine, although it is actually a podocarp (Podocarpaceae), not a true pine (Pina ...
(''Lagarostrobos franklinii''), merbau (''
Intsia bijuga ''Intsia bijuga'', commonly known as Borneo teak, Johnstone River teak, Kwila, Moluccan ironwood, Pacific teak, scrub mahogany and vesi, is a species of flowering tree in the family Fabaceae, native to the Indo-Pacific. It ranges from Tanzania ...
''), ironbark (''
Eucalyptus ''Eucalyptus'' () is a genus of over seven hundred species of flowering trees, shrubs or mallees in the myrtle family, Myrtaceae. Along with several other genera in the tribe Eucalypteae, including '' Corymbia'', they are commonly known as ...
'' 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 ''Lagarostrobos franklinii'' is a species of conifer native to the wet southwestern corner of Tasmania, Australia. It is often known as the Huon pine or Macquarie pine, although it is actually a podocarp (Podocarpaceae), not a true pine (Pina ...
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 Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
and is highly resistant to rot and
termite Termites are small insects that live in colonies and have distinct castes ( eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blat ...
s. It is most commonly used for fence posts and house stumps. Eastern red cedar ('' Juniperus virginiana'') and black locust (''
Robinia pseudoacacia ''Robinia pseudoacacia'', commonly known in its native territory as black locust, is a medium-sized hardwood deciduous tree, belonging to the tribe Robinieae of the legume family Fabaceae. It is endemic to a few small areas of the United Sta ...
'') 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 The Western United States (also called the American West, the Far West, and the West) is the region comprising the westernmost states of the United States. As American settlement in the U.S. expanded westward, the meaning of the term ''the We ...
.
Tōtara ''Podocarpus totara'' (; from the Maori-language ; the spelling "totara" is also common in English) is a species of podocarp tree endemic to New Zealand. It grows throughout the North Island and northeastern South Island in lowland, montane and ...
and
pūriri ''Vitex lucens'', or pūriri, is an evergreen tree endemic to New Zealand. History Pūriri was first collected (by Europeans) at Tolaga Bay by Banks and Solander during Cook's first visit in 1769. The plant was excellently described by Solan ...
were used extensively in
New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island coun ...
during the
Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
an colonial era when native forests were "mined", even as fence posts of which many are still operating. Tōtara was used by the
Māori Māori or Maori can refer to: Relating to the Māori people * Māori people of New Zealand, or members of that group * Māori language, the language of the Māori people of New Zealand * Māori culture * Cook Islanders, the Māori people of the Co ...
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
polyphenol Polyphenols () are a large family of naturally occurring organic compounds characterized by multiples of phenol units. They are abundant in plants and structurally diverse. Polyphenols include flavonoids, tannic acid, and ellagitannin, some ...
s,
lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity a ...
s lignans, such as gmelinol,
plicatic acid Plicatic acid is a carboxylic acid from the resin acid group. It is naturally found in ''Thuja'' and cypress resin. It is the main irritant and contact allergen present in thuja wood. (''Cf.'' pine, where the primary irritant is abietic acid.) ...
), hinokitiol, α-cadinol and other sesquiterpenoids,
flavonoid Flavonoids (or bioflavonoids; from the Latin word ''flavus'', meaning yellow, their color in nature) are a class of polyphenolic secondary metabolites found in plants, and thus commonly consumed in the diets of humans. Chemically, flavonoids ...
s, 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.
Tannin Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. The term ''tannin'' (from Anglo-Norman ''tanner'' ...
s, which have also shown to act as protectants, are present in the
bark Bark may refer to: * Bark (botany), an outer layer of a woody plant such as a tree or stick * Bark (sound), a vocalization of some animals (which is commonly the dog) Places * Bark, Germany * Bark, Warmian-Masurian Voivodeship, Poland Arts, e ...
of trees. Treatment of timber with natural extractives, such as hinokitiol,
tannin Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. The term ''tannin'' (from Anglo-Norman ''tanner'' ...
s, 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 China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...
, where it was used as a preservative for wood ships. The oil penetrates the wood, and then hardens to form an impermeable
hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, ...
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, which generally have antimicrobial properties. 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 Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
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.
crate A crate is a large shipping container, often made of wood, typically used to transport or store large, heavy items. Steel and aluminium crates are also used. Specialized crates were designed for specific products, and were often made to be reusa ...
s and
pallet A pallet (also called a skid) is a flat transport structure, which supports goods in a stable fashion while being lifted by a forklift, a pallet jack, a front loader, a jacking device, or an erect crane. A pallet is the structural founda ...
s), 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 Bamboos are a diverse group of evergreen perennial flowering plants making up the subfamily Bambusoideae of the grass family Poaceae. Giant bamboos are the largest members of the grass family. The origin of the word "bamboo" is uncertain, ...
can be buried in mud to help protect them from insects and decay. This practice is used widely in
Vietnam Vietnam or Viet Nam ( vi, Việt Nam, ), officially the Socialist Republic of Vietnam,., group="n" is a country in Southeast Asia, at the eastern edge of mainland Southeast Asia, with an area of and population of 96 million, making ...
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


Introduction and history

Probably the first attempts made to protect wood from decay and insect attack consisted of brushing or rubbing preservatives onto the surfaces of the treated wood. Through trial and error the most effective preservatives and application processes were slowly determined. In the Industrial Revolution, demands for such things as telegraph poles and
railroad tie A railroad tie, crosstie (American English), railway tie ( Canadian English) or railway sleeper ( Australian and British English) is a rectangular support for the rails in railroad tracks. Generally laid perpendicular to the rails, ties tran ...
s (UK: railway sleepers) helped to fuel an explosion of new techniques that emerged in the early 19th century. The sharpest rise in inventions took place between 1830 and 1840, when Bethell, Boucherie, Burnett and Kyan were making wood-preserving history. Since then, numerous processes have been introduced or existing processes improved. The goal of modern-day wood preservation is to ensure a deep, uniform penetration with reasonable cost, without endangering the environment. The most widespread application processes today are those using artificial pressure through which many woods are being effectively treated, but several species (such as spruce, Douglas-fir, larch, hemlock and fir) are very resistant to impregnation. With the use of incising, the treatment of these woods has been somewhat successful but with a higher cost and not always satisfactory results. One can divide the wood-preserving methods roughly into either non-pressure processes or pressure processes.


Non-pressure processes

There are numerous non-pressure processes of treating wood which vary primarily in their procedure. The most common of these treatments involve the application of the preservative by means of brushing or spraying, dipping, soaking, steeping or by means of hot and cold bath. There is also a variety of additional methods involving charring, applying preservatives in bored holes, diffusion processes and sap displacement.


Brush and spray treatments

Brushing preservatives is a long-practised method and often used in today's carpentry workshops. Technological developments mean it is also possible to spray preservative over the surface of the timber. Some of the liquid is drawn into the wood as the result of capillary action before the spray runs off or evaporates, but unless puddling occurs penetration is limited and may not be suitable for long-term weathering. By using the spray method, coal-tar creosote, oil-borne solutions and water-borne salts (to some extent) can also be applied. A thorough brush or spray treatment with coal-tar creosote can add 1 to 3 years to the lifespan of poles or posts. Two or more coats provide better protection than one, but the successive coats should not be applied until the prior coat has dried or soaked into the wood. The wood should be
seasoned Seasoning is the process of supplementing food via herbs, spices, salts, and/or sugar, intended to enhance a particular flavour. General meaning Seasonings include herbs and spices, which are themselves frequently referred to as "seasonings". Ho ...
before treatment.


Dipping

Dipping consists of simply immersing the wood in a bath of creosote or other preservative for a few seconds or minutes. Similar penetrations to that of brushing and spraying processes are achieved. It has the advantage of minimizing hand labor. It requires more equipment and larger quantities of preservative and is not adequate for treating small lots of timber. Usually the dipping process is useful in the treatment of window sashes and doors. Except for copper naphthenate, treatment with copper salt preservative is no longer allowed with this method.


Steeping

In this process the wood is submerged in a tank of water-preservative mix, and allowed to soak for a longer period of time (several days to weeks). This process was developed in the 19th century by John Kyan. The depth and retention achieved depends on factors such as species, wood moisture, preservative and soak duration. The majority of the absorption takes place during the first two or three days, but will continue at a slower pace for an indefinite period. As a result, the longer the wood can be left in the solution, the better treatment it will receive. When treating seasoned timber, both the water and the preservative salt soak into the wood, making it necessary to season the wood a second time. Posts and poles can be treated directly on endangered areas, but should be treated at least above the future ground level. The depth obtained during regular steeping periods varies from up to by sap pine. Due to the low absorption, solution strength should be somewhat stronger than that in pressure processes, around 5% for seasoned timber and 10% for green timber (because the concentration slowly decreases as the chemicals diffuse into the wood). The solution strength should be controlled continually and, if necessary, be corrected with the salt additive. After the timber is removed from the treatment tank, the chemical will continue to spread within the wood if it has sufficient moisture content. The wood should be weighed down and piled so that the solution can reach all surfaces. (Sawed materials stickers should be placed between every board layer.) This process finds minimal use despite its former popularity in continental
Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
and
Great Britain Great Britain is an island in the North Atlantic Ocean off the northwest coast of continental Europe. With an area of , it is the largest of the British Isles, the largest European island and the ninth-largest island in the world. It ...
.


Kyanizing

Named after
John Howard Kyan John Howard Kyan (27 November 1774 – 5 January 1850) was the inventor of the 'kyanising' process for preserving wood. He was the son of John Howard Kyan of Mount Howard and Ballymurtagh, County Wexford, and was born in Dublin on 27 November 17 ...
, who patented this process in
England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe ...
in 1833, Kyanizing consists of steeping wood in a 0.67% mercuric chloride preservative solution. It is no longer used.


Gedrian's Bath

Patented by Charles A. Seely, this process achieves treatment by immersing seasoned wood in successive baths of hot and cold preservatives. During the hot baths, the air expands in the timbers. When the timbers are changed to the cold bath (the preservative can also be changed) a partial vacuum is created within the lumen of the cells, causing the preservative to be drawn into the wood. Some penetration occurs during the hot baths, but most of it takes place during the cold baths. This cycle is repeated with a significant time reduction compared to other steeping processes. Each bath may last 4 to 8 hours or in some cases longer. The temperature of the preservative in the hot bath should be between and in the cold bath (depending on preservative and tree species). The average penetration depths achieved with this process ranges from . Both preservative oils and water-soluble salts can be used with this treatment. Due to the longer treatment periods, this method finds little use in the commercial wood preservation industry today.


Preservative precipitation

As explained in Uhlig's Corrosion Handbook, this process involves two or more chemical baths that undergo a reaction with the cells of the wood, and result in the precipitation of preservative into the wood cells. Two chemicals commonly employed in this process are copper ethanolamine, and sodium dimethyldithiocarbamate, which reacts to precipitate copper dimetyldithiocarbamate. The precipitated preservative is very resistant to leeching. Since its use in the mid-1990s, it has been discontinued in the United States of America, but it never saw commercialization in Canada.


Pressure processes

Pressure processes are the most permanent method around today in preserving timber life. Pressure processes are those in which the treatment is carried out in closed cylinders with applied pressure or vacuum. These processes have a number of advantages over the non-pressure methods. In most cases, a deeper and more uniform penetration and a higher absorption of preservative is achieved. Another advantage is that the treating conditions can be controlled so that retention and penetration can be varied. These pressure processes can be adapted to large-scale production. The high initial costs for equipment and the energy costs are the biggest disadvantages. These treatment methods are used to protect ties, poles and structural timbers and find use throughout the world today. The various pressure processes that are used today differ in details, but the general method is in all cases the same. The treatment is carried out in cylinders. The timbers are loaded onto special tram cars, so called ''buggies'' or ''bogies'', and into the cylinder. These cylinders are then set under pressure often with the addition of higher temperature. As final treatment, a vacuum is frequently used to extract excess preservatives. These cycles can be repeated to achieve better penetration. LOSP treatments often use a vacuum impregnation process. This is possible because of the lower viscosity of the white-spirit carrier used.


Full-cell process

In the full-cell process, the intent is to keep as much of the liquid absorbed into the wood during the pressure period as possible, thus leaving the maximum concentration of preservatives in the treated area. Usually, water solutions of preservative salts are employed with this process, but it is also possible to impregnate wood with oil. The desired retention is achieved by changing the strength of the solution. William Burnett patented this development in 1838 of full-cell impregnation with water solutions. The patent covered the use of zinc chloride on water basis, also known as ''Burnettizing''. A full-cell process with oil was patented in 1838 by John Bethell. His patent described the injection of tar and oils into wood by applying pressure in closed cylinders. This process is still used today with some improvements.


Fluctuation pressure process

Contrary to the static full-cell and empty-cell processes, the fluctuation process is a dynamic process. By this process the pressure inside the impregnation cylinder changes between pressure and vacuum within a few seconds. There have been inconsistent claims that through this process it is possible to reverse the pit closure by spruce. However, the best results that have been achieved with this process by spruce do not exceed a penetration deeper than . Specialized equipment is necessary and therefore higher investment costs are incurred.


Boucherie process

Developed by Dr. Boucherie of France in 1838, this approach consisted of attaching a bag or container of preservative solution to a standing or a freshly cut tree with bark, branches, and leaves still attached, thereby injecting the liquid into the sap stream. Through transpiration of moisture from the leaves the preservative is drawn upward through the sapwood of the tree trunk. The modified Boucherie process consists of placing freshly cut, unpeeled timbers onto declining skids, with the stump slightly elevated, then fastening watertight covering caps or boring a number of holes into the ends, and inserting a solution of copper sulfate or other waterborne preservative into the caps or holes from an elevated container. Preservative oils tend to not penetrate satisfactorily by this method. The hydrostatic pressure of the liquid forces the preservative lengthwise into and through the sapwood, thus pushing the sap out of the other end of the timber. After a few days, the sapwood is completely impregnated; unfortunately little or no penetration takes place in the heartwood. Only green wood can be treated in this manner. This process has found considerable usage to impregnate poles and also larger trees in Europe and North America, and has experienced a revival of usage to impregnate bamboo in countries such as Costa Rica, Bangladesh, India and the state of Hawaii.


High-pressure sap displacement system

Developed in the Philippines, this method (abbreviated HPSD) consists of a cylinder pressure cap made from a 3 mm thick mild steel plate secured with 8 sets of bolts, a 2-HP diesel engine, and a pressure regulator with 1.4–14 kg/m2 capacity. The cap is placed over the stump of a pole, tree or bamboo and the preservative is forced into the wood with pressure from the engine.


Incising

First tested and patented by Kolossvary, Haltenberger, and Berdenich of Austria in 1911 and 1912 (U.S. patents. 1,012,207 and 1,018,624) with several improvements from O. P. M. Goss, D. W. Edwards and J. H. Mansfield among others, this process consists of making shallow, slit-like holes in the surfaces of material to be treated, so that deeper and more uniform penetration of preservative may be obtained. The term ''incising'' or perforating comes from the Latin ''incidere'', a compound of ''in'' and ''caedere'' (to cut). Incisions made in sawed material usually are parallel with the grain of the wood. This process is common in North America (since the 1950s), where Douglas-fir products and pole butts of various species are prepared before treatment. It is most useful for woods that are resistant to side penetration, but allow preservative transport along the grain. In the region in which it is produced, it is common practice to incise all sawed Douglas-fir or more in thickness before treatment. Unfortunately, the impregnation of spruce, the most important structural timber in large areas in Europe, has shown that unsatisfactory treatment depths have been achieved with impregnation. The maximum penetration of is not sufficient to protect wood in weathered positions. The present-day incising machines consist essentially of four revolving drums fitted with teeth or needles or with lasers that burn the incisions into the wood. Preservatives can be spread along the grain up to in radial and up to in tangential and radial direction. In North America, where smaller timber dimensions are common, incision depths of have become standard. In Europe, where larger dimensions are widespread, incision depths of are necessary. The incisions are visible and often considered to be wood error. Incisions by laser are significantly smaller than those of spokes or needles. The costs for each process type are approximately for spoke/conventional all-round incising €0.50/m2, by laser incising €3.60/m2 and by needle incision €1.00/m2. (Figures originate from the year 1998 and may vary from present day prices.)


Microwaving

An alternative increases the permeability of timber using microwave technology. There is some concern that this method may adversely affect the structural performance of the material. Research in this area has been conducted by the Cooperative Research Centre at the University of Melbourne, Australia.


Charring

Charring of timber results in surfaces which are fire-resistant, insect-resistant and proof against weathering. Wood surfaces are ignited using a hand-held burner or moved slowly across a fire. The charred surface is then cleaned using a steel brush to remove loose bits and to expose the grain. Oil or varnish may be applied if required. Charring wood with a red-hot iron is a traditional method in
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the n ...
, where it is called or (literally 'fire cypress').


See also

*
Leaky homes crisis The leaky homes crisis is an ongoing construction and legal crisis in New Zealand concerning timber-framed homes built from 1988 to 2004 that were not fully weather-tight. The problems often include the decay of timber framing which, in extreme ...
in New Zealand * Nanotoxicology * Impregnation resin


References


External links


Non-CCA


Non-CCA Wood Preservatives: Guide to Selected Resources - National Pesticide Information Center


Arsenate


Case Studies in Environmental Medicine - Arsenic Toxicity

CPSC Test coatings to reduce arsenic emissions from pressure treated wood


Borate




Analysis of Borate as an Insecticide in Wood


Sodium silicate




Albite (sodium aluminum silicate)
mineral


Miscellaneous



* ttps://www.epa.gov/ingredients-used-pesticide-products/chromated-arsenicals-cca Information from the U.S. Environmental Protection Agency
The American Wood Protection Association (AWPA, formerly the American Wood Preservers' Association)

American Lumber Standards Committee (ALSC)


{{Woodworking Wood Structural engineering Preservatives