Forestry & Forest Sciences · Forest Products & Utilization · also Wood Seasoning and Preservation, Timber Preservation, Wood Protection
Drying timber without defects and protecting it from decay, insects and fire, by chemical, biological and modified-wood methods.
In the exams: ICAR AIEEA-PG Group 09 (biological and chemical wood preservation); ICAR AICE-JRF 59 Unit 7 (wood seasoning and preservation techniques); IFoS Forestry Paper II Section B
Freshly felled wood is full of water, often holding as much water as wood substance or more, and it is vulnerable to fungi, insects and, in the sea, marine borers. Before it can be used reliably, most wood must be seasoned, that is dried in a controlled way to a moisture content suited to its use, and wood that will be exposed to decay or insect hazards must be protected, either by choosing naturally durable heartwood, by keeping it dry through design, or by treating it with preservatives or modifying it. Seasoning and preservation turn perishable, unstable logs into a material that can last for decades in buildings, poles, sleepers, boats and furniture, and they allow fast-grown, non-durable plantation species to replace scarce durable timbers.
This subject covers air seasoning; kiln seasoning, including kiln types and schedules; seasoning defects and their control; solar and other low-cost methods; the agents that destroy wood (fungi, insects and marine borers); the main groups of preservatives (oil-type, water-borne and organic solvent); treatment methods without and with pressure, including the full-cell and empty-cell processes; sap-displacement and diffusion methods; retention, penetration and treatment standards; fire retardants; biological and environmentally safer approaches, including modified wood; and the safety and environmental rules that now govern preservatives. The structure of wood, moisture content, the fibre saturation point and natural durability are explained in wood-anatomy-properties and are used here without repetition; the biology of wood-destroying insects and fungi is treated in forest-entomology and forest-pathology; and felling, extraction and handling of logs in forest-operations-harvesting.
Seasoning and preservation are examined in Unit 7 of the ICAR AICE-JRF forestry syllabus (wood seasoning and preservation techniques), in the ICAR AIEEA-PG forestry group (biological and chemical wood preservation), and in forest resources and utilization in Section B of Paper II of the Indian Forest Service forestry optional. Questions ask for the principles and methods of air and kiln seasoning, the causes and control of seasoning defects, the classes and examples of preservatives, the steps of the Bethell, Rueping and Lowry processes, the Boucherie process, calculations of retention, and the safety and environmental issues of preservatives.
Preservative registrations and permitted uses differ between countries and change often; several older preservatives are now banned or restricted. This manual describes classes and principles; any treatment must follow current national standards, product approvals and labels.
Seasoning reduces the moisture content of wood to a level close to the equilibrium moisture content of its intended service conditions. Dry wood is lighter, cheaper to transport, stronger and stiffer, more dimensionally stable in use, resistant to decay and stain fungi (which need moisture above roughly 20%), easier to machine, glue and finish, and ready to accept many preservatives and coatings. Wood used green shrinks, distorts, opens joints and loosens fastenings as it dries in service. The aim of seasoning is to dry wood as quickly and cheaply as possible without causing defects, which arise when wood shrinks unevenly.
Air seasoning (air drying) uses natural air movement and ambient temperature and humidity to dry wood in stacks in the open or under cover. Drying proceeds by evaporation from the surface and movement of moisture from the interior to the surface; the rate depends on air temperature, relative humidity and air speed over the wood, and on the species, thickness and initial moisture content. Air seasoning cannot reduce moisture content below the equilibrium moisture content of the outdoor air, which in much of India is typically in the range of about 12–18% depending on season and region, and it is slow: thin softwood boards may dry in weeks to a few months, while thick hardwood planks may take a year or more.
Correct stacking is the key to good air seasoning. The stacking yard should be well drained, clear of vegetation and debris, and open to prevailing winds. Stacks are built on firm, level foundations raised well above the ground to allow air movement underneath and keep the wood out of reach of ground moisture and termites. Boards of similar thickness and species are laid in layers separated by stickers (crossers or piling sticks) of uniform thickness and dry, durable or treated wood, placed at regular intervals along the length and aligned vertically one above the other so that the weight is carried straight down and boards do not bend. Spaces between boards in each layer allow vertical air flow. Stacks are covered with a roof to protect them from rain and direct sun, and heavy weights on top restrain warping in the upper layers. Ends of valuable boards are coated to slow end drying and reduce end splits. Logs awaiting conversion may be stored under water or water sprays (see wood-anatomy-properties for water relations) to prevent splitting, stain and insect attack.
Air seasoning needs little capital or energy and is suitable for initial drying of thick stock and large volumes, especially of refractory species that must be dried slowly. Its limitations are slow drying, which ties up capital and space; dependence on weather and season; inability to reach the low moisture contents required for interior joinery and furniture; and risk of stain, decay and insect attack during long drying periods in warm, humid climates. Air seasoning is often used to reduce moisture content before kiln drying, saving kiln time and energy.
Kiln seasoning dries wood in an enclosed chamber where temperature, relative humidity and air circulation are controlled. Raising temperature increases the rate at which moisture moves through wood and evaporates; controlling humidity keeps the surface from drying much faster than the interior, which would cause stresses and checks; and fans circulate air through the stacks to carry heat to the wood and moisture away. Kiln drying is much faster than air drying, can reach any required moisture content, kills insects and fungi present in the wood, and produces more uniform results. It requires capital, energy and skilled operation.
Compartment (batch) kilns are the most common type: a whole charge of timber is loaded, dried through a schedule and unloaded. Progressive kilns are long tunnels through which loads move on trolleys from a cool, humid end to a hot, dry end, giving continuous operation for large volumes of uniform stock. Kilns are heated by steam coils, hot water, thermal oil or direct firing, often fuelled by wood residues. Humidity is raised by steam spray or water spray and lowered by venting moist air. Dehumidification kilns use a heat pump to condense moisture from the air at relatively low temperatures, saving energy and suiting refractory species. Vacuum kilns dry at reduced pressure, which lowers the boiling point of water and speeds drying of thick or difficult stock. High-temperature kilns (above 100 °C) are used for some softwoods. Solar kilns are described in a later chapter.
A kiln schedule specifies the sequence of temperatures and humidities (usually as dry-bulb temperature and wet-bulb depression or relative humidity) used during drying. Schedules for each species and thickness are developed from experience and research and published by forest products research institutes. Most commercial schedules are moisture-content schedules, which change conditions as the moisture content of the wood falls; time schedules change conditions at set times. Early in drying, while the wood is above the fibre saturation point, conditions are mild (moderate temperature and high humidity) to avoid surface checks and collapse; as the wood dries, temperature is raised and humidity lowered. Refractory species such as many dense hardwoods require mild schedules; permeable softwoods tolerate severe schedules. Final equalising brings all boards to a similar moisture content, and conditioning (a short period of high humidity at the end) relieves casehardening stresses.
Kiln operators monitor drying with kiln samples: short boards cut from representative pieces, weighed regularly and oven-dried at the end (or a section oven-dried at the start) to calculate current moisture content, and with electrical moisture meters and probes. Wet-bulb and dry-bulb thermometers or electronic sensors record conditions, and modern kilns are computer-controlled. Records of each charge allow schedules to be refined. Stacking in the kiln follows the same principles as in air seasoning, with careful sticker alignment and baffles to force air through the stack.
Seasoning defects arise mainly because wood shrinks unevenly: the surface dries and shrinks before the interior, tangential shrinkage exceeds radial shrinkage, and reaction wood and juvenile wood shrink abnormally along the grain. Stresses develop between the drier shell and the wetter core; if they exceed the strength of the wood perpendicular to the grain, the wood cracks. Some defects are caused by too rapid drying, some by too slow drying in warm humid conditions, and some by properties of the wood itself.
Surface checks are shallow cracks on the surface, usually along rays, caused by rapid surface drying; they often close later but remain as weaknesses and spoil appearance. End checks and end splits form because moisture escapes fastest from the ends; end coating and avoiding very rapid early drying reduce them. Splits pass through the full thickness. Honeycombing (internal checking) consists of cracks inside the wood, often invisible on the surface, which develop when the core shrinks after the surface has set in an expanded state, or from collapse; it is caused by too severe conditions in the later stages of drying of refractory hardwoods.
Casehardening is a condition of stress in which the outer shell is in compression and the core in tension after drying, because the surface dried and set while the interior was still wet. Casehardened boards distort when resawn or machined; the condition can be detected by the prong test, in which a cross-section cut from the board is sawn into prongs whose inward or outward bending reveals the stress. It is relieved by conditioning with high humidity at the end of kiln drying. Collapse is the flattening of cells in wood dried from the green state at high moisture content, caused by capillary tension as free water leaves cells with impermeable walls, sometimes aided by high temperature; it produces corrugated surfaces and severe shrinkage and is common in some eucalypts and other species. Mild early drying reduces collapse, and in some species collapsed wood can be partly recovered by steaming (reconditioning).
Distortion (warp) includes cup (curvature across the width), bow (curvature along the length on the flat face), spring or crook (curvature along the length on the edge) and twist (spiral distortion). Square sections may become diamond-shaped because of different radial and tangential shrinkage. Distortion is reduced by correct stacking with aligned stickers, weights on top of stacks, using quarter-sawn material for critical uses, and avoiding reaction wood and spiral-grained stock. Fungal stain (sapstain and moulds) and chemical stains, including brown stain in some softwoods and grey or sticker stain where stickers contact boards, develop when drying is too slow in warm, humid conditions; prompt drying, good air circulation and prophylactic antistain treatment of green timber prevent them (see forest-pathology).
Solar kilns use the sun's energy to heat air in a chamber with a transparent roof or walls, often with a solar collector, black absorbing surfaces and fans powered by electricity or solar panels. They dry wood faster than air seasoning and to lower moisture contents, with less degrade, because the daily cycle of warm days and cooler, more humid nights helps relieve stresses. They are cheap to build and operate, suitable for small enterprises and rural areas in sunny climates, and have been developed and promoted by forest research institutes in India and other tropical countries. Their capacity is limited and drying depends on weather.
Water seasoning involves immersing logs or sawn timber in running or still water for several weeks before air drying. Water dissolves and leaches some sap and starch, which is said to reduce subsequent stain and insect attack and to make drying more even, and logs stored in water are protected from end splitting and attack while awaiting conversion. It is a traditional practice in some regions, especially for bamboo and some hardwoods, though its effects on drying time vary.
Girdling a standing tree by cutting through the bark and sapwood around the stem kills it gradually, so that the stem dries partly while standing. Teak in South and South-east Asia was traditionally girdled two to three years before felling, both to reduce weight so that logs would float for river transport and to reduce seasoning time and defects. The practice is now less common because it reduces growth and can permit insect attack. Other traditional methods include smoke seasoning, in which timber is dried over smoky fires, which also deposits some protective compounds, and boiling small pieces before drying.
In chemical or salt seasoning, green wood is treated with hygroscopic chemicals, such as common salt or urea, that lower the vapour pressure at the surface so that the surface dries more slowly and in step with the interior, reducing checking in refractory species. The chemicals can, however, make the wood corrosive to fastenings and hygroscopic in service, so the method has limited use. Kiln drying with suitable schedules has largely replaced it.
Decay fungi are the most important cause of deterioration of wood in service. They need moisture above about 20% (near or above the fibre saturation point for vigorous attack), oxygen, suitable temperatures and food; removing any of these prevents decay. Brown rot fungi degrade cellulose and leave modified lignin, causing rapid loss of strength and cubical cracking; they are common in softwoods and in buildings, including the dry rot fungus. White rot fungi degrade lignin as well as cellulose, leaving bleached, fibrous wood, and are common in hardwoods. Soft rot fungi attack very wet wood and wood with high preservative loadings. Sapstain fungi and moulds discolour wood with little strength loss. The biology and recognition of these fungi are described in forest-pathology.
Insects that damage wood in service include subterranean termites, which attack wood in or near the ground and can reach timber in buildings through mud tubes; drywood termites, which live entirely within dry timber in warm climates; powderpost beetles (Lyctinae and other Bostrichidae), which attack starch-rich sapwood of hardwoods and bamboo; furniture beetles in temperate regions; and longhorn beetles such as the house longhorn, which attacks softwood in buildings in some regions. Freshly felled logs are attacked by ambrosia beetles, bark beetles and borers. The biology of these insects is described in forest-entomology. Termites are the main hazard for timber in ground contact in the tropics, and preservatives used in such conditions must be effective against them as well as against fungi.
Timber in seawater, such as piles, jetties and boats, is attacked by marine borers. Shipworms are molluscs (family Teredinidae, such as Teredo and Bankia) that enter as tiny larvae and grow into long, worm-like animals boring tunnels lined with shell through the wood, often destroying piles internally while the surface looks sound. Gribbles are small crustaceans (Limnoria) that bore shallow tunnels in the surface, which is eroded by wave action. Attack is most severe in warm seas. Protection requires naturally resistant timbers, which are few, or heavy preservative treatments rated for marine use, often with dual treatments effective against both molluscs and crustaceans.
Wood exposed outdoors without protection weathers: ultraviolet light degrades lignin at the surface, rain leaches the breakdown products, and the surface turns grey, roughens and checks, though weathering alone proceeds slowly (a few millimetres per century). Fire is a further hazard, discussed in the chapter on fire retardants. Chemical attack, mechanical wear and some bacteria also cause deterioration in particular conditions. Design that keeps wood dry, sheds water, allows ventilation and protects end grain is the first line of defence against all biological agents.
A wood preservative should be toxic to the organisms it is meant to control; penetrate wood adequately; remain in the wood without excessive leaching or evaporation (permanence); be safe to handle and use, with acceptable effects on people and the environment; not corrode fastenings or equipment; not weaken wood or interfere with gluing, painting or other processing where these are needed; be available and economical; and, where relevant, be clean and odourless. No single preservative meets all requirements for all uses, so different types are chosen for different hazards.
Creosote, a distillate of coal tar, was for over a century the most important preservative for railway sleepers, poles, piles and marine timbers. It is highly effective against fungi, insects and marine borers, water-repellent and resistant to leaching, but it is oily, has a strong odour, cannot be painted, and contains polycyclic aromatic hydrocarbons, some of which are carcinogenic; its use is now restricted in many countries to industrial applications such as sleepers and utility poles. Pentachlorophenol, an organochlorine dissolved in oil, was widely used for poles but was listed for elimination under the Stockholm Convention on Persistent Organic Pollutants in 2015 and is banned or being phased out in many countries. Copper naphthenate is an oil-borne preservative used for poles and field treatment in some countries.
Water-borne preservatives are salts dissolved in water that react with or fix in wood after treatment. Copper-chrome-arsenic (CCA), known in India as ASCU (arsenic-copper-chrome), was for decades the most widely used water-borne preservative: copper protects against fungi, arsenic against insects and copper-tolerant fungi, and chromium fixes the components in the wood, making them highly resistant to leaching; treated wood is clean, odourless and paintable. Because of concerns about arsenic and chromium, CCA was withdrawn from most residential uses in the United States from the end of 2003 and is restricted in the European Union and elsewhere, though it remains permitted for some industrial uses in many countries. Copper-chrome-boron (CCB) replaces arsenic with boron. Copper-based alternatives without arsenic or chromium include alkaline copper quaternary (ACQ) and copper azole, now widely used for residential and agricultural timber. Boron compounds (borax and boric acid) are effective against fungi and insects, have low toxicity to mammals and diffuse well into green wood, but leach readily, so they are used mainly for wood protected from wetting, such as interior framing and bamboo.
Light organic solvent preservatives (LOSPs) dissolve fungicides and insecticides, such as azoles (propiconazole, tebuconazole), copper or zinc naphthenates and synthetic pyrethroids (such as permethrin and cypermethrin), in a light petroleum solvent. They penetrate dry wood well without swelling it or raising the grain, and are used for joinery, window frames and machined components that must keep their dimensions and be painted. The solvents are flammable and volatile, raising emission and safety concerns, and water-based micro-emulsions of the same active ingredients are increasingly used. Insecticides alone, such as synthetic pyrethroids, are used for protection against termites and borers in some applications, including soil treatment under buildings where permitted.
Non-pressure methods apply preservative without pressure vessels. Brushing and spraying deposit a thin surface layer, giving short-term protection, and are used for remedial and field treatment of cut ends and bore holes. Dipping in a preservative bath for seconds to minutes gives somewhat better surface protection, and is used for antistain treatment of green timber and for joinery with LOSPs. Steeping or soaking for days to weeks gives deeper penetration in permeable species. The hot and cold (open tank) process heats wood in preservative (or oil) to expand and drive out air from the cells, then cools it (or transfers it to cold preservative); as air in the cells contracts, preservative is drawn in. It gives good penetration in permeable sapwood and has been used for fence posts and poles where pressure plants are not available.
Pressure treatment in a closed cylinder forces preservative deep into wood, achieving the penetration and retention needed for severe hazards such as ground contact, poles, sleepers and marine use, with good control and consistency. The wood must normally be dried below the fibre saturation point (often to about 25–30% or lower) so that cells contain air rather than free water. Two main types of process are distinguished by whether they aim to fill the cells with preservative or leave them coated but largely empty.
In the full-cell or Bethell process, wood is placed in the cylinder and an initial vacuum is drawn to remove air from the wood; the cylinder is then filled with preservative while the vacuum is maintained; pressure is applied (commonly around 7–14 bar, depending on species and specification) until the required absorption is reached; the pressure is released and the preservative pumped out; and a final vacuum removes excess from the surfaces. Because air was removed beforehand, the cells fill with preservative, giving the maximum retention for a given penetration. The full-cell process is used with water-borne preservatives such as CCA and copper-based products, where the concentration of the solution controls retention, and with creosote for marine piles and other severe hazards where maximum retention is required.
Empty-cell processes aim for deep penetration with lower net retention, mainly with oil-type preservatives such as creosote, by using air trapped in the wood to expel excess preservative after treatment. In the Rueping process, initial air pressure is applied to the wood before the preservative is introduced and maintained while the cylinder is filled; preservative pressure is then applied, forcing liquid in and compressing the air inside the cells; when pressure is released, the expanding air forces much of the preservative back out, and a final vacuum recovers more. The Lowry process is similar but starts at atmospheric pressure with no initial air pressure or vacuum, so it recovers less preservative than the Rueping process. Empty-cell processes coat the cell walls with preservative while leaving the cell cavities largely empty, saving preservative and reducing bleeding of oil from treated poles and sleepers. Modified processes, such as oscillating pressure for refractory or green wood, and incising (making small cuts in the surface to open pathways) improve penetration in difficult species.
Pressure processes require dry wood, but drying takes time and money, and some uses, such as poles and fence posts in rural areas, need simple methods that work on freshly felled green wood. Sap-displacement and diffusion methods use the water in green wood as a pathway for preservatives, avoiding the need to dry first.
In sap-displacement methods, preservative solution is introduced at one end of a freshly felled pole or log with its bark on, and pushed or drawn through the sapwood, displacing the sap, which emerges at the other end. In the Boucherie process, developed in France in the nineteenth century, a cap or rubber sleeve is fitted tightly over the butt end of a freshly felled pole and connected by a pipe to a tank of water-borne preservative placed several metres above, so that the hydrostatic head forces the solution through the sapwood towards the top until preservative appears at the top end. Modified versions use pumps or compressed air to apply pressure (pressure Boucherie). The method treats sapwood thoroughly and has been used in India and elsewhere for poles of species such as eucalyptus, chir pine and bamboo (in which preservative is forced through the culm). It must be applied within a short time after felling, before air enters the vessels or tracheids and blocks flow, and works only on sapwood.
Diffusion methods rely on the movement of dissolved preservative from a concentrated surface layer into the water-saturated wood. Green or freshly cut wood is dipped, sprayed or coated with a concentrated solution or paste of a diffusible preservative, usually boron compounds, and then close-stacked under cover (wrapped or covered to prevent drying) for several weeks, during which the preservative diffuses inward. Double diffusion uses two chemicals applied in succession that react inside the wood to form a less soluble compound, reducing leaching. Diffusion is particularly suitable for bamboo, for refractory species that are difficult to treat by pressure, and for small-scale enterprises. Its products, if boron-based, must be protected from leaching in service.
Other practical methods include butt treatment of poles and posts by standing them in a preservative bath, bandages or pastes applied to the groundline of poles already in service, and remedial treatments by drilling holes into existing timbers and filling them with diffusible rods, liquids or fumigants to extend service life. These methods are important for utilities maintaining large numbers of poles and for heritage timber structures.
Retention is the amount of preservative active ingredient (or total preservative) held in the treated wood, expressed as kilograms per cubic metre of wood (kg/m³). For water-borne preservatives, retention is calculated from the uptake of solution and its concentration: retention = (mass of solution absorbed × concentration) ÷ volume of wood treated. For example, if a charge of 0.25 m³ of wood absorbs 50 kg of a 4% solution, the preservative retained is 2 kg and the retention is 8 kg/m³. Required retentions are specified for each preservative and use; higher retentions are required for more severe hazards such as ground contact, freshwater and marine use than for above-ground uses. Retention can also be checked by chemical analysis of samples taken from the treated zone.
Penetration is the depth to which preservative has entered the wood. It is checked by taking cores with an increment borer from treated pieces and applying chemical spot tests that change colour where the preservative is present (for example tests for copper or boron), or by visual inspection for oily preservatives. Standards specify a minimum depth of penetration or require full penetration of the sapwood. Penetration matters as much as retention: a high retention concentrated in a thin surface layer may be breached by checks that open after treatment, exposing untreated wood. Heartwood of many species is refractory and resists penetration; sapwood is usually more permeable.
Species differ greatly in treatability, the ease with which they can be penetrated by preservatives. Treatability classes describe this, from easily treatable (permeable sapwood, such as pine sapwood) to refractory or extremely difficult (such as the heartwood of many dense hardwoods and spruce). Treatability depends on anatomy: open vessels and unaspirated pits favour penetration, while tyloses, aspirated pits, extractives and gums hinder it. For refractory species, incising, longer treatment cycles, diffusion methods or the use of naturally durable heartwood are alternatives.
Treatment standards specify which preservatives may be used, the required retention and penetration for each use, and the methods of testing. In India, IS 401 is the code of practice for preservation of timber, with related standards for particular preservatives, treatment of bamboo and of poles and sleepers. Internationally, hazard or use classes classify service conditions: the European standard EN 335 defines use classes from 1 (interior, dry) to 5 (in contact with seawater), and the American Wood Protection Association uses use categories from UC1 (interior, dry) through UC4 (ground contact, with sub-categories for increasing severity) to UC5 (marine). Specifying wood by use class links the hazard to the required treatment, so that the purchaser obtains appropriately treated wood. Quality control at treating plants, including records of each charge and testing of samples, ensures that standards are met.
Wood burns, but it behaves predictably in fire. When large timber sections are exposed to fire, the surface chars, and the char layer insulates the wood beneath, so that the charring front moves inward at a fairly steady rate, commonly of the order of 0.6 to 0.8 mm per minute for many softwoods and hardwoods, with denser hardwoods charring more slowly. The unburned core retains its strength, so large timber members can maintain their load-bearing capacity in fire for a predictable time, often longer than unprotected steel, which loses strength rapidly at high temperatures. Fire design of timber structures uses charring rates to size members for required fire resistance periods. Small sections, thin panels and surfaces, by contrast, ignite easily and spread flame, which is the main concern addressed by fire-retardant treatments.
Fire-retardant treatments reduce the ease of ignition, the rate of flame spread over the surface and the heat released. Most act by promoting charring at lower temperatures, so that wood decomposes to char and water rather than to flammable gases, reducing fuel for flaming combustion; some release non-combustible gases or water that dilute flammable gases, and some form glassy or foamed protective layers on the surface. Fire retardants do not make wood non-combustible; they slow its involvement in fire and its contribution to fire growth.
Impregnated fire retardants are salts forced into wood by vacuum-pressure treatment, similar to preservative treatment. Common chemicals include ammonium phosphates, ammonium sulphate, boric acid and borax, and guanylurea phosphate, often in proprietary combinations; some formulations are leachable and suitable only for interior use, while others are designed to be leach-resistant for exterior use. Surface coatings include intumescent paints, which swell when heated to form an insulating foam layer, and non-intumescent coatings. Fire-retardant-treated wood and panels are classified by standard fire tests of flame spread and heat release, such as reaction-to-fire classes in Europe and flame spread indices in North America, and are required in some building uses such as escape routes and public buildings.
Fire-retardant treatments can increase the hygroscopicity of wood, causing high moisture content and corrosion of metal fasteners in humid conditions; some reduce strength, especially at elevated temperatures over long periods (as was found with some treated plywood roof sheathing in hot attics); they may interfere with gluing and finishing; and some leach outdoors. Selection of an appropriate formulation and redrying after treatment address these problems.
Concerns about the toxicity of arsenic, chromium, creosote and pentachlorophenol, about leaching of preservatives into soil and water, and about disposal of treated wood at the end of its life have driven the search for safer methods. Approaches include less toxic preservatives, better use of natural durability, design for durability, biological methods and wood modification, which improves the properties of wood itself without biocides.
Protection by design keeps wood dry and well ventilated: overhanging roofs, raised foundations, drainage, avoiding water traps and end-grain exposure, using coatings that shed water, and separating wood from soil. Where wood is kept dry, decay cannot occur, and many non-durable species perform well without treatment. Naturally durable heartwood can be used in more hazardous situations, and sapwood can be removed. Choosing the right material for the use class often avoids the need for chemicals altogether.
Wood modification changes the chemical or physical structure of the cell wall to improve durability and dimensional stability without leaving toxic biocides. Thermal modification heats wood to about 180–230 °C in the absence of oxygen (in steam, nitrogen or oil) for several hours; hemicelluloses are degraded and the wood becomes less hygroscopic, more stable and more resistant to decay, but darker and more brittle, with reduced strength. Acetylation reacts wood with acetic anhydride, replacing hydroxyl groups in the cell wall with acetyl groups, greatly reducing water uptake and swelling and giving high durability. Furfurylation impregnates wood with furfuryl alcohol, derived from agricultural residues, which polymerises in the cell walls. Densification compresses wood, sometimes with heat and resin, to increase density and hardness. Modified woods are used for decking, cladding, windows and outdoor furniture, and their environmental benefits must be weighed against the energy and chemicals used in processing.
Biological approaches include using antagonistic fungi, such as species of Trichoderma, or bacteria to protect green timber against stain and mould fungi; using natural extracts with fungicidal or insecticidal properties, such as neem oil, tannins, essential oils, chitosan and heartwood extracts from durable species; and combining low doses of biocides with non-biocidal additives such as water repellents and antioxidants. These approaches are promising but generally less effective and less durable than conventional preservatives in severe hazards, and must be tested by standard methods before use. Traditional practices such as smoking bamboo and leaching starch also provide some protection.
Many wood preservatives are biocides designed to kill organisms, and some are toxic to people and the environment. Arsenic is a known human carcinogen, hexavalent chromium (present in CCA concentrates before fixation) is toxic and carcinogenic, creosote contains carcinogenic polycyclic aromatic hydrocarbons, pentachlorophenol is persistent and contaminated with dioxins, and some organic solvents are flammable and harmful. Hazards arise to workers at treatment plants and in handling treated wood, to users through skin contact or ingestion (especially children in contact with treated playground equipment, which prompted the restrictions on CCA in residential uses), and to the environment through spills, drippage from freshly treated wood, leaching from wood in service and improper disposal.
Wood preservatives are regulated as pesticides or biocides in most countries. In the European Union, the Biocidal Products Regulation (Regulation (EU) No 528/2012) requires active substances and products to be approved after assessment of risks to health and the environment, and has led to the withdrawal of many older substances. In the United States, preservatives are registered by the Environmental Protection Agency under pesticide law, and the residential use of CCA was ended by voluntary agreement from the end of 2003. Internationally, the Stockholm Convention on Persistent Organic Pollutants lists pentachlorophenol for elimination. In India, insecticides and fungicides used as preservatives are registered under the Insecticides Act, 1968 through the Central Insecticides Board and Registration Committee, and treatment practices follow Bureau of Indian Standards codes. Users must check which preservatives are permitted for which uses in their country.
Good practice at treatment plants includes enclosed and bunded storage of chemicals and treatment cylinders to contain spills; drip pads where freshly treated wood is held until fixation and drippage are complete; closed-loop systems that recycle solutions and drippage; protective clothing, gloves, eye protection and respirators for workers; monitoring of workplace exposure and of soil and water around plants; training; and records of each charge. Treated wood should be labelled with the preservative, retention and use class.
Users of treated wood should wear gloves and dust masks when cutting it, avoid using it where it contacts food or drinking water or where it could be chewed by livestock, not use sawdust from it as animal bedding or mulch, and never burn it in open fires, stoves or fireplaces, since burning CCA-treated wood releases arsenic and burning creosoted wood releases harmful smoke. At the end of its life, treated wood should be disposed of according to local regulations, which may classify it as hazardous waste; options include controlled landfill, incineration in plants with emission controls, and recycling of some treated wood into specified products. Life-cycle thinking, considering the impacts of treatment against the benefits of longer service life and reduced harvesting, guides the choice between treated, modified, durable and alternative materials.
Learn the principles of air and kiln drying, stacking rules, kiln types and the logic of schedules; the causes and control of each seasoning defect; the classes and examples of preservatives with their advantages and problems; the steps and purposes of the Bethell, Rueping and Lowry processes and of the Boucherie and diffusion methods; how to calculate retention; use classes and treatability; and the safety and environmental rules. Practise explaining why each step exists, for example why a final vacuum is drawn or why kiln schedules start mild.
A forester five years in may manage timber depots, sawmills or treatment units: organising log storage and stacking, operating or supervising kilns and solar dryers, specifying treatment for poles, sleepers, fencing and construction timber, checking retention and penetration certificates, treating bamboo for housing and handicrafts by diffusion or Boucherie methods, and ensuring safe handling and disposal of preservatives. Practical skills include using moisture meters, reading kiln schedules, sampling for spot tests and keeping treatment records.
A senior forester shapes policy and investment in wood processing: promoting seasoning and treatment so that plantation and lesser-used species can replace scarce durable timbers, setting standards and procurement specifications for treated wood in public works and utilities, phasing out hazardous preservatives in line with regulation, and supporting small enterprises with solar kilns and safe treatment facilities.
A senior researcher develops drying schedules and technologies, evaluates new preservatives and modified woods, studies treatability and its anatomical basis, assesses leaching and environmental fate of preservatives, contributes to standards and use-class systems, and advises governments and industry on safer, more sustainable wood protection.
No kiln schedules, preservative concentrations, retention requirements or treatment pressures are prescribed beyond typical ranges and illustrative examples; these must be taken from current national standards (such as IS 401 and related codes) and product approvals. Preservative registrations and permitted uses differ by country and change; CCA and creosote status in particular should be checked locally. Proprietary products are not named. The anatomy, moisture relations and natural durability of wood are in wood-anatomy-properties, and the biology of wood-destroying organisms in forest-entomology and forest-pathology.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
These notes are free to read here, with no account.
What the Laisha app adds:
Free for ten days, then a subscription. We would rather write that here than have you find it later.