Introduction :Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers (which are strong in tension) embedded in a matrix of lignin which resists compression. Wood is produced as secondary xylem in the stems of trees (and other woody plants). In a living tree it performs a support function, enabling woody plants to grow large or to stand up for themselves. It also mediates the transfer of water and nutrients to the leaves and other growing tissues. Wood may also refer to other plant materials with comparable properties, and to material engineered from wood, or wood chips or fiber.
The following are some of the wood timbers used in kerala
(P.S : Click on The Images To See In full Size )
The following are some of the wood timbers used in kerala
(P.S : Click on The Images To See In full Size )
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Seasoning Of Timber : Seasoning is the name given to the methods of drying timber
There are two methods by which timber can be dried:
(i) natural drying or air drying, and
(ii) artificial drying.
Air drying
Air-drying is the drying of timber by exposing it to the air. The technique of air-drying consists mainly of making a stack of sawn timber (with the layers of boards separated by stickers) on raised foundations, in a clean, cool, dry and shady place. Rate of drying largely depends on climatic conditions, and on the air movement (exposure to the wind). For successful air-drying, a continuous and uniform flow of air throughout the pile of the timber needs to be arranged. Coating the planks with any substance that is relatively impermeable to moisture can control the rate of loss of moisture; ordinary mineral oil is usually quite effective. Coating the ends of logs with oil or thick paint, improves their quality upon drying. Wrapping planks or logs in materials, which will allow some movement of moisture, generally works very well provided the wood is first treated against fungal infection by coating in petrol/gasoline or oil. Mineral oil will generally not soak in more than 1-2 mm below the surface and is easily removed by planning when the timber is suitably dry.
Natural seasoning of timber
Kiln drying
The process of kiln drying consists basically of introducing heat. This may be directly, using natural gas and/or electricity or indirectly, through steam-heated heat exchangers, although solar energy is also possible. In the process, deliberate control of temperature, relative humidity and air circulation is provided to give conditions at various stages (moisture contents or times) of drying the timber to achieve effective drying. For this purpose, the timber is stacked in chambers, called wood drying kilns, which are fitted with equipment for manipulation and control of the temperature and the relative humidity of the drying air and its circulation rate through the timber stack
Kiln drying provides a means of overcoming the limitations imposed by erratic weather conditions. In kiln drying as in air drying, unsaturated air is used as the drying medium. Almost all commercial timbers of the world are dried in industrial kilns. A comparison of air drying, conventional kiln and solar drying is given below:
1. Timber can be dried to any desired low moisture content by conventional or solar kiln drying, but in air drying, moisture contents of less than 18% are difficult to attain for most locations.
2. The drying times are considerably less in conventional kiln drying than in solar kiln drying, followed by air-drying.
1. This means that if capital outlay is involved, this capital is just sitting there for a longer time when air-drying is used. On the other hand, installing an industrial kiln, to say nothing of maintenance and operation, is expensive.
2. In addition, wood that is being air-dried takes up space, which could also cost money.
3. In air-drying, there is little control over the drying elements, so drying degrade cannot be controlled.
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Seasoning Of Timber : Seasoning is the name given to the methods of drying timber
There are two methods by which timber can be dried:
(i) natural drying or air drying, and
(ii) artificial drying.
Air drying
Air-drying is the drying of timber by exposing it to the air. The technique of air-drying consists mainly of making a stack of sawn timber (with the layers of boards separated by stickers) on raised foundations, in a clean, cool, dry and shady place. Rate of drying largely depends on climatic conditions, and on the air movement (exposure to the wind). For successful air-drying, a continuous and uniform flow of air throughout the pile of the timber needs to be arranged. Coating the planks with any substance that is relatively impermeable to moisture can control the rate of loss of moisture; ordinary mineral oil is usually quite effective. Coating the ends of logs with oil or thick paint, improves their quality upon drying. Wrapping planks or logs in materials, which will allow some movement of moisture, generally works very well provided the wood is first treated against fungal infection by coating in petrol/gasoline or oil. Mineral oil will generally not soak in more than 1-2 mm below the surface and is easily removed by planning when the timber is suitably dry.
Natural seasoning of timber
Kiln drying
The process of kiln drying consists basically of introducing heat. This may be directly, using natural gas and/or electricity or indirectly, through steam-heated heat exchangers, although solar energy is also possible. In the process, deliberate control of temperature, relative humidity and air circulation is provided to give conditions at various stages (moisture contents or times) of drying the timber to achieve effective drying. For this purpose, the timber is stacked in chambers, called wood drying kilns, which are fitted with equipment for manipulation and control of the temperature and the relative humidity of the drying air and its circulation rate through the timber stack
Kiln drying provides a means of overcoming the limitations imposed by erratic weather conditions. In kiln drying as in air drying, unsaturated air is used as the drying medium. Almost all commercial timbers of the world are dried in industrial kilns. A comparison of air drying, conventional kiln and solar drying is given below:
1. Timber can be dried to any desired low moisture content by conventional or solar kiln drying, but in air drying, moisture contents of less than 18% are difficult to attain for most locations.
2. The drying times are considerably less in conventional kiln drying than in solar kiln drying, followed by air-drying.
1. This means that if capital outlay is involved, this capital is just sitting there for a longer time when air-drying is used. On the other hand, installing an industrial kiln, to say nothing of maintenance and operation, is expensive.
2. In addition, wood that is being air-dried takes up space, which could also cost money.
3. In air-drying, there is little control over the drying elements, so drying degrade cannot be controlled.
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Timber Preservation : Timber has been used as a structural material for millennia and is in many ways ideal for the job. It can be described as strong, beautiful and sustainable – basically, wood is good! However, wood-rotting fungi and the wood-boring larvae of certain insects (also known as woodworm) feed by breaking down wood cells and, if left unchecked, ultimately destroy the timber in your home. Fungi and woodworm consequently threaten the structural integrity of your home, leading to substantial financial costs and potential danger for inhabitants.
For the preservation of timbers different types of preservatives can be used :
Natural preservatives
Naturally rot-resistant woods :These species are resistant to decay in their natural state, due to high levels of organic chemicals called extractives, mainly polyphenols. Extractives are chemicals that are deposited in the heartwood of certain tree species as they convert sapwood to heartwood. Huon pine (Lagarostrobos franklinii), merbau (Intsia bijuga), ironbark (Eucalyptus spp.), tōtara (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.
Tung oil :Tung oil has been known about for hundreds of years in China, where it was used as a preservative for wood ships. The oil penetrates the wood, then hardens to form an impermeablehydrophobic layer up to 5 mm into the wood. As a preservative it is effective for exterior work above and below ground, but the thin layer makes it less useful in practice. It is not available as a pressure treatment. Some manufacturers recommend tung oil as a stabiliser for CCA.
Heat treatments : There is ongoing research as to whether heat treatments can be used to make timber more durable. By heating timber to a certain temperature, it may be possible to make the wood-fibre less appetising to insects. Although unlikely to be as effective as chemical preservatives, anecdotal evidence suggests some consumers would prefer chemical-free timber preservation methods.
Heat treatment can also improve the properties of the wood with respect to water: 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.
Mud treatment : Wood and bamboo can be buried in mud to help protect it from insects and decay. This practice is used widely in Vietnam to build farm houses consisting of a wooden structural frame, a bamboo roof frame and bamboo with mud mixed with rice hay for the walls. While wood in contact with soil will generally decompose more quickly than wood not in contact with soil, 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.
Chemical preservatives Chemical preservatives can be classified into three broad categories: water-borne preservatives, oil-borne preservatives, and light organic solvent preservatives (LOSPs). These are discussed in more detail below.
Chromated copper arsenate (CCA) :In CCA treatment, copper is the primary fungicide, arsenic is a secondary fungicide and an insecticide, and chromium is a fixative which also provides ultraviolet (UV) light resistance. Recognized for the greenish tint it imparts to timber, CCA is a preservative that was extremely 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.
Alkaline copper quaternary : Alkaline copper quaternary (ACQ) is a preservative made of copper, a fungicide, and a quaternary ammonium compound (quat), an insecticide which also augments the fungicidal treatment is a wood preservative that has come into wide use in the USA, 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 :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 USA, 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.
Other copper compounds : These include copper HDO (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.
Micronized copper technology : Particulate (micronised or dispersed) copper preservative technology has recently been introduced in the USA and Europe. In these systems, the copper is ground to micro sized particles and suspended in water rather than being dissolved in a chemical reaction as is the case with other copper products such as ACQ and Copper Azole. There are currently two particulate copper systems in production. One system uses a quat biocide system (known as MCQ) and is a take-off of ACQ. The other uses an azole biocide (known as MCA or μCA-C) and is a take-off of Copper Azole.
Borate preservatives : Boric acid, oxides and salts (borates) are effective wood preservatives and are supplied under numerous brand names throughout the world. 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 readily be leached out. Therefore they should not be used where they will be exposed to rain, water or ground contact. 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.
Sodium silicate-based preservatives : Sodium silicate is produced by fusing sodium 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. One company, Timber Treatment Technology, LLC, has found that infusing timber with a chemical solution containing sodium silicate with a specified energy level applied yields wood that not only does not provide flake or layering on the wood, nor does it wash out as others have done in the past; and it provides processed timber that received a class A fire classification. Their processed wood also paints and stains as new wood does. TTT, LLC, sells these products under the name TimberSIL. Other uses include fixing pigments in paintings and cloth printing, and for preserving eggs.
Potassium silicate-based preservatives :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 preservatives : 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.
Toxic Oil-borne preservatives : These include pentachlorophenol and creosote. They are toxic, have an unpleasant odour and are generally not used in consumer products.
Coal-tar creosote : Creosote is a tar-based preservative that has been commonly used for telephone poles and railroad ties. Creosote is one of the oldest wood preservatives, and was originally derived from a wood distillate. These days virtually all creosote is manufactured from the distillation of coal tar. It often collects inside chimneys and may cause a fire hazard. Creosote is regulated as a pesticide and is not usually sold to the general public. It is still used for railroad ties (also called railway sleepers and cross ties) and utility poles.
Linseed oil :Linseed oil is used to preserve Wood fences, log cabins, and wood furniture. (Such woods as willow, pine, oak, etc.) The function of linseed oil as a preservative is believed to be related to its action as a water repellent and drying agent rather than a direct biocidal activity.
Other emulsions : Light organic solvent preservatives (LOSP) This class of timber treatments use white spirit, or light oils such as kerosene, as the solvent carrier to deliver preservative compounds into timber. Synthetic pyrethroids are typically used as an insecticide, such as permethrin, bifenthrin or deltamethrin. In Australia and New Zealand, the most common formulations use Permethrin as an insecticide, and Propaconazole and Tebuconazole as fungicides. While still using a chemical preservative, this formulation contains no heavy-metal compounds.
New technologies
Glass Fortified Wood: Glass Fortified Wood (glass wood) is lumber that has gone through a process that intermixes a non-toxic sodium silicate (water glass) based formula throughout the wood fibers protecting the wood from fire, rot and insect damage. With glass encapsulating the wood fibers, the lumber becomes harder and the strength is increased. Glass wood can be used for in ground contact applications, in water applications and it is Class-A fire retardant.
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 has been the most studied.
The physical properties of any material are determined by its chemical structure. Wood contains an abundance of chemical groups called free hydroxyls. Free hydroxyl groups readily absorb and release water according to changes in the climatic conditions to which they are exposed. This is the main reason why wood's dimensional stability is impacted by swelling and shrinking. It is also believed that the digestion of wood by enzymes initiates at the free hydroxyl sites - which is one of the principal reasons why wood is prone to decay
For the preservation of timbers different types of preservatives can be used :
Natural preservatives
Naturally rot-resistant woods :These species are resistant to decay in their natural state, due to high levels of organic chemicals called extractives, mainly polyphenols. Extractives are chemicals that are deposited in the heartwood of certain tree species as they convert sapwood to heartwood. Huon pine (Lagarostrobos franklinii), merbau (Intsia bijuga), ironbark (Eucalyptus spp.), tōtara (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.
Tung oil :Tung oil has been known about for hundreds of years in China, where it was used as a preservative for wood ships. The oil penetrates the wood, then hardens to form an impermeablehydrophobic layer up to 5 mm into the wood. As a preservative it is effective for exterior work above and below ground, but the thin layer makes it less useful in practice. It is not available as a pressure treatment. Some manufacturers recommend tung oil as a stabiliser for CCA.
Heat treatments : There is ongoing research as to whether heat treatments can be used to make timber more durable. By heating timber to a certain temperature, it may be possible to make the wood-fibre less appetising to insects. Although unlikely to be as effective as chemical preservatives, anecdotal evidence suggests some consumers would prefer chemical-free timber preservation methods.
Heat treatment can also improve the properties of the wood with respect to water: 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.
Mud treatment : Wood and bamboo can be buried in mud to help protect it from insects and decay. This practice is used widely in Vietnam to build farm houses consisting of a wooden structural frame, a bamboo roof frame and bamboo with mud mixed with rice hay for the walls. While wood in contact with soil will generally decompose more quickly than wood not in contact with soil, 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.
Chemical preservatives Chemical preservatives can be classified into three broad categories: water-borne preservatives, oil-borne preservatives, and light organic solvent preservatives (LOSPs). These are discussed in more detail below.
Chromated copper arsenate (CCA) :In CCA treatment, copper is the primary fungicide, arsenic is a secondary fungicide and an insecticide, and chromium is a fixative which also provides ultraviolet (UV) light resistance. Recognized for the greenish tint it imparts to timber, CCA is a preservative that was extremely 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.
Alkaline copper quaternary : Alkaline copper quaternary (ACQ) is a preservative made of copper, a fungicide, and a quaternary ammonium compound (quat), an insecticide which also augments the fungicidal treatment is a wood preservative that has come into wide use in the USA, 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 :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 USA, 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.
Other copper compounds : These include copper HDO (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.
Micronized copper technology : Particulate (micronised or dispersed) copper preservative technology has recently been introduced in the USA and Europe. In these systems, the copper is ground to micro sized particles and suspended in water rather than being dissolved in a chemical reaction as is the case with other copper products such as ACQ and Copper Azole. There are currently two particulate copper systems in production. One system uses a quat biocide system (known as MCQ) and is a take-off of ACQ. The other uses an azole biocide (known as MCA or μCA-C) and is a take-off of Copper Azole.
Borate preservatives : Boric acid, oxides and salts (borates) are effective wood preservatives and are supplied under numerous brand names throughout the world. 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 readily be leached out. Therefore they should not be used where they will be exposed to rain, water or ground contact. 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.
Sodium silicate-based preservatives : Sodium silicate is produced by fusing sodium 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. One company, Timber Treatment Technology, LLC, has found that infusing timber with a chemical solution containing sodium silicate with a specified energy level applied yields wood that not only does not provide flake or layering on the wood, nor does it wash out as others have done in the past; and it provides processed timber that received a class A fire classification. Their processed wood also paints and stains as new wood does. TTT, LLC, sells these products under the name TimberSIL. Other uses include fixing pigments in paintings and cloth printing, and for preserving eggs.
Potassium silicate-based preservatives :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 preservatives : 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.
Toxic Oil-borne preservatives : These include pentachlorophenol and creosote. They are toxic, have an unpleasant odour and are generally not used in consumer products.
Coal-tar creosote : Creosote is a tar-based preservative that has been commonly used for telephone poles and railroad ties. Creosote is one of the oldest wood preservatives, and was originally derived from a wood distillate. These days virtually all creosote is manufactured from the distillation of coal tar. It often collects inside chimneys and may cause a fire hazard. Creosote is regulated as a pesticide and is not usually sold to the general public. It is still used for railroad ties (also called railway sleepers and cross ties) and utility poles.
Linseed oil :Linseed oil is used to preserve Wood fences, log cabins, and wood furniture. (Such woods as willow, pine, oak, etc.) The function of linseed oil as a preservative is believed to be related to its action as a water repellent and drying agent rather than a direct biocidal activity.
Other emulsions : Light organic solvent preservatives (LOSP) This class of timber treatments use white spirit, or light oils such as kerosene, as the solvent carrier to deliver preservative compounds into timber. Synthetic pyrethroids are typically used as an insecticide, such as permethrin, bifenthrin or deltamethrin. In Australia and New Zealand, the most common formulations use Permethrin as an insecticide, and Propaconazole and Tebuconazole as fungicides. While still using a chemical preservative, this formulation contains no heavy-metal compounds.
New technologies
Glass Fortified Wood: Glass Fortified Wood (glass wood) is lumber that has gone through a process that intermixes a non-toxic sodium silicate (water glass) based formula throughout the wood fibers protecting the wood from fire, rot and insect damage. With glass encapsulating the wood fibers, the lumber becomes harder and the strength is increased. Glass wood can be used for in ground contact applications, in water applications and it is Class-A fire retardant.
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 has been the most studied.
The physical properties of any material are determined by its chemical structure. Wood contains an abundance of chemical groups called free hydroxyls. Free hydroxyl groups readily absorb and release water according to changes in the climatic conditions to which they are exposed. This is the main reason why wood's dimensional stability is impacted by swelling and shrinking. It is also believed that the digestion of wood by enzymes initiates at the free hydroxyl sites - which is one of the principal reasons why wood is prone to decay