Wood
Wood a material of plant origin characterised by a hard fibrous structure and composed mainly of cellulose, hemicellulose, and lignin. The high content (22-29%) of lignin contributes unique rigidity to the structure and thus distinguishes wood from all other plant materials. Wood is produced in a living tree for support, conduction, and storage of food. The support enables a tree to remain erect despite the height to which the tree grows. Wood also performs the role of conduction which consists of transporting water from the ground to the upper parts of the tree. Food is stored in certain parts of the wood until required by the living tree.
Near the bark there is a light coloured wood which is called sapwood. It contains living cells and has an active part in the life process of the tree. It is located next to the cambium. The sapwood layer may vary in thickness and in the number of growth rings contained in it.
The central portion of the wood is generally darker in colour which is called heartwood. It consists of inactive cells formed by changes in the living cells of the inner sapwood rings, presumably after their use for sap conduction and other life processes have largely ceased. The heartwood is the preferred portion of wood, because in many woods the discolouring substances are toxic to insects and fungi which decay wood. Accordingly, heartwood often offers greater resistance to decay than sapwood.
Wood in its original solid form has adequate properties for many uses. In Bangladesh it is widely used for furniture, cabinet, house, boat, ship, railway sleeper, electric transmission poles, fuel, etc. In the comminuted and reconstituted form, it can provide materials with a wide range of properties. [MA Sattar]
Wood preservation protecting wood, wood products or timbers from deterioration, decomposition or damage due to pest attacks through application of chemical substances. Wood is usually deteriorated by fungi (white rot, brown rot, dry rot, soft rot fungi), insects (termites, beetles, carpenter ants), marine borers (molluscans, crustaceans), and a host of other agents.
No single preservative is available which can meet all desirable requirements. Wood preservatives may be oil-borne, water-based or a mixture of different substances. Creosote and pentachlorophenol (PCP) are the organic oil-based preservatives, earlier used for treating of wood poles, piles and railway sleepers in Bangladesh. Nowadays creosote is used only for railway sleepers.
Chromated copper arsenate type C (CCA-C) is considered as effective inorganic water-based preservative and is being used for treating electric wood poles, anchor logs and corsairs in Bangladesh. Chromated copper boron (CCB) is another water-borne preservative, used for indoor use such as for electric meter boards, wooden packings, doors and windows, furniture etc. The third type is a preservative paste, formulated by mixing the oil-borne and water-borne types, such as heavy creosoted boron (HCB).
The life of preservative-treated timber products depends on penetration, retention and degree of fixation of preservatives used. The penetration and retention of a preservative, however, depends on the preservation method. The different methods being used in Bangladesh include washing and coating; brushing, spraying and dipping; soaking; Boucherie process; hot and cold bath process; diffusion process; and pressure process. The full cell pressure process is the best for the highest penetration and retention of preservative and is being commercially followed in Bangladesh. In this process air inside the wood is removed by initial vacuum and then empty cells of woods are filled up with preservative chemical by applying pressure on preservative solution. In Bangladesh fixative type of preservative, such as CCA-C is used for ground and water contact timber products and CCB is preferred for indoor use including hardwood and bamboo. [Arun Kumar Lahiry]
Wood seasoning' removal of the moisture from wet or green wood, in other words, drying of wood. The moisture remains in wood either as free water in intercellular spaces or as chemically bound water in cell walls. When the moisture content is above 30%, the wood is usually termed as 'green wood'.
The quality and durability of wood are enhanced by seasoning. Adequate seasoning increases the life of wood and usually affords certain amount of protection from pests and microorganisms. Different natural and artificial seasoning methods are practiced in different countries of the world, notable among these are: air seasoning, steam drying, steam conditioning, Boulton drying, chemical drying, water seasoning, solar kiln-drying, kiln-drying, etc.
Air-seasoning method involves natural air and temperature by putting the timbers in the open or under shade for a long time. It is a traditional method and requires at least one year including one winter season for the round and sawn timbers thicker than 25 mm in climatic conditions of Bangladesh. Thin lumbers, planks, boards, fuel woods are often dried under direct sunshine for quick drying. Air-seasoning is economical for only very durable and refractory timbers, otherwise the non durable timbers are often attacked by fungi, insects, and molds and uniform seasoning is not achieved.
In steam drying the timbers or timber products are heated in a closed steam bath and the heated timbers are allowed for further air-drying.
Steam-vacuum process is more advanced process than steam drying because the heated timber in steam bath is further evacuated, as a result some moisture is evaporated out through vacuum.
Water seasoning is not really a seasoning method but an indigenous method traditionally used in Bangladesh. By this process the green timber or bamboo is kept in water for several weeks to months and then air-dried or kiln-dried for use or for preservative treatment. As a result of water treatment some reserve food materials are removed or decomposed, consequently wood becomes less attractive to insects and becomes more porous for quick drying.
In solar kiln-drying process, solar heat is arrested in a glass fabricated green house roofed by black painted tin, then the hot air inside the kiln or house in circulated by using a motor fan. Thereby the thin sawn timbers inside the kilns are dried slowly. In the past such kilns were installed in Bangladesh for drying crossarms, meter boards, planks, etc, but those are not in use now.
In kiln-drying process high temperature is generated, humidity and wind force are controlled in a heat insulated closed chamber or kiln for drying the timber. It is considered as the best method of all due to the fact that timber can be dried uniformly within short possible time.
In Bangladesh dry kilns or seasoning kilns are used for drying wood poles, anchor logs, crossarms, sawn timbers for doors, windows, cabinet, furniture, etc. Generally soft and light wood dries easily and quickly but those comparatively hard, heavy, impermeable due to the presence of deposits and tyloses (anatomical abnormality which blocks the pores) dry slowly.
Besides the Bangladesh Forest Industries Development Corporation (BFIDC), several private enterprises have kilns located at Khulna, Chittagong, Kaptai, Dhaka, Gazipur an Srimangal. [Arun Kumar Lahiry]
Pests of wood living organisms, particularly certain invertebrates causing damage to wood. Some wood boring insects infest freshly felled logs often boring deeply into the wood. The insects do not eat wood, but feed on fungus, often called ambrosia fungus, which grow in the tunnel made by the insect. This group includes the species of Platypus, Crossotarsus, (Platypodidae: Coleoptera), Xyleborus, and Webbia (Scolytidae: Coleoptera). The powderpost beetles, Heterobostrychus, Sinoxylon and Dinoderus (Bostrychidae: Coleoptera) attack dried wood. They bore only in the sapwood containing starch and reduce the wood to a fine flour-like powder. Long-horned beetles, including Hoplocerambyx spinicornis, Batocera (Cerambycidae: Coleoptera) bore in the wood, and their tunnels are usually filled with fibrous wood dust. The Carpenter bees, Xylocopa (Xylocopidae: Hymenoptera) make tunnels in the wood for making their nests. A number of species of termites attack wood on the ground, in storage depots or in use, feeding on the wood materials.
The common insect damage can be prevented by the preservative treatment of wood, residual spraying of insecticides, providing physical barriers with finishes, quick extraction and drying of logs, submersion in water, use of resistant wood, etc. The remedial measures involve fumigation with toxic gases, heat sterilization in kiln, cold sterilization in freezer, etc. Some marine arthropods and molluscs damage harbour fixures, piling, boats, and floating timber in salt water. The most notable are the gribbles, Limnoria spp. (Crustacea) and the shipworms, Teredo and Bankia (Bivalvia: Mollusca). [Md Wahid Baksha]
