CONSTRUCTION MATERIAL
TIMBER
DIFFERENT TYPES OF PRESERVATIVES FOR TIMBER
1. COAL TAR
The timber surface is coated with hot coal tar with the help of brush. The coal tar becomes workable when heated. The process is known as the tarring. The coal tar has unpleasant smell and appearance. It makes timber unsuitable for painting. Hence the tarring is adopted for frames of doors and windows, rough timber work etc. and it is found to be most useful for parts embedded in ground because of its cheapness and effective resistance. The coal tar is fire resistant.
2. ASCU
The ascu is special preservative which is developed at the Forest Research Institute Dehradun. Its composition is as follows-
(a). Part by weight of hydrated arsenic pentoxide, (As2O5, 2H2O)
(b). Part by weight of blue vitriol or copper sulphate, (CuSO4, 5H2O)
(c). Part by weight of potassium dichromate, (K2Cr2O7) or sodium dichromate (Na2Cr2O7, 2H2O)
This material is available in powder form. To prepare a solution of this material, six parts by weight of ascu are mixed in 100 parts by weight of water. The solution is then sprayed or applied on timber surface. This preservative gives timber protection against the attack of white ants. The surface treated with this preservative can be painted, polished, varnished or waxed. The solution is odourless.
(a). Part by weight of hydrated arsenic pentoxide, (As2O5, 2H2O)
(b). Part by weight of blue vitriol or copper sulphate, (CuSO4, 5H2O)
(c). Part by weight of potassium dichromate, (K2Cr2O7) or sodium dichromate (Na2Cr2O7, 2H2O)
This material is available in powder form. To prepare a solution of this material, six parts by weight of ascu are mixed in 100 parts by weight of water. The solution is then sprayed or applied on timber surface. This preservative gives timber protection against the attack of white ants. The surface treated with this preservative can be painted, polished, varnished or waxed. The solution is odourless.
3. CHEMICAL SLATS
These are water borne preservatives and they are mostly salts dissolved in water. The usual salts used are copper sulphate, mercury chloride, sodium fluoride and zinc chloride. The solutions are prepared from these salts and they are applied on the timber surface can be painted or varnished after drying. These preservatives have good penetration and the timbers treated with these preservatives will show an immediate increase in weight of 2400 to 4800 N per m3. After drying the net increase in weight will come down to about 50 to 300 N per m3.
4. OIL PAINTS
The timber surface is coated with 2 or 3 coats of oil paint. The wood should be seasoned. Otherwise sap will be confined and it will lead to the decay of timber. The oil paints preserve timber from moisture and make it durable.
5. SOLIGNUM PAINTS
These paints preserve timber from white ants as they are highly toxic in nature. They can be mixed with colour pigments and applied in hot state with the help of brush. The timber surface may therefore be given the desired colour or appearance.
6. CREOSOTE OIL
In this case the timber surface is coated with creosote oil. The process is known as the creosoting or Bethel's method of preservation of timber. The creosote oil is obtained by the distillation of tar. The creosoting is carried out as follows-
(a). The timber is thoroughly seasoned and dried.
(b). It is then placed in an air tight chamber.
(c). The air is pumped out from the chamber.
(d). The creosote oil is then pumped under a high pressure off about 0.70 to 1 N/mm2 and a temperature of about 50*C.
(e). After a period of about 1 to 2 hours when timber has sufficient absorbed creosote oil it is taken out of chamber.
The creosote oil is one of the best antiseptic substance poisonous for wood attacking fungi. It is a black or brown liquid weakly affected by water neither volatile nor hygroscopic, harmless to wood or metal, inflammable, with an unpleasant odour and having low wood penetrating ability to the extent of 1 mm to 2 mm only.
The creosoting practically doubles the life of timber and it is generally adopted for piles, poles, railway sleepers, etc. Depending upon the net retention and type of timber the creosote treated timber will normally increase in weight by 800 to 3200 N per m3. The creosote oil is highly toxic in nature and gives out highly unpleasant smell. The process of creosoting proves to be costly. The creosote oil should not be used for interior surface of dwelling houses, foodstuff storage premises, in underground installation and near inflammable surface.
In this case the timber surface is coated with creosote oil. The process is known as the creosoting or Bethel's method of preservation of timber. The creosote oil is obtained by the distillation of tar. The creosoting is carried out as follows-
(a). The timber is thoroughly seasoned and dried.
(b). It is then placed in an air tight chamber.
(c). The air is pumped out from the chamber.
(d). The creosote oil is then pumped under a high pressure off about 0.70 to 1 N/mm2 and a temperature of about 50*C.
(e). After a period of about 1 to 2 hours when timber has sufficient absorbed creosote oil it is taken out of chamber.
The creosote oil is one of the best antiseptic substance poisonous for wood attacking fungi. It is a black or brown liquid weakly affected by water neither volatile nor hygroscopic, harmless to wood or metal, inflammable, with an unpleasant odour and having low wood penetrating ability to the extent of 1 mm to 2 mm only.The creosoting practically doubles the life of timber and it is generally adopted for piles, poles, railway sleepers, etc. Depending upon the net retention and type of timber the creosote treated timber will normally increase in weight by 800 to 3200 N per m3. The creosote oil is highly toxic in nature and gives out highly unpleasant smell. The process of creosoting proves to be costly. The creosote oil should not be used for interior surface of dwelling houses, foodstuff storage premises, in underground installation and near inflammable surface.
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