The method of preservative treatment to be used depends on the use of wood in construction. The general methods adopted are as follows:
1. Charring
In this method, the ends of the timber are burnt to about 1.5 cm and then quenched in water. This charred end act as a protective coat. This is particularly done to prevent dry rot and attacks by insects.
2. Tarring
This is applying a coat of tar or tar mixed with pitch. It is generally applied to rough types of works such as ends of door and window frames that are to be fixed to the walls, timber fences, etc.
3. Creosoting
Creosote oil is one which has wood preservative oil in it. The timber to be treated is placed in cylinders and closed tightly. Creosote oil under a pressure of 9 bars and at a temperature of 50°C is pumped into the cylinder until the desired absorption is reached. By this treatment the timber is preserved against rot and attack by white ants.
4. Treatment Using Preservatives
Some of the methods where preservatives are directly used are as follows:
(i) Surface Application Method
Surface Application Method is also known as the brush and spray method. Solvents or emulsions are used for this purpose. The timber to be treated is brushed or sprayed. At least two coats should be applied. This method is adopted only when naturally resistant timbers are used.
(ii) Soaking Treatment
Soaking Treatment is also known as steeping. In this treatment the timber is submerged in the preservative solution till the required absorption is obtained. Here, a water-soluble type preservative is used to avoid excessive evaporation loss. This treatment is recommended for timbers containing a lot of sapwood and for light and medium density timbers.
(iii) Hot and Cold Treatment
Hot and Cold Treatment is one in which the timber is placed in a steel tank immersed in a preservative (creosote) and heated to about 85–90°C. Then the tank is cooled slowly after some time with the timber immersed in the solution. This treatment is adopted for timbers containing sapwood and easily treatable heartwood.
(iv) Pressure and Vacuum Treatment
Pressure and Vacuum Treatment is of two types, viz., full cell process and empty cell process.
In the full cell process, the timber to be treated is placed in a closed vessel and is subjected to a low vacuum for about an hour. During this process, a preheated preservative is introduced till the vessel is full. A pressure up to 14 kg/sq.cm is gradually applied and kept constant for several hours. The pressure is then reduced and the solution is drained out. Timbers for marine works, railway sleepers, etc. are treated by this process.
In the empty cell process the quantity of preservative required is less and hence cheaper than the full cell process. The timber to be treated is stacked in a cylinder. After closing the door, the timber in the cylinder is then subjected to an air pressure of 1.75–5 bars from half an hour to one hour. The pressure is maintained while the cylinder is filled with preservatives. When the cylinder is completely filled, a pressure of 5–12 bars is applied and this pressure is maintained till the required absorption has taken place. The cylinder is drained off and vacuum is applied to remove the preservative dripping from the timber. This method is used for preserving timber of mixed species and timber containing sapwood and heartwood.
5. Treatment by Diffusion
This treatment is carried out for timber which has moisture content of over 50%, i.e., for green timber, which is felled recently. Extremely soluble boron compounds are applied to the surface of the timber. The timber pieces are stacked and covered with an impermeable cover to prevent evaporation. After a period of 1–2 months, the boron diffuses into the wet timber and acts as a preservative.
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