Preservation of timber is carried out to increase the life of timber. Preservation is done using different types of preservatives. Methods and different materials used for preservation of timber is discussed.
Increasing life makes timber more durable and it can be used for longer periods. Preservation also helps the timber to get rid of insects and fungi etc. If preservation is not done, then wood will be diseased and damaged badly as shown in figure below.
Properties of Good Preservative for Timber
The preservative used to protect the timber should contain following requirements or properties.
- It should be effortlessly and cheaply available.
- It should not contain any harmful substances, gases etc.
- It should cover larger area with small quantity. Hence, it should be economical.
- Decorative treatment or any surface treatment should be allowed on timber after the application of preservative.
- Strength of timber should not be affected by the preservative.
- It should not contain any unpleasant smell.
- It should not get affected by light, heat, water etc.
- It should not get affected by fungi, insects etc. and should also efficient to kill them.
- It should not generate flame when contacts with fire.
- It should not corrode metals when it makes a contact with them.
- The depth of penetration of preservative in wood fibers should be minimum 6mm to 25mm.
Different Types of Preservatives for Timber
- Coal tar
- Chemical slats
- Oil paints
- Solignum paints
- Creosote oil
Coal Tar for Preservation of Timber
Coal tar is heated and obtained liquid hot tar is applied on timber surface using brush. Coal tar contains unpleasant smell and does not allow paint on it.
So, it is used for door frames, window frames etc. It is very cheap and has good fire resistance.
ASCU Preservative for Timber
ASCU is a special preservative which is available in powder form. It is dissolved in water to get preservative solution.
It should be added 6 parts by weight of ASCU in 100 parts by weight of water. The final solution is applied on timber by spraying. This solution does not contain any odor. It is useful mainly to get rid of from white ants.
ASCU contains hydrated arsenic pent oxide, copper sulphate or blue vitriol and sodium dichromate or potassium dichromate in it. After applying ASCU, the timber can be coated with paint, varnished etc.
Chemical Slats for Preservation of Timber
Chemical salts like copper sulphate, mercury chloride and zinc chloride are used as preservative which can be dissolved in water to get liquid solution. They are odorless and do not generate flames when contact with fire.
Oil Paints Preservatives for Timber
Oil paints are suitable for well-seasoned wood. They are generally applied in 2 or 3 coats. Oil paints prevents timber from moisture. If timber is not seasoned, then oil paints may lead to decay of timber by confining sap.
Solignum Paints for Preservation of Timber
Solignum paints are applied in hot condition using brush. They are well suitable for preserving timber from white ants. Solignum paints can be used by adding color pigments so, the timber has good appearance.
Creosote Oil for Preservation of Timber
Creosote oil is prepared by the distillation of tar. It is black or brown in color. It contains unpleasant smell. It is applied in a special manner.
Firstly, the timber is well seasoned and dried. Then, it is placed in airtight chamber and inside air is pumped out. Finally creosote oil is pumped into the chamber with high pressure about 0.7 to 1 N/mm2 at a temperature of 50oC.
After allowing it for 2 hours, the timber absorbs creosote oil sufficiently and taken out from the chamber.
Creosote oil is flammable so, it is not used for timber works in fireplaces. It is generally used for wood piles, poles, railway sleepers etc.
Methods of Timber Preservation
- Injecting under pressure
- Dipping and stepping
- Hot and cold open tank treatment
Brushing of Timber Preservatives
Brushings the simplest method of applying preservatives. For well-seasoned timber, oil type preservatives are applied with good quality brushes.
For better results, the applied preservative should in hot condition. Multiple coats should be applied and certain time interval should be maintained between successive coats.
Spraying of Timber Preservatives
Spraying is an effective technique than brushing. In this case, preservative solution is sprayed on to the surface using spray gun. It is time saving and quite effective.
Preservative Injecting Under Pressure
The preservative is injected into the timber under high pressure conditions. Generally, creosote oil is applied in this manner which is already discussed above. It is costly treatment process and required special treatment plant.
Dipping and Stepping Method of Timber Preservation
Dipping is another type of preserving in which, timber is dipped directly in the preservative solution. Hence, the solution penetrates the timber better than the case of brushing or spraying.
In Some case, the stepping or wetting of timber with preservative solution is allowed for few days or weeks which is also quite effective process.
Charring Method of Timber Preservation
Charring is nothing but burning of timber surface, which is quite an old method of preservation of timber.
In this method, the timber surface is wetted for 30 minutes and burnt up to a depth of 15mm from top surface. The burnt surface protects the inner timber from white ants, fungi, etc.
This method is not suitable for exterior wood works so, it is applied for wood fencing poles, telephone pole bottoms etc.
Hot and Cold Open Tank Treatment of Timber
In this method, the timber is placed in an open tank which contains preservative solution. This solution is then heated for few hours at 85 to 95 degree Celsius. Then, the solution is allowed to cool and timber gets submerged with this gradual cooling. This type of treatment is generally done for sap wood.