Because the sapwood of trees contain water, food, salts, air and warmth, wood is often attacked by fungi, termites, insects and other pestiferous organisms. Since woods usually contain varying amounts of these substances, the rate of attack differs with wood type. This attack lowers the quality of wood and consequently reduces its service life. In order to preserve the service life of wood, there is therefore a need to protect wood against an attack. Usually, this is done through any suitable method of wood preservation.
Wood preservation is the process of applying certain substances to wood in order to protect it against attack by destructive organisms and therefore prolong its service life.
Types of Timber Preservative
A substance which is applied to wood in order to prevent its deterioration and hence lengthen its service life is called a wood preservative. A wood preservative works essentially by making the food material in wood poisonous to the destructive agents.
The three principal types of preservatives are:
- Solvent Chemicals: The solvent type chemicals need to be dissolved in liquids other than water so as to form solutions, which can then be applied on wood.
- Water borne Chemicals: The water borne chemicals are dissolved in water to form the required solutions.
- Tar oils: A common example of tar oil is creosote. Creosote is black and it is often used to paint wooden electric and telephone poles.
Properties of a Good Timber Preservative
The following characteristics are desired in a good wood preservative.
- It must be capable of rendering the wood completely poisonous to destructive organisms, which feed on wood.
- In normal use, it should be incapable of inflicting injury on human beings, domestic animals or pets.
- It must not have any odour.
- It must be able to remain in the wood for a very long period after it has been applied.
- It should be able to maintain its chemical composition for a long period.
- It should be capable of being simply and easily applied on wood.
- It is also desirable that a wood preservative be capable of good penetration into the wood.
- Also, it should be easily obtainable and not very expensive.
Wood Preservation Methods
Wood preservatives are applied through any of the following methods:
- Pressure impregnation: Pressure impregnation involves forcing the preservative into the wood, which is placed in a specially designed pressure cylinder.
- Steeping: In the steeping method, a container is filled with cold preservative. Wood is then wholly, or partially (as in the case of poles) immersed in it. The period the wood spends in the preservative depends on the thickness of the wooden piece but normally it ranges from some hours to some weeks.
- Dipping: In the dipping method, the time the wood spends in the preservative is usually short lasting a few minutes at the most. Apart from this time difference, dipping is not much different from steeping.
- Hot and cold tank: Wood is submerged in a tank filled with preservative. The tank is then heated for some hours and then allowed to cool. In the process of cooling, atmospheric pressure forces the preservative into the wood.
- Spraying and brushing: Using this method, preservative is applied on the surface of wood through the use of a suitable spraying device or brush. This does not ensure penetration of the wood by the preservative. However it is the easiest method of applying wood preservative.
Pressure impregnation, hot and cold tank treatment and steeping are the most effective methods. Dipping is not much effective while spraying and brushing are the least effective methods.