Timber Preservation

Considering the numerous challenges faced in securing a safe and desired piece of timber, preservation techniques come in handy. Preservation is the positioning of the food supply in the timber (especially in the sapwood) by applying a toxic liquid to it. This is intended to prevent the fungi and insects from attacking the timber. The ideal requirements for a timber preservative include the following;

·         It must be toxic to the fungi and insects but safe to animals and humans

·         It should be permanent and not be bleached out by sunshine or leached out by rain

·         It should be economical and easy to obtain

·         It should not corrode or affect metal in any way

·         It should be easy to handle and apply

·         It should as far as possible be odourless

·         It shouldn’t affect the subsequent finishing of timber e.g. painting and polishing

·         It should be non-flammable

Whereas, these are the ideal requirements of a preservative, it’s important to note that most preservatives will not embody all these mentioned above.

There are 3 main types of preservatives available and these are;

1. Tar oils; these are derived from coal and are dark brown or black in colour. They are fairly permanent, cheap, effective and easy to apply (this is the commonest in Uganda’s Construction Industry). However, they shouldn’t be used internally as they are inflammable and possess a strong lingering odour. They should never be used around food stuffs, as the odour will contaminate them. The timber once treated will not accept any further finish e.g. paints. Its main uses are for the treatment of external timber such as: fences, sheds, telegraph poles etc.

2. Water-soluble preservatives; these are toxic chemicals mixed with water and are suitable for use in both internal and external situations. The wood can be painted subsequently and are odourless and non-flammable.

3. Organic solvent preservatives; these consist of toxic chemicals mixed with a spirit that evaporates after the preservative has been applied. They have similar characteristics and area of usage as water soluble preservatives. However, some of the solvents used are inflammable and some have strong odour. They are the most expensive type but considered to be superior because of their excellent preservative properties.

Methods of application

To a greater extent, it’s been revealed by a number of Research experts that it’s the method of application rather than the preservative that governs the degree of protection obtained. And this is because each application method gives a different depth of preservative penetration and the greater the depth, the higher the degree of protection.

Methods of application are classified into 2 groups and these are;

Non-pressure treatments

· Brushing. The preservative is just brushed on; giving very limited penetration of the preservative into the timber.

· Spraying. The preservative is sprayed on; giving a very limited penetration of the preservative into the timber.

· Dipping. Timber is immersed in a container full of preservative and after a certain length of time; the timber is taken out and allowed to drain. Depth of penetration is dependent on the length of time the timber is immersed.

· Steeping. Known as hot and cold method. Timber is immersed in large tanks containing preservative which is heated for about 2hours then allowed to cool. During the heating, air in the timber cells expands and escapes as bubbles to surface and on cooling; preservative is sucked into the spaces left by the air. This is the best non-pressure method by far because a fairly good penetration is achieved.

Pressure treatments

This is the most effective method as full penetration of cells is achieved.

·Empty-cell process. The timber is placed in a sealed cylinder where the air is subjected to pressure which causes the air in the timber cells to compress. From there on; the preservative is run into the cylinder and the pressure increased further forcing the preservative into the cylinder.

The pressure is maintained at this high level until desired amount of penetration is achieved. Pressure is then released and surplus preservative is pumped back into a storage container leaving only the cell walls coated.

·Full-cell process. Timber is placed into a sealed cylinder as before described only that this time, the air is drawn out instead of compressing it; creating a vacuum in the cylinder as well as a partial vacuum in the timber cells.

At this stage, the preservative is introduced into the cylinder and when full, the vacuum is released and the preservative is sucked into the timber cells by the partial vacuum. This method is ideal for timbers used in wetlocations e.g. marine tanks, docks, ports, piers, jetties etc. as the water cannot penetrate into the timber cells  because they are already full of preservative.

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