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  • Writer's pictureBenny Kuriakose

What Are the Methods of Timber Treatment?

All timber pests are fundamentally nature’s forest scavengers. Once a tree is felled, whether by age or by axe, the timber, from a natural standpoint is, “dead” and ready for destruction. Most timber preservatives are essentially fungicides and need the addition of an insecticide to make them suitable for use against termites and other insects. Timber should be adequately seasoned (dried) before treatment with a preservative. Pressure/vacuum impregnation is the most effective method of application. Timbers vary in their susceptibility to attack by insects and fungi with some being naturally long-lasting. Others are less durable but it is important to note that all timber that is kept dry (i.e. below a moisture content of 20%) will not sustain a fungal attack.

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There are three main types of timber preservatives: tar oils, waterborne and organic solvent-borne. Waterborne and organic solvent types are most widely used for the protection of building timbers. Their primary function is to penetrate and be retained usually in a ‘shell’ of the outer few millimetres of timber, to guard against stain, decay, mould growth and insect attack. The preservative treatment for timber is to be decided based on its end use i.e., – whether for external or internal works. Many of the water soluble and organic solvent preservatives used in India such as durspan, diazinon, termex etc. are banned in many developed countries. This article explains the different timber treatment methods in detail.

Types of Timber preservatives:

  • Tar Oil Preservatives

Creosote, a by-product of coal, is the most commonly used tar oil. Because of its smell and dark colour, and inability to take any painted finish, it is generally only suitable for external purposes such as sleepers, fence posts, piers and transmission poles. It reduces moisture absorption and offers very effective protection against fungal and insect attack.

  • Water-Borne Preservatives

The two types of water-borne preservatives used, boron compounds for green timber and combinations of copper, chromium and arsenic (CCA) for seasoned timber are both water-soluble. Most water-borne preservatives are applied by vacuum/pressure impregnation. After impregnation, the more commonly used copper-chromium-arsenic (CCA) become fixed and insoluble in the wood, resulting in high durability. They have high resistance to attack by fungi and insects. The preservative gives light coloured wood a light grey/green hue and is liable to cause some slight swelling. They are usually odourless and can be decorated over.

CCAs are relatively safe. The chemicals are fixed to the wood fibres during the treatment process so that they will not leach out. But the criticism over the chemical arsenic in the past few years has led to its ban in countries like the United states and thus, prompted the development of alternatives.

Boron compounds are effective against timber rotting fungi and termites. However, it is effective only against green timber. This is because, other kinds of timber have low moisture content and thus, cannot easily diffuse the preservative. Since the preservative is not chemically fixed within the timber, boron treated timber is not used for outdoor use where it is exposed to prolonged rain.

  • Organic Solvent Type Preservatives

Organic solvent preservatives can be applied by vacuum/pressure impregnation treatments. They do not cause any change in the moisture content or the dimension of the timber. They have good penetration and they are very effective against fungal attacks. They are non-corrosive and most types can be painted over when the solvent has evaporated. They contain ingredients of low water solubility that are dissolved in volatile solvents such as white spirit or solvent naphtha that evaporate, leaving the fungicide and insecticide in the timber.

Application Of Preservatives:

Various methods can be used for the application of preservatives. For any chemical protection, a suitable concentration of the appropriate preservative must be applied. The use of a wrong type of preservative or a low concentration will lead to failure and mistrust. If treated timber is to be used in the open, then non-leachable preservatives should be used.

There are many traditional timber treatments used in India. One such is the application of cashew shell oil, which has a palliative effect. Another method is to apply saturated saltwater on timber. For the timber idols in temples, special preparation for treatment is done using various vegetable additives.

  • Brushing And Spraying

Brushing and spraying of timber are rarely carried out as a preservative treatment. These methods have only a temporary effect, because of the very low penetration of the preservatives. The zone of timber can be easily broken by cutting, drilling or abrasion.

  • Open–Tank Method

A simple method that has a good outcome and is economical is the open-tank method. Timber cut to required sizes is soaked in a solution of water-soluble preservatives for a sufficient period of time. The solution gets into the timber by diffusion through the ends and sides. The duration of treatment depends mainly on the kind of preservative chosen, the timber species, and the condition of timber. The size of the tank is determined by the length of the piece to be treated. Application of enamel paint is not a preservative treatment. Its only advantage is that it repels water.

  • Pressure Treatment

Pressure treatment of timber is carried out either with water based preservatives or with creosote. It is expensive but efficient. The material must be air dried to ensure the necessary penetration, for the pressure treatment. The treatment is done in a pressure plant equipped with pressure cylinders, gauges, storage tanks, vacuum and pressure pumps. The preservative is forced into the timber by means of the pressurized cylinder. Pressure treatment creates a shell of protection around a piece of lumber – the outermost half inch or so is infused with the preservative, but the center of has little or none. For this reason, if you cut a piece of pressure treated wood, you should apply timber preservatives to the surfaces exposed by the timber.


Untreated timber deteriorates quickly when exposed outdoors. In their natural unfinished state, the timber is more prone to fungal and insect attacks. This is why timber preservation or treatment becomes a vital part in construction. One should carefully choose from the different types of timber treatments available, according to the purpose intended for the wood. Checking the quality of timber after the treatment is also essential during construction to ensure that it is safe from rots and insect attacks.


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