Preservation are used to protect timber against deterioration due to fungi and attacks of termites, borers, and marine organisms. Most efficient means of preserving timber are, good seasoning and free circulation of air. Protection against moisture is afforded by oil-paint provided that the timber is perfectly dry when first painted otherwise the filling up of the outer pores only confines the moisture and causes rot. A pre-requisite for satisfactory treatment is that the timber shall be seasoned so that the timber shall be seasoned so that the outer layers have a moisture content of less than 20 percent. For exposed timber the only remedy at present available is impregnation by substances poisonous to fungi, these substances being either of the oil or chemical types.



 Description of Preservatives

Oil type preservatives

Coal Tar Creosote is a fraction of coal tar distillate and is the most important preservatives of the oil type having been in use for a very long time and is specially suitable for the treatment of timber for exterior use, e.g. railway sleepers, poles, piles, etc. Although creosote has been fairly satisfactory but after sometimes in exposed situation, it tends to leach out, and has an unpleasant odour; is not clean to handle and the timber treated with it cannot be painted or polished. Surface application of creosote has little, if any, value, and for satisfactory protection a deep impregnation of the preservative must be obtained.

Coal tar is a good preservative (but not so effective as the creosote derived from it) and is more suitable for surface applications. It is less toxic to wood destroying agencies and being very viscous does not penetrate the wood deeply. It should be applied hot. Coal tar is sometimes mixed with ordinary creosote. All timbers embedded in masonry or in contact with masonry should be well tarred before erection with three coasts of hot coal tar into which quick lime powder has been thoroughly incorporated in the proportion of 1 kg of lime to 5 litres of tar. Framed joints must be coated with paint before frames are put together. When the end of a beam or any woodwork is buried in masonry or brickwork, an air space of 6 mm should be left at the ends, sides and top.


Chemical Type Organic Solvent Preservatives

These preservatives are used after dissolving them in suitable organic solvents such as, naptha, kerosene and white spirit. They are clean to handle and are more or less permanent but some of them are inflammable and care is necessary in handling the solution. These preservatives are applied cold and in most cases the timber treated with them can be painted or polished. DDT is an example.



 Water Soluble Preservatives

They are comparatively cheaper and the timber treated with these preservatives can be painted or varnished when dry. The (and washed out, being water soluble) when the timber is exposed to wet conditions (i.e. the preservative gets gradually depleted owing to the dissolving effect of water), though more recent preservatives employ acidified solutions which after impregnation deposit the chemical in the wood as an insoluble salt. This leaching of the preservatives can be to some extent, minimized if a water proof paint coating is applied on the treated timber and is properly maintained. Most of these chemicals are suitable for inside locations only for protecting timber not in contact with ground, and are not suitable for works underground and severe conditions of exposure.

“Ascu” has been quite successful. It is in powder form and 1 part of the powder dissolved in 16 parts of water (by weight) gives a solution for ordinary use. The solution can be applied with a brush or the wood soaked into it. The treated wood can be painted or polished. Zinc chloride has some fire-retardant properties also.
The most common method of applying a preservative is by brush, but this gives only limited protection. Better results are obtained with a hot solution or by spraying, but dipping or steeping is much more effective. Still better results can be obtained by using what is known as the open hot tank and cold process. impregnating timber by applying the preservative under pressure is the most effective method.

Timber to be treated should be dried to an appropriate moisture content and whenever possible all work on the timber should be completed and it should be fully fabricated and all cuttings and drilling done before preservative treatment is applied. Where subsequent cutting or working is unavoidable, preservative should be liberally applied to the freshly worked surfaces.



 Fire-Proofing Timber

For fire-proofing timber, the method recommended is the pressure impregnation of the timber with large quantities of chemicals, the most common of which are ammonium disphoshate, sodium aresenate, sodium tetraborate. Fire-resistant paints are also available. White washing is effective to some extent in retarding the action of fire. It is not possible to make timber fire-proof, chemicals and paints only retard the action of fire. Timber can be rendered non-inflammable in that it will not flame or glow but merely char, and will not, therefore, assist in the propagation of fire. A dense wood offers greater resistance to fire than a lighter one. Presence of resins and oils in wood increase combustibility. No wood work of any kind should be laid within 60 cm of a fire-place or a flue.


All portions of timber abutting against masonry or concrete or embedded in ground shall be painted with a wood primer or two coats of boiling tar.