Method of Seasoning of Timber

Timber can be seasoned naturally or artificially -

1. Natural or Air seasoning - The log of wood is sawn into planks of convenient sizes and stacked under a covered shed in cross-wise direction in alternate layers so as to permit free circulation of air. The

duration for drying depends upon the type of wood and the size of planks. The rate of drying is however very slow. Air seasoning reduces the moisture content of the wood to 12–15 per cent. It is used very extensively in drying ties and the large size structural timbers.



2. Artificial Timber - The prevalent methods of artificial seasoning are as follows:

Water Seasoning - The logs of wood are kept completely immersed in running stream of water, with their larger ends pointing upstream. Consequently the sap, sugar, and gum are leached out and are replaced by water. The logs are then kept out in air to dry. It is a quick process but

the elastic properties and strength of the wood are reduced.

Boiling in water or exposing the wood to the action of steam spray is a very quick but expensive process of seasoning.

Kiln Seasoning is adopted for rapid seasoning of timber on large scale to any moisture content. The scantlings are arranged for free circulation of heated air with some moisture or superheated

steam. The circulating air takes up moisture required from wood and seasons it. Two types of kilns, the progressive and the compartment are in use. For most successful kiln-seasoning the timber should be brought to as high a temperature as it will stand without injury before drying is begun otherwise the moisture in the hot outer fibers of the

wood will tend to flow towards the cooler interior. With kiln drying there is a little loss in strength of timber, usually less than 10 per cent. Also, the wood is more thoroughly and evenly dried, thus reducing the hygroscopicity of the wood.

Chemical or Salt Seasoning-" An aqueous solution of certain chemicals have lower vapour pressures than that of pure water. If the outer layers of timber are treated with such chemicals the vapour pressure will reduce and a vapour pressure gradient is setup. The interior of timber,

containing no salts, retains its original vapour pressure and, therefore, tends to dry as rapidly as if there had been no treatment. The result is to flatten the moisture gradient curves, to reduce

the slope of the curves, and consequently to reduce the internal stresses induced during drying. Since it is these stresses which are responsible for defects such as checks, etc. a chemically treated timber will exhibit fewer defects. Common salt or urea are generally used; the latter is

preferred as the corrosive action of common salt is a drawback.

Electric Seasoning - The logs are placed in such a way that their two ends touch the electrodes. Current is passed through the setup, being a bad conductor, wood resists the flow of current, generating heat in the process, which results in its drying. The drawback is that the wood may split.