When a tree is newly felled, it contains about 50% of its own dry weight as water. This water is in the form of sap and moisture. The water is to be removed before the timber can be used for any engineering purpose. In other words, the timber is to be dried. This process of drying is known as the seasoning of timber and the moisture should be extracted during seasoning under controlled conditions as nearly as possible at a uniform rate from all parts of the timber.

Objectives of seasoning are as follows: i. To allow timber to burn readily, if used as fuel. ii. To decrease the weight of timber and thereby to lower the cost of transportation and handling. iii. To impart hardness, stiffness, strength and better electrical resistance to timber. iv. To increase the resisting power of timber, as most of the causes of timber are more or less related to moisture. v. To make timber easily workable and to facilitate operations during conversions. vi. To maintain the shape and sized of the components of the timber articles which are expected to remain unchanged in form. vii. To make the timber safe from attack of fungi and insects. viii. To reduce the tendency of timber to crack, shrink and warp.

Methods of seasoning:

The methods of seasoning can be broadly be divided into the following two categories:

A. Natural seasoning:

In this method, the seasoning of timber is carried out by natural air and hence it is also sometimes referred to as air seasoning.

Following procedure is adopted in the air seasoning:

i. The timber in log form is not usually fit for the process of seasoning. Hence it is cut and sawn into suitable sections of plank or scantlings.

ii. The timber pieces can be either stacked horizontally or vertically.

iii. The ground, where stack is to be constructed, is cleared and it is levelled for good drainage.

iv. The platform of stack is made slightly higher, about 300mm, than the ground level. For this purpose, the rows of bricks or concrete pillars are constructed. The pillars may also be made of creosoted wood or wood coated with coal tar. The tops of pillars should be in the same horizontal plane. The pillars should be durable.

v. The timber pieces are sorted out according to lengths and thicknesses. They are then arranged in layers, one above the other. The care should be taken to see that all members in particular layer are of the same thickness. If this precaution is not taken, there are chances for timber to become warped or cracked.

vi. Each layer is separated by spacers of sound dry wood. The usual dimensions of spacers vary from 35mm x 25mm to 50mm x 35mm, the larger dimension being the width. The spacers are to be carefully placed in correct vertical alignment. vii. The distance between spacers depends on the sizes of timber members to be seasoned. It is less for thin sections and more for thick sections. It usually varies form 450mm to 600mm.

viii. The length of stack is equal to length of timber pieces. The width and height of stack are restricted to about 1500mm and 3000mm respectively. A distance of about 25mm is kept between adjacent layers.

ix. The stack is to be protected from fast blowing wind, rain and extreme heat of sun. hence the stack should preferably be covered by a roof of suitable material.

x. Similar stacks may be constructed. The minimum distance between adjacent stacks should be at least 600mm.

B. Artificial Seasoning

Following are the reasons for adopting the artificial seasoning over the natural seasoning:

i. The defects such as shrinkage, cracking and warping are minimized.

ii. The drying is controlled and there are practically no chances for the attack of fungi and insects.

iii. The drying of different surfaces is even and uniform.

iv. It considerably reduces the period of seasoning.

The various methods of artificial seasoning are as follows:

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

b. 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.

c. Kiln 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.

d. Chemical Seasoning: This is also known as the salt seasoning. In this method, the timber is immersed in a solution of suitable salt. It is then taken out and seasoned in the ordinary way. The interior surface of timber dries in advance of exterior one and chances of formation of external cracks are reduced.

e. Electrical 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.