Seasoning is the process of reducing the moisture content by drying under controlled conditions a nearly as possible at uniform rate from all parts of timber in order to prevent the timber from posse fermentation and making suitable for use.
A well-seasoned piece of timber contains moisture content in the range of 10-12%.
The objectives of Timber seasoning are as follows :
(i) Reduce the shrinkage and warping after placement in structure.
(ii) Increase strength, durability, resilience and workability and dimensional stability.
(iii) Reduce its tendency to split and decay.
(iv) Make it suitable for painting.
(v) Reduce its weight.
(vi) Make timber burn readily if used as fuel.
(vii) Make it suitable for glueing.
Methods of Seasoning :
(a) Natural seasoning
It is very cheap and simple method in which natural air is used to remove moisture.The basic principle is to stack the timber so that plenty of air can circulate around each piece of timber.
The timber is stacked with wide spaces between each piece horizontally, and with strips of wood between each layer ensuring that there is a vertical separation too.Air can then circulate around and through the stack, to slowly remove moisture.
In some cases, weight can be placed on top of the stacks to prevent warping of the timber as it dries .
Overhead cover to protect from effects of direct sunlight and driving weather has to be provided.
It is a very slow process and drying of different phases may not be uniform.
(b) Artificial seasoning
In this method timber is seasoned in a chamber with regulated heat, controlled humidity and proper air circulation. Therefore, specific conditions for different species can be maintained.
Methods of artifical seasoning
(a) Kiln Seasoning
Kiln is an airtight chamber. The process can be summarized as :
(i) Timber to be seasoned is placed inside Kiln, then fully saturated air with a temperature 35°C to 38°C is forced inside it.
(ii) The heat gradually reaches inside timber, then relative humidity is gradually reduced and temperature is increased, and maintained till desired degree of moisture content is achieved.
(iii) The kiln used may be stationary or progressive.
In progressive kiln the carriages carrying timber travel from one end of kiln to other end gradually, The hot air is supplied from the discharging end so that temperature increase in gradual from charging end to discharging end. This method is used for seasoning on a larger scale.
(b) Electrical Seasoning
In this method, high frequency alternating currents are used. The timber when green offers little resistance to flow of current, but the resistance increases as the timber dries internally and leads to production of heat.
It is the most rapid method of seasoning. Due to uniform rise in temperature and consequently uniform evaporation of moisture, results in uniform quality of timber. However, the capital and running costs are very high. So it is uneconomical to season timber on commercial basis
The method consists of immersing the timber completely in water and then boiling it for 3-4 hours, then timber is taken out and allowed to dry very slowly under a shed or using steam. The method is very quick and causes lesser shrinkage, but it reduces the strength and elasticity of timber.
The procedure is expensive and cumbersome and cannot be adopted on a large scale.
(d) Chemical Seasoning (Salt Seasoning)
In this method, the timber is immersed in a solution of suitable salt eg. Urea, Sodium nitrate or sodium chloride (dehydrating agents) and then taken out and air dried. Interior surface of timber dries in advance of exterior one and chances of formation of external cracks are reduced.
(e) Water Seasoning
Timber is completely immersed in running stream of water, with their larger ends pointing upstream. Therefore the sap, sugar, and gum are leached out and are replaced by water. Then the timber/wood is air dried.
It is a quick process but the elasticity and strength are reduced.