Seasoning of timber is the process by which moisture content in the timber is reduced to required level. By reducing moisture content, the strength, elasticity and durability properties are developed. A well-seasoned timber has 12- 15% moisture content in it. The purpose of seasoning is to reduce the moisture content in the wood to a level appropriate to the use of the articles made from the wood.
Reasons for Seasoning
- To prevent shrinkage, splitting, checking and warping.
- To achieve greater stiffness and strength.
- To allow penetration of preservatives
- To obtain a surface that will accept paint, polish or glue.
- To protect against decay.
Moisture Content of Timber
- Water is stored in wood in two main forms:
- As free water in the vessels and/or cells, used to move nutrients within the tree.
- As cell (or bound) water, which is an integral part of the cell walls
Initial seasoning first removes the free water contained from within the cells to a moisture content of 25 to 35 percent. The cell walls still contain moisture. This state is known as the fibre saturation point. With further seasoning the combined moisture within the walls of the cells begins to dry out and the cells and timber begin to shrink, so this part of the process must be carried out with careful control over drying rate.The timber continues to dry out until it is brought to the moisture content required usually about 12-15%.
Methods of Seasoning of Timber
There are two methods of Seasoning of timber which are explained below
1. Natural seasoning
2. Artificial seasoning
Natural Seasoning of Timber
Natural seasoning is the process in which timber is seasoned by subjecting it to the natural elements such as air or water. Natural seasoning may be water seasoning or air seasoning.
Water Seasoning
Water seasoning is the process in which timber is immersed in water flow which helps to remove the sap present in the timber. It will take 2 to 4 weeks of time and after that the timber is allowed to dry.
Air Seasoning
In the process of air seasoning timber logs are arranged in layers in a shed. The arrangement is done by maintaining some gap with the ground. So, platform is built on ground at 300mm height from ground. The logs are arranged in such a way that air is circulated freely between logs. By the movement of air, the moisture content in timber slowly reduces and seasoning occurs. Even though it is a slow process it will produce well-seasoned timber.
Artificial Seasoning of Timber
Natural seasoning gives good results but takes more time. So, artificial seasoning of timber is developed nowadays. By artificial seasoning, timber is seasoned with in 5-6 days. Here also different methods of artificial seasonHeading 3ing are there and they are as follows.
- Seasoning by Boiling
- Chemical seasoning
- Kiln seasoning
- Electrical seasoning
Seasoning by Boiling
Seasoning of timber is also achieved by boiling it in water for 3 to 4 hours. After boiling timber is allowed to drying. For large quantity of timber boiling is difficult so, sometimes hot steam is passed through timber logs in enclosed room. It also gives good results. The boiling or steaming process develops the strength and elasticity of timber but economically it is of heavier cost.
Chemical Seasoning
In case of chemical seasoning, timber is stored in suitable salt solution for some time. The salt solution used has the tendency to absorb water from the timber. So, the moisture content is removed and then timber is allowed to drying. It affects the strength of the timber.
Kiln Seasoning
In this method timber is subjected to hot air in air tight chamber. The hot air circulates in between the timber logs and reduces the moisture content. The temperature inside the chamber is raised with the help of heating coils. When the required temperature is obtained moisture content and relative humidity gets reduced and timber gets seasoned. Even though it is costly process it will give good results strength wise.
Advantages of Kiln Drying
(a) Kiln drying permits drying to the exact M.C. required, which may be much less than that of air dried timber.
(b) The carefully controlled conditions can minimize degrade and seasoning defects which occur while drying.
(c) The heating of the timber while in the kiln kills eggs, larvae and adult borers which may be present in the wood.
(d) Although capital costs may be higher, greater turnover is possible by reductions in drying time, and less timber yard storage space required.
AUTHOR:
S. C. Sahoo, Scientist, Indian Plywood Industries Research &Training Institute, (IPIRTI)