SEASONING

Trees felled recently will contain large amounts of sap and moisture. Seasoning is the process of removing all the sap and moisture content under controlled conditions to avoid any splitting and distortion in the wood.

Objectives of Seasoning

Although the primary aim is to remove sap and moisture, the other objectives of seasoning are as follows:

1. As most causes of decay and other problems are more or less related to moisture, the main objective is to remove moisture thereby increasing the resisting power of timber.
2. To impart hardness, stiffness and strength, which improve the adoptability of timber for many building purposes.
3. To improve the resisting power of timber against electrical power and attack by fungi and insects.
4. To make the timber easily workable.
5. To totally remove the possibility of shrinking, warping and splitting.
6. To maintain the size and shape of the components of the timber structure of articles that are expected to be unchanged during the life span.
7. To make timber suitable for treating with paints, polishes, preservatives, etc.
8. To make the timber suitable for effectively joining with the use of glues.
9. To reduce the weight of the timber for easy handling and transportation to other places.
10. To give high order of durability for more years after construction.

Methods of Seasoning

There are two broad approaches to seasoning, viz.,

1. Natural seasoning
2. Artificial seasoning

1. Natural Seasoning

There are two methods of natural seasoning, viz., air seasoning and water seasoning.

(i) Air Seasoning

It is also known as air drying. In this method timbers intended for seasoning are stacked in a dry place under cover about 30 cm above the floor level. Here the longitudinal and cross pieces are arranged one upon the other leaving space for free circulation of air (Fig. 10.9). It is better to provide a foundation to keep the timber clean off the ground. Wood seasoned by such a process can be used for carpentry work after two years.

Figure 10.9 Typical air seasoning of timber

(ii) Water Seasoning

This method of seasoning timber is to completely immerse the logs of wood in water soon after cutting. This is better done in a running stream of water with the longer ends of the logs being kept pointing upstream. By this arrangement the sap, sugar, gum, etc. are leached out of the logs and in turn replaced by water. After adequate soaking the logs are dried in an open place. Although it is a quick process, it reduces the durability of the timber.

2. Artificial Seasoning

Artificial seasoning may be done by kiln seasoning, chemical seasoning and electrical seasoning. By artificial seasoning the moisture content can be brought under control from 4% to 12%. This is a quick process.

(i) Kiln Seasoning

In this system, complete control of temperature and humidity is maintained with proper air circulation and ventilation system. The timber is kept inside an airtight chamber. As a first step fully saturated air at 35–38°C is circulated. Then the humidity is reduced slowly followed by an increase of temperature till the moisture in the timber is reduced to the degree of moisture required. Then the kiln is slowly cooled. This process takes about a fortnight during which the timber is kept on a trolley. The quality of wood obtained by this method is inferior to those seasoned by natural methods.

(ii) Chemical Seasoning

This method of seasoning is also known as salt seasoning. In this method the timber is immersed in salt solution. After a specific period it is taken out and seasoned in the ordinary way. In this method of seasoning the interior surface of the timber dries before the exterior dries out. Here the chances of formation of external cracks are reduced.

(iii) Electrical Seasoning

This method is based on the principle that heat is produced when poor conductors are placed in the field of high frequency. Here, an induction coil producing field of high frequency is set up. The timber is made to pass through the coil. Due to electric induction the moisture is removed instantly and the wood is dried quickly. By this process the moisture gets evaporated uniformly and results in superior quality of timber. Because of high cost this method is not generally recommended