Broadly speaking, seasoning is of two types viz.,
(i) Natural or Air Seasoning,
(ii) Artificial Seasoning.
Further, artificial seasoning is of various types namely., water seasoning, seasoning by boiling, kiln seasoning, chemical seasoning, electrical seasoning, smoke-drying, charrying, etc.
The various methods of seasoning of timber are briefly described below:
(i) Natural or Air seasoning.
In this method, converted timber in the form of planks or posts or scantlings is stacked horizontally or vertically above 30 cm of ground under a covered shed. Care should be taken to see that sawn timber scantlings are placed in cross-wise directions in alternate layers to permit free circulation of air and they should also be protected from the sun and rain. The time of seasoning depends upon the type of wood, thickness of sawn members and weather. Generally, for every 25 cm thickness of sawn timber, soft wood takes about 2 to 3 months and hard wood about 12 months.
This method of seasoning s simple, cheap and requires little supervision. However, the rate of drying is slow and it is difficult to exercise controls over temperature and humidity, and hence desired seasoning quality is not attained.
(ii) Artificial Seasoning.
The various methods of artificial seasoning are adopted against natural seasoning because of the following reasons:
(a) The process of drying is controlled and hence not liable to be attacked by insects and fungi.
(b) Period of seasoning can be reduced depending upon the need.
(c) Since the desired moisture content in timber can be attained, so it monomises the defects such as shrinkage, cracking, wraping, etc.
(d) Drying of different surfaces is even and uniform due to better control of air, humidity and temperature.
The different methods of artificial seasoning are described below:
A. Water seasoning.
In this process, timber after felling in the form of logs of suitable sizes, is totally immersed in a running stream of water for a period of 3 to 4 weeks. The thicker end of the log is kept pointing upstream. The sap in timber is then washed away by water. The log is then kept out in the free access of air to dry out. Timber is thus seasoned. It is a quick method and renders timber less liable to warp and crack. It also removes organic materials contained in sap of timber. Although it is a quick process but it reduces the elasticity and durability of timber and makes it weak and brittle.
B. Seasoning by boiling or steaming.
In this method, the timber is first immersed in water and then water is boiled for 3 to 4 hours. In steam seasoning, timber is exposed to the action of steam spray. The timber is thus dried out slowly and seasoned. This is very quick method of seasoning and reduces the shrinkage. However, it is expensive method and also affects the strength and elasticity of timber.
C. Kiln seasoning or Hot air seasoning.
In this method, timber can be seasoned to any moisture content and hence is commonly adopted for rapid seasoning of timber on a large scale. In this process, timber is stacked inside the chamber and hot air is forced for free circulation for about three days. The sap and moisture content are dried out to the desired extent. This process of drying out can be carried either in stationary kilns(by stacking timber in kiln) or in progressive kiln (by moving the timber from one end to the other end of kiln). This method of kiln seasoning gives a well seasoned timber as it controls three important conditions, namely, air circulation, relative humidity and temperature. However, this method is expensive and can be used for timber sawn to small pieces under skilled supervision.
D. Chemical seasoning or Salt seasoning.
In this method, the timber is immersed in solution of suitable salt which absorbs the moisture content from inside of timber. The timber is then taken out and seasoned by ordinary way i.e. by air seasoning. Since the interior surface of timber dries in advance of exterior one and, therefore, the chances of formation of external cracks are reduced.
E. Electric seasoning.
It is an established fact that a green timber offers less resistance to the flow of electric current as compared to a dry timber. In other words, a hard timber offers more resistance to the flow of current than a soft wood since resistance of current is inversely proportional to the moisture content present in the timber. Therefore, high frequency alternating currents which produce heat are used to dry out the timber. This is the most rapid method of seasoning but being very costly cannot be used for seasoning of timber on a large scale.
In this method, timber is dried out over a fire of straw or twig. Care should be taken in applying heat gradually to prevent splitting. Timber seasoned by this method is hardened, more durable and proof against worm attacks. This method is generally used for bending planks in boat building.
G. Charring or Scorching.
In this process the ends of piles or posts are burnt to charcoal in order to remove moisture content. This method is used for piles, posts and other members that are to be kept under water or ground. However, this produces dry-rot in green timber.
Find More About,
Structure of a Timber Tree
Properties or Characteristics of Timber
Defects of Timber
Diseases of Timber - Dry and Wet Rot
Decay of Timber
Seasoning of Timber
Methods of Seasoning of Timber
Preservation of Timber