Explain seasoning of timber.

              Timber contains water. Timber does require a definite percentage of water in it, in order to retain its fibers strongly adhered to each other. 
               Timber releases a part of this moisture in dry and hot atmosphere. And absorbs moisture in cold and humid conditions. This absorption and release, makes the timber bent, warped, shrunk or swollen etc. This change in the shape of a member can endanger the structure of which it is a part of.
               It has been found that if the percentage of moisture is controlled to a certain level, there is a least transfer of water between the timber and the atmosphere. And thereby the structure is prevented from damaging or deforming. This percentage to be controlled depends upon tropical conditions where the timber is to be used.

            ‘Controlling water percentage in timber to an extent that least moisture transfers take place between timber and environment, for the given ambient conditions’, is called as ‘Seasoning’.

             Moisture content  =         Weight of water in timber.

                                                     Weight of dry timber.       

            Moisture content in the felled tree is 50 %. The desired moisture content for timber in Asian countries is between 10 to 20 %. For Mumbai, a humid place, it is 17 to 20%. For Nagpur, a dry place, it is 10 to 12%. And for Pune, a moderate place, it is 14 to 16%.

            In order to control moisture in sawn timber sections, there are two methods:

1.             Natural / air seasoning method.

2.             Artificial seasoning methods.

 1. Natural / air seasoning method.  

Swan timber sections are stacked horizontally on a 30cm raised platform as shown. These are arranged in layers one above other. The layers are separated by sound dry wood spacers. Gap of about 50cm is kept between two adjoining members for free air circulation. Roof covers the stack and the timber is not exposed to direct sunlight or dew. The process of seasoning is very slow & takes even upto 2 years. Capital cost and operating costs are least in this method.

2) Artificial seasoning:

There are various methods of artificial seasoning as follows:

a)     Kiln seasoning

b)    Chemical seasoning

c)     Electrical seasoning

d)    Water seasoning

e)     Boiling

Brief information of above methods is as under:

a) Kiln seasoning:

In this method, the drying of timber is carried-out inside an airtight chamber or oven. Timber is placed in such a way that free air circulates around. The steam is first forced inside in order to control the termites. Then the temperature is raised and maintained till the desired degree of moisture content is attained. The kiln seasoning, though costly, gives well seasoned timber as it controls three important conditions, namely, circulating air, humidity and temperature.

b) Chemical seasoning: This is also known as the salt seasoning. In this the timber is immersed in a solution of suitable salt. The salt is hygroscopic in nature and attracts the water content of timber. This water is then wiped out by cloth. But this method affects the elasticity and strength of wood.

c) Electrical seasoning: In this method, the use is made of high frequency alternating currents. The timber, when it is green, offers less resistance to the flow of electric current. The resistance increases as the wood dries internally which also results in the production of heat. This is the most rapid method of seasoning. But the initial and maintenance costs are so high that it becomes uneconomical to season timber on commercial base by this method.   

d) Water seasoning: In this method the timber is cut in suitable sizes and immersed wholly in water, preferably in running water. The timber is taken out after 2 to 4 weeks. During this period the sap contained in wood is washed away. Then the timber is allowed to dry under a shed having free circulation of air. And the timber is seasoned.

e] Boiling: In this method the timber is immersed in water and then it is boiled. This is a very quick method. The timber is boiled for three to four hours. Then it is dried very slowly in shed. The periods of seasoning and shrinkage are reduced by this method, but its affects the elasticity and Strength of wood. This method is costly as compared to others.