When a tree is newly felled, it contains about 50% or more of its own weight as water. This water is in the form of sap and moisture. The water is to be removed before the timber can be used for any engineering purpose. In other words, the timber is to be dried. This process of drying of timber is known as the seasoning of timber.
Notes to Remember
The moisture should be extracted during seasoning under controlled conditions as nearly as possible at a uniform rate from all parts of the timber.
The moisture which cannot be extracted during seasoning, is uniformly distributed throughout the mass. If the drying is irregular, the shrinkage of timber will also be irregular and it will set up internal stresses between the fibres. When these stresses become excessive and are capable of overcoming the cohesion of fibres, the timber warps and the shakes are formed.
The wood is a hygroscopic material. The capacity of wood to absorb water vapours from air is called hygroscopicity of wood. The dry wood absorbs the moisture from the surrounding air. Now, the air humidity is not constant and hence the wood moisture content also varies accordingly. The fluctuations in wood moisture content from zero to fibre saturation point cause corresponding volume changes in wood leading to cracking, warping, swelling and shrinkage of wood.
By the process of seasoning, the excess water of timber is extracted in such a way that the moisture content of seasoned timber corresponds to the required moisture content of timber for the environments in which it is to be used.
Seasoned timber should be protected from exposure to the rain and excessively high humidity.