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Wood drying

Keywords: Wood drying    Publish Time: 12-10-2010

Wood drying (also seasoning lumber or wood seasoning) reduces the moisture content of wood prior to its use.

There are two main reasons for drying wood:

Woodworking - When wood is used as a construction material, whether as a structural support in a building or in woodworking objects, it will absorb or desorb moisture until it is in equilibrium with its surroundings. The equilibration process (usually drying) causes unequal shrinkage in the wood, and can cause damage to the wood if the equilibration process occurs too rapidly. The process of equilibration needs to be controlled in order to prevent damage to the wood.
Wood burning - When wood is burned, it is usually the best to dry it first. Damage due to shrinkage is not a problem here, and the drying process may proceed more rapidly than in the case of drying for woodworking purposes. Moisture affects the burning process, with unburnt hydrocarbons going up the chimmney. Although if a 50% wet log is burnt at high temperature, with good heat extraction from the exhaust gas leading to a 100C exhaust temperature, only about 5% of the energy of the log is wasted evaporating and heating the water vapour. With condensers, the efficiency can be further increased, but for the normal stove, the key to burning wet wood is to burn it very hot, perhaps starting fire with dry wood.
For some purposes, wood is not dried at all, and is used "green". Often, wood needs to be in an equilibrium with the air outside as for construction wood or the air indoors as for wooden furniture. Wood is either air-dried or kiln-dried. Usually, the wood is sawn prior to drying, but not always, as when the whole log is dried.

Case hardening describes lumber or timber which has been improperly kiln-dried. If dried too quickly, wood shrinks heavily on the surface, compressing its still damp interior. This results in unrelieved stress. Case hardened wood may warp considerably and dangerously when the stress is released by sawing.


From:   http://en.wikipedia.org/wiki/Wood_drying