Wood drying (also seasoning lumber or wood seasoning) reduces the moisture content of wood before 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. Equilibration (usually drying) causes unequal shrinkage in the wood, and can cause damage to the wood if equilibration occurs too rapidly. The equilibration must be controlled to prevent damage to the wood.
- Wood burning: when wood is burned, it is usually best to dry it first. Damage from shrinkage is not a problem here, and the drying may proceed more rapidly than in the case of drying for woodworking purposes. Moisture affects the burning process, with unburnt hydrocarbons going up the chimney. If a 50% wet log is burnt at high temperature, with good heat extraction from the exhaust gas leading to a 100 °C exhaust temperature, about 5% of the energy of the log is wasted through 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 must be in equilibrium with the air outside, as for construction wood, or the air indoors, as for wooden furniture.
Wood is air-dried or kiln-dried. Usually, the wood is sawn before drying, but not always, as when the whole log is dried.
Case hardening describes lumber or timber that has been improperly kiln-dried. If dried too quickly, wood shrinks much at the surface, compressing its damp interior. This results in unrelieved stress. Case-hardened wood may warp considerably and dangerously when the stress is released by sawing.