Trees contain considerable amount of water, often referred to as sap. When a tree is cut down and sawn into timber the wood immediately begins to lose its moisture. The process of losing moisture from green timber can be very unpredictable due to variations in air temperature and
humidity. Low temperature and high humidity slows the process of drying while high temperature and low humidity quickens it. When drying timber is exposed to a combination of the two conditions, the result is usually defects within the timber. A skilled saw miller or timber merchant could control the drying process to minimize such defects and enhance timber quality, a process referred to as seasoning. Seasoning aims to dry timber uniformly with minimum deformation in the shortest possible time to a moisture level similar to the surrounding air. This is called equilibrium moisture content (EMC). Seasoning is done by exposing timber to circulating air and controlled heat over a given period of time.
The most important reasons for seasoning timber are to:
Increase stability: In order to minimize dimensional changes, wood should be dried to the moisture content it will assume in use. The use of green timber in any construction can result in severe deformation as the timber dries.
Reduce inception of decay or stain fungi: Drying timber to a moisture content of less than 20%, or below fiber saturation point prevents the onset of decay and stain fungi.
Reduce weight: The weight of wood is reduced by about 35% or more when seasoned which is of practical importance as it reduces handling and transportation costs.
Increase strength: As the wood dries its stiffness, hardness and resistance to bending increases. Most species of wood increase their strength characteristics by 50% or more when seasoned to a moisture content of 15%.
Allow preservation treatment, gluing and finishing: In order to treat timber with some preservatives proper penetration is only achieved if the timber is dry. Similarly, the successful gluing or finishing of timber is dependent on timber being dry.