Wood Seasoning is the process of drying timber to remove the bound moisture contained in the walls of the wood cells to produce seasoned timber.
Also, it can be defined as the process of drying the timber to a suitable moisture content so that it is in equilibrium with the prevailing atmospheric pressure.
Wood Seasoning principles:
- The water is to be removed before the timber can be used for any purpose.
- Moisture should be extracted during seasoning under control conditions at a uniform rate from all parts of the timber.
- Drying should be regular to overcome the shrinkage, cracking, warping, and swelling of the timber.
- Maintain uniform internal stress between fibers.
- Maintain a level of equilibrium moisture content under the given climatic condition of temperature and relative humidity.
- The relationship between climatic conditions and moisture content in timber has been established from tests on various types of timber.
- The moisture content of seasoned wood is approximately equal to the average humidity of the surroundings.
Factors to be considered during seasoning:
- Seasoning time.
- Moisture content in wood.
- Heat and temperature.
- Air circulation.
- Types of species.
- The thickness of wood etc.
Classification of timber according to their ease of seasoning
Highly refractory woods
These woods can be easily damaged and are slow and difficult to dry. They are heavy structural timbers with high density. Therefore, they require significant protection and care against rapid drying conditions for the best results.
Examples are Sal (Shorea robusta) Bel (Aegle marmelos) , Jamun (Syzygium cumini), etc.
Moderately refractory woods
These timbers are partly damaged during seasoning. To make such timbers free from defects they should be seasoned with moderately rapid drying conditions. These woods are suitable for making furniture.
Examples are Siris (Albizia spp), Tooni (Toona ciliata), sissoo (Dalbergia sissoo),
These woods have the potential to withstand rapid seasoning. They can be seasoned even by applying high temperatures (more than 100 °C) in industrial kilns. Rapid drying is necessary as they may develop discoloration (blue stain) and mold on the surface if not.
Examples are low-density wood such as Semal(Bombax ceiba), Pinus (Pinus spp), Mango (Mangifera indica), etc.
- Seasoned timber tends to have superior dimensional stability.
- Drying reduces the weight of wood, and as a result, there is a decrease in shipping costs.
- It reduces or eliminated further shrinkage, checking, and warping.
- Increase in stiffness, bending strength, and compression strength.
- It increases strength and nail-holding power.
- It decreases susceptibility to infection by blue stain and other fungi (fungi can only grow on timber with moisture content above 20%)
- It reduces the chance of attack by insects.
- It improves the capacity of wood to take preservative and fire-retardant treatment and to hold paint.
- Electrical resistance and thermal insulation properties of wood are increased after drying.
- Increase strength, durability, and workability.
- Finishing characteristics of end products of wood improve after drying.
- Gluing by the adhesive is effective in dry wood.
- Dry wood burns better as fuel wood.
The major disadvantage of wood seasoning is wood defects. Basically, there are two types of defects that arise during seasoning:
- Defects from shrinkage anisotropy, resulting in warping: cupping, bowing, twisting, crooking, spring, and diamonding.
- Defects from uneven drying, resulting in the rupture of the wood tissue, such as checks, end splits, honeycombing, and case hardening.
Other disadvantages can be discussed as follows;
In air seasoning,
- It is a very slow process.
- It keeps the valuable land as well as timber blocked for longer periods (and hence in some cases may be uneconomical).
- Moisture content can only be brought to a certain limit (16-17 percent).
- Seasoning may be non-uniform in all sections of timber.
- Air seasoned timber is more amenable to attacks of insects and fungi.
In kiln seasoning,
- This method is costly, though less space is required.
- It requires skilled labor.
- Continuous attention is needed so as to check seasoning defects such as warping, cracks (internal and surface), and split ends.
- It gives a little weaker timber.