Wood Seasoning is the process of drying timber in order to remove moisture from the cells of wood walls. It is very important for the commercial use of timber as it prevents it from decaying and increases strength.
Wood seasoning can be done by many methods which basically involve natural air seasoning or kiln seasoning.
Air seasoning
The process of removing moisture from timber before it is used and put into service by drying it in natural air.
In air seasoning, the air is the only medium that carries heat to the timber. Air removes the moisture evaporated from the timber by the heat supplied to it.
For uniform drying of timber following points should be taken into consideration:
a. Piling or staking, of timbers.
b. Protection of the stack.
A. Piling or stacking of timber
The piling of timber is very important as poor, uneven piling or staking results in crooked and twisted pieces. Timber can be stacked in either of the two ways:
- Horizontal Stacking
- Vertical staking
Horizontal Stacking :
It is a common method of stacking and is suitable for all forms of timber such as swan timber, veneers, etc.
While preparing horizontal stacks of timber, given points should be considered:
- Sanitation: The site should be level and clean.
- Foundation: It should be strong and durable and raised 30-45 cm above the ground.
- Orientation: Length should be perpendicular to the prevailing wind in the case of non-refractory wood and parallel in the case of refractory wood.
- Sorting: Longer planks are kept at bottom of the stack and shorter ones at the top.
- Dimension: Length depends on the material to be stacked while width should not be more than 1.5m and height should not be more than 3m.
- Crossers: Crossers are small battles or thin pieces of wood, 2.5cm or square. they should be kept to separate one layer of seasoning timber from another. This allows uniform circulation of air on all sides of timber and prevents the green wood from warping and twisting during seasoning.
Cross-sections of crossers – 2.5cm*4cm or 4cm*5cm.
Vertical Stacking:
This method is used mainly for the rapid surface drying of certain species of non-refractory wood such as Semal(Bombax ceiba), Pinus (Pinus spp), Mango (Mangifera indica), etc.
These species are liable to develop mold and discoloration, soon after conversion. Thus, surface moisture is removed rapidly to prevent germination and growth of mold and fungi which cause the staining of the wood.
Vertical stacking involves the following steps:
- The timber is exposed to the sun and wind after conversion as soon as possible.
- After the time has become partly dry it can be stacked by
- Horizontal method for further drying or it can be left standing in a vertical position till the completion of its drying.
- The planked stand should be turned frequently.
B. Protecting the Stack
According to the types of wood on the basis of their ease to dry, different protection measures should be applied to different types of wood.
In the case of,
- Refractory wood – The seasoning shed is made with a tight roof and side shelter with the provision of ventilation in a closed room.
- Medium Refractory wood – Seasoning shed for partial shade, in which three sides are closed and one is left open (northern side).
- Non-refractory wood – Seasoning shed to save from rain only which gives full opportunity to sun and wind.
Kiln Seasoning:
It is the process of drying wood using artificial heat. In this method, the timber is stacked in the chamber and artificial heat is provided to dry the timber.
The following factors affect Kiln Seasoning:
- The temperature of circulating air.
- The humidity of circulating air and rate of circulating air.
Basic Principles of Kiln seasoning:
- Temperature can be maintained according to our willingness.
- Applicable to any type of species.
- Relative humidity and air circulation can be controlled.
- Relative higher temperature kills insects, pests, and pathogens.
- Outer climate and wind do not affect it.
Types of seasoning kilns:
- Steam heated Kilns
- Furnace Kilns
- Solar heated kilns
- Dehumidification Kilns
Steam heated Kilns
In these Kilns, heating is by steam pipe coils inside a compartment underneath the kiln, and circulation is brought about by fans, placed either outside or inside.
There are water sprays and condensers also kept, to control the humidity. These Kilns are suitable for the drying of both furniture timbers and packing case wood.
Furnace kilns
• When steam is not available and the cost of running a stem boiler is considered too high,
a simple type of kiln having its own furnace can be installed. Furnace kilns are two types:
i. Direct heating types
In this method, smoke is used to dry the timber. Because of unburnt smokes, the timber becomes discolored but it is purely superficial and easily removed. This kiln is used to dry hardwood for furniture manufacture.
ii. Indirect heating type
In this method, the heat combustion gases and smoke pass through sheet iron ducts, which are placed inside the kiln and heat the air circulation inside the drier.
It has the advantage that the timber to be dried does not come in contact with smoke and therefore there is no chance of surface blacking of timber during drying.
Solar heated Kilns
In this method, solar energy is used to provide heat for seasoning timber of various sizes.
Dehumidification Kilns
In this method, electrical heat is used to provide heat to the stack and heated air is circulated by a fan. Green timber generates warm and moist air which is sucked by the humidifier.
Other types of seasoning
Veneers Seasoning
A closed metal chamber is used to dry the veneers by using fans in this method.
Salt Seasoning
It involves soaking the timber in the salt before stacking for air seasoning.
Chemical Seasoning
It involves soaking the timber in suitable chemical solutions before stacking it for seasoning. These solutions absorb water from the timber.
Electrical seasoning
Electricity is the main source of heat for drying the timber in a closed room.
