Seasoning of timber

Seasoning is the name given to the methods of drying timber

There are two methods by which timber can be dried:
(i) natural drying or air drying, and
(ii) artificial drying.

Air drying

Air-drying is the drying of timber by exposing it to the air. The technique of air-drying consists mainly of making a stack of sawn timber (with the layers of boards separated by stickers) on raised foundations, in a clean, cool, dry and shady place. Rate of drying largely depends on climatic conditions, and on the air movement (exposure to the wind). For successful air-drying, a continuous and uniform flow of air throughout the pile of the timber needs to be arranged. Coating the planks with any substance that is relatively impermeable to moisture can control the rate of loss of moisture; ordinary mineral oil is usually quite effective. Coating the ends of logs with oil or thick paint, improves their quality upon drying. Wrapping planks or logs in materials, which will allow some movement of moisture, generally works very well provided the wood is first treated against fungal infection by coating in petrol/gasoline or oil. Mineral oil will generally not soak in more than 1-2 mm below the surface and is easily removed by planning when the timber is suitably dry.

Natural seasoning of timber

Kiln drying

The process of kiln drying consists basically of introducing heat. This may be directly, using natural gas and/or electricity or indirectly, through steam-heated heat exchangers, although solar energy is also possible. In the process, deliberate control of temperature, relative humidity and air circulation is provided to give conditions at various stages (moisture contents or times) of drying the timber to achieve effective drying. For this purpose, the timber is stacked in chambers, called wood drying kilns, which are fitted with equipment for manipulation and control of the temperature and the relative humidity of the drying air and its circulation rate through the timber stack

Kiln drying provides a means of overcoming the limitations imposed by erratic weather conditions. In kiln drying as in air drying, unsaturated air is used as the drying medium. Almost all commercial timbers of the world are dried in industrial kilns. A comparison of air drying, conventional kiln and solar drying is given below:

1. Timber can be dried to any desired low moisture content by conventional or solar kiln drying, but in air drying, moisture contents of less than 18% are difficult to attain for most locations.
2. The drying times are considerably less in conventional kiln drying than in solar kiln drying, followed by air-drying.
1. This means that if capital outlay is involved, this capital is just sitting there for a longer time when air-drying is used. On the other hand, installing an industrial kiln, to say nothing of maintenance and operation, is expensive.
2. In addition, wood that is being air-dried takes up space, which could also cost money.
3. In air-drying, there is little control over the drying elements, so drying degrade cannot be controlled.

Kiln seasoning
Kiln seasoning animation
Treatment and Preservation of Timber
Strength Grading of Timber