Moisture content of wood

Moisture content of wood

 Water is stored in wood in two main forms:

·                     As free water in the vessels and/or cells, used to move nutrients within the tree.

·                     As cell (or bound) water, which is an integral part of the cell walls.

The process of seasoning removes all of the free water and most of the bound water. In the removal of the bound water, the wood cells change in size and shape, so this part of the process must be carried out with careful control over drying rate.

When the timber is first cut, the initial reduction in moisture content is a result of free water loss. This usually occurs without any significant dimensional changes to the timber as the loss of moisture represents the drainage of pores in the timber. If the environmental conditions are favourable, the moisture loss continues until all the free water is released to the atmosphere. This point is known as the fibre saturation point (fsp). The fibre saturation point varies a little with each piece of timber, but it is generally taken to be at a moisture content of between 25% and 30%. The loss of free water will occur relatively quickly in small cross-sections of timber, even if the timber is exposed to rain.

After all of the free water has been lost, the timber is still classed as unseasoned as its moisture content is above 25%. Further drying is required to bring the moisture content below 15% so that it can be classed as seasoned. This additional reduction in moisture content, below fibre saturation point, requires a reduction in the amount of bound water in the wood's cell wall, which in turn requires more energy and occurs more slowly than the loss of free water. It also results in a reduction in the size of the cell walls, which causes the timber to shrink in size.