Wood drying

Wood in the living tree is saturated with water. Once felled, the timber will naturally start to dry, however there is great art to drying timber without inducing cracks and other defects. Timber is typically dried by kiln drying or air drying, and these can take from just days to many months! This also depends on wood species. For some end uses the target moisture content is significantly lower than for others.

Wood in service must be dried to a moisture content suitable to the environment in which it will be installed. This has a number of advantages. Firstly, as wood dries it becomes stronger and harder. If the moisture content is low enough, it is protected against decay since fungi need high moisture content. Finally, and most importantly, if it is installed at a moisture content to match the environment it will not swell or shrink significantly. It is fair to say that more problems in the utilisation of wood are attributed to moisture content that any other.

Wood drying is an energy-intensive process and one that needs great care if the timber is not to be degraded by distortion and splitting. Whilst some drying can be achieved by natural air drying for zero energy cost, it is a process that is dependent on the variability of the weather. For example, a period of hot sunny weather can irreparably damage a stack of freshly sawn oak. Better results are obtained by artificial drying in special chambers where air at a precise temperature and humidity is blown across the boards in a stack which is separated by stickers. The rate of drying and stresses within the timber can be carefully monitored and controlled. Research continues at improving the accuracy and energy efficiency of drying chambers.

In parallel with wood drying, there is much interest in the measurement of moisture content. Rapid and accurate measurement is essential in manufacturing processes. Many problems remain such as how to measure moisture in modified wood.

 

Useful links

Equilibrium  Moisture Content

The Shrinkulator

KilnServices Limited

Back to The Material