IMPORTANT NOTE

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• Movement of water within the wood to the board surface
• Evaporation of water from the board surface

Movement of water within wood can occur through a number of processes including capillarity, vapour diffusion, bound water diffusion, combined vapour diffusion and bound water diffusion and at temperatures in excess of 100 degrees centigrade mass flow. Diffusion processes are increased with increasing temperature and therefore it is possible to increase the rate of water movement within the board to the board surface, assuming there is a lower concentration of diffusing molecules at the board surface, simply by increasing the temperature within the kiln. The loss of water from the board surface to the air is by evaporation. Evaporation is temperature dependent and relies on a relative humidity of the air surrounding the timber. If hot dry air is used for drying then the surface layers of the drying timber may dry rapidly and develop case hardening or checks. To avoid excessive evaporation from board surfaces the air in the kiln must be kept relatively humid. Kiln drying therefore involves careful control of both air temperature and humidity within the kiln. Air circulation as in air drying also plays an important role in the control of humidity as it ensures that the air in contact with the timber is renewed frequently thereby preventing the development of a stagnant layer of excessively humid air from occurring around the timber.