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Air drying (end racking) of teak (Tectona grandis), Chiang Mai, Thailand, prior to furniture manufacture (Picture courtesy of Dr. Philip Evans)
Introduction
While softwoods are usually kiln dried, hardwoods are usually air dried to their final EMC or air dried to FSP followed by kiln drying to their final EMC. The main reason for air drying hardwoods is their susceptibility to degrade when kiln dried. Softwoods on the other hand can be kiln dried with fewer problems.
Air drying involves the open piling of freshly processed timber in stacks out of doors or in open sheds so that the wood surfaces are exposed to the surrounding atmosphere. During air drying there is little control over the factors that influence drying and hence the rate of drying is very much controlled by local atmospheric conditions. One guiding principle during air drying of timber is to ensure adequate air circulation through the stack. In practice this is achieved by separating each board within the stack using sticks or stickers. The rate of drying can be controlled to some extent by altering the size of the sticks. Drying can be slowed by using thinner sticks or increased by using wider sticks. During air drying, air circulating within the stack absorbs moisture, is cooled and in this process drops to the bottom of the stack (see figure below). If a space or chimney is constructed within the centre of the stack then this cooler air can exit the stack and fresh air will be drawn in at the top of the stack. If this process occurs correctly then air movement and drying throughout the stack should be continuous and uniform.
 Stacking should be arranged so as to allow a space or chimney through the centre thus allowing moisture laden air to fall and thus encourage a continuous flow of air through the stack. (UCNW 1977)
Other factors that affect the success of air drying include the following;
Back to Drying
URL: http://www.anu.edu.au/Forestry/wood/drying/air.drying.html
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