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Movement of Moisture Within The Wood

When moisture evaporates from the surface of a piece of wet wood the moisture concentration in the outer layers is lowered and moisture begins to move from the wetter interior to the drier surface. For the consideration of practical drying it will suffice to accept the movement as being a combination of capillary flow and vapour diffusion. The structure of wood offers resistance to the passage of moisture an some species, especially in dense hardwoods such as Eucalypts, this resistance is very considerable. If the evaporation from the surface occurs at a faster rate than the moisture from the interior zones flows to these surfaces, the moisture gradient within the wood becomes progressively steeper. As the outer layers dry below the fibre saturation point their tendancy to start shrinking is resisted by the wetter interior so that a state of stress develops, with the outer layer in tension and the inner zones in compression. If the stresses become to severe the outer layers may rupture, ie., surface checking may occur, or they may become stretched beyond the elastic limit without breaking and the wood is then said to be casehardened.

In both air and kiln drying the establishment of a reasonable moisture gradient is unavoidable and indeed desirable, for in any particular piece of wood at a given temperature the rate of movement of moisture up to the surface is proportional to the steepness of the gradient. The skill in timber drying really lies in allowing the gradient to become as steep as the wood will tolerate without suffering damage, and in controlling the rate of evaporation to match the rate at which moisture is reaching the surface (Pratt 1974).


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