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surrounding wood. These knots cannot become loose and as a result they check. Low relative humidity intensifies this defect and, therefore, it is advisable to use high relative humidity. 2.6.5. Casehardening Ranta-Maunus et al. (2001) define casehardening as: "the tendency of dried wood to deform (into a cup shape) after re-sawing and equalising of the moisture content". This defect appears when the core of the board has a moisture content above the fibre saturation point, while the board shell (the surface zone) has reached a moisture content level below the fibre saturation point (Avramidis, personal communication). This happens during drying as moisture is reduced at higher rates near the surface of the board. At this point, the shell needs to shrink but the core remains the same in size since shrinkage occurs below the fibre saturation point. The result is that tension stresses develop in the shell and compression stresses in the core. As drying continues, the core's moisture content drops below the fibre saturation point and, therefore, the core tends to shrink but this is prevented because of the non-shrinking shell. What follows is a strength reversal during which tension stresses develop in the core and compression stresses develop in the shell. This phenomenon is called casehardening and it can cause severe deformations if the boards are further processed. Conditioning treatment (steaming) can cause the opposite stresses to appear (tension in shell - compression in core) and, therefore, it can be used after drying to eliminate casehardening. Additionally, it reduces moisture content variation within boards (Simpson & Boone, 1997). However, there is an optimal amount of conditioning, after which reverse casehardening (presence of stresses opposite to those of casehardening) develops (Avramidis, 2003). Bramhall & Wellwood (1976) mention that keeping relative humidity at high levels can eliminate casehardening because the board shell will not dry too quickly. At later stages of drying, temperature can be raised and relative humidity decreased so as to complete the drying process quicker. Skaar (1988) 24PDF Image | KILN DRYING PROBLEMS AND ISSUES IN CANADA
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