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water. Because the pores are blocked, no air can move to fill the spaces that were occupied by the water. Tensions then develop pulling the cell walls together. Figure 5. Western red cedar seriously affected by collapse (Source: Ward & Simpson, 1997). For species that commonly develop collapse, it is recommended that the wood be air-dried before kiln drying. Frequently, the effects caused by collapse can be remedied by steaming the wood between 80°C and 100°C at 100% relative humidity (Forestry Commission of N.S.W., 1983). During this process the cell walls return to their original shape after they become softened. There are two types of internal checks: honeycomb and shake. Honeycomb is internal checking that generally occurs along the rays. It happens because of tension stresses in the board core. In order for honeycomb to appear, the core of the board must have high moisture content and the temperature must be kept high for a long period. Consequently, a solution to this defect would be to keep temperatures low until the moisture content in the board core drops below the fibre saturation point (Ward & Simpson, 1997). Shake is separation of growth rings. It is not considered a drying defect. Checks can occur on the end grain face of knots and are called checked knots. They follow the direction of rays and can be explained by the differential shrinkage of wood within and outside knots (Bramhall & Wellwood, 1976). The phenomenon is observed on intergrown knots, which are connected to the 23PDF Image | KILN DRYING PROBLEMS AND ISSUES IN CANADA
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