Effect of preheating on internal checking in boards of different dimension and grain orientation inEucalyptus regnans

Eucalyptus regnans resulted in a decrease in both area of internal checking and number of checks after drying. Grain orientation was significant in determining the extent of reduction. For tangential grain, checking decreased linearly with increasing temperature; for intermediate grain, checking was relatively constant to 70 °C before declining at 90 °C ; and for radial grain, checking increased at 50 and 70 °C before declining at 90 °C . At 90 °C , reduction in area of checking and number of checks averaged 89.8 and 53.3% respectively for tangential grain, 62.7 and 62.2% for intermediate grain and 69.1 and 53.9% for radial grain, a reduction overall of 75.1 and 56.2%. Shrinkage in 5 × 10 cm cross-sections was also modified by preheating such that increased shrinkage in board width and reduced shrinkage in board thickness tended to be associated with reduced checking. Since these relationships were largely irrespective of grain orientation, they were attributed to specimen geometry. However, the major factor in check reduction was an increase in the rate of moisture loss, or ostensible permeability. When the effect of rate of moisture loss was nullified in regression analysis, check reduction tended to be related to an increase in tangential shrinkage overall and a decrease in radial shrinkage, irrespective of grain orientation. There was little effect on check reduction in smaller sized material (5 × 5 and 2.5 × 10 cm cross-sections) through preheating, although initial checking in this material was very low by comparison with 5 × 10 cm cross-sections.

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