Effect of Board Dimensions and Grain Orientation on Internal Checking inEucalyptus regnans

Abstract

Dried boards of collapse-prone mountain ash showed severe internal checking in 5 x 10cm cross-sections but little or no checking in matched 5 x 5 or 2.5 x 10cm cross-sections. Differences in checking were interpreted as being caused by drying stresses in association with collapse and resulting differential shrinkage in 5 x 10cm material, the last constituting shrinkage disproportionately high in thickness and low in width. Such shrinkage characterised all 5 x 10cm boards, irrespective of grain orientation, where area of checking was positively related to shrinkage in thickness and, to a lesser extent, negatively related to shrinkage in width. Possible mechanisms of internal check development are discussed. In 5 x 10cm boards, since number of checks was primarily related to shrinkage from green to 12% EMC and area of checking was mainly correlated with shrinkage from 12% EMC to oven dry, it was suggested that checks formed early in drying probably underwent a substantial increase in size in the later drying stages. Checking was also highly negatively correlated with the rate of drying, i.e. the greater the putative permeability the lower the checking. This, coupled with its negative association with density and positive relationship with moisture content, demonstrated that internal checking was closely associated with collapse and so, notwithstanding the influence of drying stress, could be substantially regarded as an internal manifestation of that property.