PREHEATING GREEN BOARDS OF MOUNTAIN ASH (EUCALYPTUS-REGNANS F MUELL)

Preheating green board sections of Eucalyptus regnans in water produced significant changes in shrinkage and checking behaviour during subsequent drying. Of the treatments employed, gradual heating was most successful in reducing shrinkage, followed by sudden heating and gradual heating extended. Internal checking (S(vi)) responded best to preheating with significant reductions evident for all treatments; optimum water temperatures, as determined by paired t-test analyses, were 50, 70 and 80-degrees-C for gradual heating, 50-degrees-C for sudden heating, and 80-degrees-C for gradual heating extended. Overall, the most statistically significant reduction in S(vi) occurred at 50-degrees-C. External shrinkage tended to increase with preheating, although there was some indication of a reduction at higher temperatures during gradual heating. Surface checking almost invariably rose, irrespective of treatment, although variation with temperature was observed among treatments. Amongst the age groups represented, reduction in internal checking was greatest for 16-year old material and less, but still significant, for 31-year old material; there was no significant change for 23-year old trees, a result attributable to one tree from Tasmania which, in contrast to all others, showed an increase in S(vi). This point is discussed. When shrinkage data was plotted against calculated 'heat', irrespective of treatment, significant relationships were evident only for change in external and change in total shrinkage. For change in internal checking, however, maximum peaks occurred at approximately equal logarithmic intervals along the heat scale, possibly suggesting a sequence of critical heating points.