Preheating and Continuous and Intermittent Drying in Boards of Eucalyptus regnans F. Muell. I. Effect on Internal Checking, Shrinkage and Collapse

Matched, green board-sections of Eucalyptus regnans were pretreated by gradual heating in water at temperatures from 50 to 90°C and dried by either continuous or intermittent drying regimes. For both schedules, number of internal checks was negatively related to preheating temperature and declined by ca. 45% at 90°C. Number of checks was significantly lower for intermittent drying than for continuous drying, but such differences narrowed with increasing preheating temperature. Area shrinkage of internal checking also declined with increasing temperature and demonstrated a reduction of nearly 50% at 90°C. For continuous drying, internal recovery after reconditioning, as represented by area and number of internal checks, increased significantly with increasing pretreatment temperature. An explanation of the mechanisms thought to be involved in the reduction of internal checking by preheating is presented. It is suggested that an increase, rather than a decrease, in shrinkage may contribute to this reduction. Difference in shrinkage between that determined by linear measurements and that by image analysis was attributed to stress relief associated with cross-cutting after oven drying