Microwave Pretreatment of Backsawn Messmate Boards to Improve Drying Quality: Part 1

Abstract

The findings of an investigation into the microwave (MW) modification and conventional kiln drying of backsawn/flatsawn messmate stringybark (Eucalyptus obliqua L'Herit) are summarized. The project was conducted in two parts. Part 1, the subject of this article, investigates whether low-intensity microwave modification results in more or less check formation than occurs during conventional drying of eucalypts. Part 2 of the investigation will determine the acceleration in kiln drying that can be achieved through microwave modification of the wood structure and in turn improving permeability. Significant improvement in microwave modified samples compared to controls in relation to check (internal/honeycombing and surface) formation during drying was observed, with Schedule 1 (70 kWh/m3) having 55% of samples without checks compared to the control samples at 31%. The study also found that there is a significant reduction in surface checking (number and depth) in MW modified boards compared to the controls. The control samples exhibited the highest proportion of surface checking, with 65% possessing one or more surface checks, of which 82% extended more than one quarter the width of the sample. This was in marked contrast to Schedule 2 (75 kWh/m3), which resulted in 43% having one or more surface checks, of which 25% extended more than one quarter the width of the sample. The control samples also returned the highest rate of internal/honeycombing checking with 28% of samples affected, while samples pretreated according to MW Schedule 2 yielded the lowest rate, with 11% affected.