Relationships between drying defect parameters and some growth characteristics in kiln-dried South African grown Eucalyptus grandis poles

Abstract

Eucalyptus grandis wood is susceptible to serious drying defects and all possible measures before and during kiln drying of poles need to be taken to minimize the defects. In order to identify factors that may affect drying quality of poles, the objective of this study was to assess relationships between surface check total length and depth, MC gradient, honeycomb percentage of pole cross section area, extent of collapse, sapwood depth, heartwood percentage of pole cross-sectional area, green MC and pole diameter. The defects were measured on 3 loads of 10 kiln-dried E. grandis poles, using a different drying schedule for each load. After measuring surface check length using a measuring tape and check depth using feeler gauges and a ruler, poles were cross cut at the theoretical ground line (TGL) 1.5 m from the butt end, to measure the MC gradient between the shell and core of poles. Digital image analysis of cross sections of discs cut at TGL was used to measure sapwood depth, and the areas of honeycomb, heartwood and disc cross section. Collapse was assessed using qualitative methods. Results showed that surface check total length was positively correlated with sapwood depth, and the average depth of surface checks was greater than the average sapwood depth. MC gradient between the shell and core was positively correlated with green MC while the transformed honeycomb percentage and collapse class were both positively correlated with heartwood percentage. It was concluded that pole drying defects are related to sapwood depth, green MC and heartwood percentage, and if pre-sorting of poles by these characteristics is done and appropriate kiln schedules are used, drying defects can be minimised.