Effect of kiln drying schedule on the quality of South African grown Eucalyptus grandis poles

Abstract In specific cases, kiln drying prior to preservative treatment is the prescribed method of utility pole seasoning in South Africa. The slow drying nature of Eucalyptus grandis poles makes rapid kiln drying potentially detrimen-tal due to development of defects such as excessive surface checking, honeycomb and collapse. To control these defects while maintaining the fastest drying rate possible, the effect of drying schedule on the drying quality of E. grandis poles was investigated. Full length pole drying was simulated in thirty 2.1 m long butt sections of poles of which the top ends were end sealed. Three industrially used schedules that differed in dry and wet bulb temperature and duration of drying (Tdb, Twb and time) were chosen: 80°C/59°C/8 days, 80°C/69°C/10 days and 70°C/59°C/12 days. Surface check length of each pole was measured after drying. Cross-sections cut at the theoretical ground line (TGL), 1.5 m from the butt end, were used to measure the moisture content (MC) gradient between the shell and core of poles. Digital image analysis of cross-sections of discs cut at TGL was used to measure the area of honeycomb and disc cross-section. Collapse was assessed qualitatively. With all three schedules, MC’s in the pole cores at the end of drying were still very high. Some poles were above 25 % in the outer 25 mm of the treatable zone. The 70°C/59°C/12 days schedule generated the highest surface check total length. A reduction in the wet bulb depression o