Influence of Grain Direction on Penetration, Retention, and Leaching of CCA(C) in Sapwood and Heartwood of Kenyan-Grown Eucalyptus saligna and Acacia mearnsii
Penetration, retention, and leaching of CCA (C) in relation to grain orientation were tested in small (50mm x 50mm x 200mm) sapwood and heartwood samples of Eucalyptus saligna and Acacia mearnsii. In both sapwood and heartwood of the two species, the results showed a consistent pattern: Transverse > Radial > Tangential. Whereas penetration, retention, and leaching were significantly high in the transverse grain direction for both species, magnitudes recorded for radial and tangential grain orientations were low. However, radial conductivity appeared to be consistently better that tangential conductivity of the preservative. Compared to the results for sapwood, penetration, retention, and leaching were significantly lower in heartwood samples of both species. It was noted that in heartwood samples of both species, percent leaching of the preservative was higher than in sapwood samples, irrespective of grain orientation. That was attributed to the high lignin contents in heartwood of E. saligna and A. mearnsii interfering with the fixation of CCA elements. High penetration of the preservative in the transverse grain direction is because of higher conductivity through large interconnecting vessels in the two species. Radial and tangential conduction relied on low volumes of parenchyma cells (rays) and small vessel and fibre pits. Low fluid conductivity in heartwood is mainly due to anatomical, physical and chemical changes that accompany transformation of sapwood to heartwood, hence low void spaces for preservative penetration. From the results, it becomes apparent that different treatment schedules must be worked out for effective treatment of posts and poles of E. saligna and A. mearnsii (where only an outer sapwood envelope treatment is required), and for sawn material with transverse, radial, and tangential faces exposed, in which case treatment would have to be evenly balanced.