Natural durability and waterborne preservative treatability of tamarack

Eastern larch or tamarack (Larix laricina (du Roi) K. Koch) lumber from New Brunswick was evaluated for heartwood natural durability by laboratory decay tests and for preservative treatability with chromated copper arsenate (CCA-C) and ammoniacal copper arsenate (ACA). The CCA fixation and leaching characteristics of tamarack sapwood and heartwood were also evaluated. The natural durability of tamarack heartwood exposed to two brown-rot fungi, Gloeophyllum trabeum and Postia placenta, was moderate to low and generally inferior to that of heartwood of spruce (Picea spp.) and white pine (Pinus strobus L.) and similar to that of red pine (Pinus resinosa Ait.) sapwood and heartwood. The decay resistance of tamarack was variable, and not related to extractive content. CCA-C penetration into both sapwood and heartwood was poor. ACA fully penetrated the sapwood, but penetrated the heartwood only marginally better than the CCA-C. The rate of CCA fixation, as defined by rate of reduction of hexavalent chromium varied greatly between sapwood and heartwood and between different heartwood samples. Heartwood fixed much faster than sapwood, averaging 2 to 3 days at 21°C and 5 to 6 hours at 50°C as compared to 15 to 20 days and 30 to 35 hours, respectively, for sapwood. The fixation rate was directly related to the hot water soluble extractive content of the wood. The quality of fixation, as defined by resistance to leaching of the CCA components, was much lower for the faster reacting heartwood. The combination of poor penetration and low CCA stability in tamarack heartwood suggests that this preservative is not appropriate for this species.