Biocide Retention Variation and Gradients in Commercially Treated 4 by 4s

Abstract

Biocide retentions for copper-based wood preservatives used for residential ground-contact applications are established through field stake tests using small research stakes cut from selected sapwood and then treated in laboratory cylinders. The results from the relatively short-term field tests are then used to specify retentions for commercially treated timbers that have a range of densities and heartwood content. More than 120 4 by 4s that had been commercially treated with four copper-based systems readily available were purchased, with the posts carefully selected for good biocide penetration and a wide outer sapwood band, and analyzed for biocide retentions. Also, biocide gradients from five soluble copper and three particulate copper commercially treated 4 by 4s, all having the same labeled retention, were determined. All four systems gave a range of analyzed retentions, with some fraction of the timbers of each preservative system having a relatively low biocide level. In contrast to previous reports, the biocide gradient for wood treated with a soluble copper formulation was less steep than in wood treated with a particulate copper system.