This method covers the determination of the minimum amount of preservative that is effective in preventing decay of selected species of wood by selected fungi under optimum laboratory conditions.Conditioned blocks of wood are impregnated with solutions, emulsions, or dispersions of a preservative in water or suitable organic solvent to form one or more series of retentions of the preservative in the blocks. After periods of conditioning or weathering, the impregnated blocks are exposed to recognized destructive species of both brown-rot and white-rot wood-destroying fungi. At least one brown-rot and one white-rot fungus should be used in screening tests with new formulations.
In general, the accuracy of the preservative efficacy determination will increase as more fungi are used. The minimum amount of preservative that protects the impregnated blocks against decay by a given test fungus is defined as the threshold retention for that organism. A minimum of three species of both brown-rot and white-rot fungi should be used when refining the thresholds of new preservative formulations. Failure to protect is evidenced by loss of mass from the treated wood blocks, as indicated by a loss of weight. Blocks treated with preservatives of known efficacies should be included in tests with new formulations.