Bioassay of Residual Preservative Protection in Wood

A simple bioassay that had proved effective as an indicator of the relative level of protection by pentachlorophenol proved to be similarly effective for assessing protective levels in wood treated with ammoniacal copper arsenate (ACA) and copper chrome arsenate (CCA). Bonneville Power Administration has adopted the method in preference to chemical analyses for predicting when penta-sprayed cedar poles should be resprayed. A sample of wood to be analyzed is placed on nutrient agar 30 mm from an inoculum of a decay fungus (Poria placenta) and 15 mm from the edge of the culture chamber, and the protection residing in the preservative in indicated by the extent to which the linear growth of the fungus is retarded in three weeks by chemical diffused from the wood. The most sensitive measure of retardation was the distance, in line with the sample, remaining between the margin of the fungus growth and the chamber edge (zone of limitation, ZOL). The bioassay measurements were correlated with amounts (retentions) of penta, ACA, and CCA. Retentions of penta and CCA indexed by the ZOL ranged down to 0.1 and 0.25 pcf, respectively. Results of the assay with creosoted wood from poles in service were indefinite, probably because of the variable composition of this preservative. Certain variables affecting uniformity of the assay were identified.

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