Treatment quality is in general terms dependent upon the preservative retention, and how the preservative is distributed throughout the cross-section of the treated article in question, i.e. the penetration of the preservative. Of these two fundamental qualities that are intrinsically linked to treatment quality, penetration is in general terms the most subjective. The determination of retention is relatively straight forward, assuming that an analytical method exists for the analysis of the preservative active ingredient(s) in question. Traditionally the penetration of preservatives has been assessed visually, either through being able to distinguish treated from untreated areas, e.g. as in the case of creosote, or through the use of a penetration spot test, whereby a reagent reacts with one of the preservative components to elicit a colour change. These methods are used extensively on account of their simplicity and convenience and are standardized in many countries. With the introduction of metal-free preservative systems, particularly active ingredients such as the triazole fungicides and synthetic pyrethroid or neonicotinoid insecticides for which spot test reagents have proven elusive, a common approach has been to include a surrogate or penetration indicator in the treating solution. A penetration indicator is a substance that penetrates the timber in a similar way to the active ingredient(s), but for which a spot test exists, enabling a visual assessment of the penetration. In Australian and New Zealand Standards technical forums the use of central one-ninth or core retentions has been raised as a means of determining penetration compliance for preservatives for which no colorimetric spot test exists. The purpose of this paper is to consider the appropriateness of this approach, through using biodeteriogens (in this case wood decay fungi) to map the preservative distribution through the cross-section of commodity sized, preservative-treated specimens.

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