Most Canadian treated decking in service has been given just a thin shell of preservative treatment but it has still performed very well. This is because copper migrated into checks and prevented germination of basidiospores on the freshly exposed untreated wood. Unlike soluble copper systems, MCQ has its copper primarily in the form of small basic copper carbonate particles. For MCQ to be effective as a shell treatment, some of this copper will need to be mobile. A field exposure test was initiated and monitored for one year. Significant amounts of copper were detected in all leachates from MCQ-treated wood. The copper leaching from MCQ-treated wood was low but steady rather than the early flush and decline in loss typical of ACQ- treated wood. Check analysis revealed copper concentrations of 0.18 mg/g and 0.99 mg/g after five months and one year, respectively. After one year of exposure the amount of copper detected in the checks greatly exceeded the 0.27 mg/g previously reported to prevent the germination of basidiospores.

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