Copper and quaternary ammonium compound (quat) distributions across lumber (gradients) and availability, as measured by amount removed by intensive leaching of wood flour, were measured in ACQ treated southern pine lumber. Samples were evaluated just after treatment and stabilization, or after 3 years of laboratory storage, or after 3 years of natural weathering exposure. The objective was to investigate how the distributions of copper, quat and monoethanolamine (MEA) changed under different exposure conditions to try to explain the long terms effects of semi-transparent wood coatings to reduce leaching, even after partial failure of the coatings. In unweathered lumber, there was a slight copper gradient from the surface to the interior of the lumber and a steeper quat gradient. Analysis of specimens after 3 years of weathering exposure confirmed the effectiveness of the coatings to reduce copper and quat leaching and showed that quat leaching from uncoated samples was substantial. There was little effect of aging indoors on availability of ACQ components. However, the availability of copper was significantly reduced after 3 years of natural exposure, even after accounting for the copper that leached during weathering. It was observed that the MEA availability was greatly reduced, presumably by preferential leaching compared to copper and quat during weathering. This appears to have reduced the amount of soluble/available copper resulting in lower long term copper leaching, especially in coated samples that were protected from high copper losses by the effect of the coating. The reduced quat leaching from coated samples is attributed to the long term effectiveness of the coatings in the earlywood portions of the samples.

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