**4. Discussion**

The color change and erosion data confirm the value of copper in wood protection for its ability to partially photo-stabilize wood as has been previously been reported [5–8]. The improved resistance to black-stain fungal colonization of the MBCC-treated samples is consistent with previous work showing that copper can inhibit these organisms. Plasma-deposited copper has been found to inhibit the growth of *A. pullulans* [27]. However, some copper tolerance has also been observed with these fungi [28]. This may indicate why only some growth was observed. The copper tolerance of the fungal strains used has not been otherwise assessed. The natural looking mid-brown color produced by the peroxide post-treatment of MBCC impregnated wood suggests that an attractive and fade resistant brown color at point of sale could be achieved. This could potentially reduce or eliminate the need for the addition of colorants, which are widely used in wood treated with MBCC-containing preservatives [9,10].

The performance against different black-stain inocula varied considerably, though the performance trends were similar. This highlights the importance of carefully choosing test isolates and in evaluating field-collected, fresh isolates, not mutilated through long storage and inclusion of more than one strain to better understand performance. The optimized growth conditions of this accelerated set up led to very rapid attack that would not be observed in field studies. Field studies on the growth of black-stain fungi on MBCC-treated wood would be useful to better understand the effect of the MBCC treatment. Co-biocides, such as the propiconazole used in micronized copper azole type C, would likely also contribute to efficacy in the field.

This study did not examine the use of peroxide dip treatment before pressure treatment with MBCC. Given the increased water uptakes observed in the leaching experiment, peroxide pre-treatment could potentially improve preservative solution uptake. This could make it easier for treaters to meet preservative penetration and retention requirements and lead to better treated products and/or more rapid treatment schedules. A more detailed assessment of the interaction between peroxide and copper carbonate should be conducted to optimize the treatment so that it maximizes reacted copper and minimizes leaching while still producing the desired brown color.

Peroxide is a common ingredient in deck cleaning products. These data sugges<sup>t</sup> that treatments may result in increased copper mobility and increased levels of reacted copper. Bleach-based deck cleaners have been associated with increased copper leaching from CCA-treated decking [29]. The authors of that study note that amount of copper leached would not be a concern in a residential context. It is unclear to what extent a peroxide-based deck cleaner would mobilize copper from wood treated with a MBCC-containing preservative.

The potential impact of increased copper leaching and reactivity on long-term durability should be investigated. The increased water uptake and copper leaching observed in this study sugges<sup>t</sup> a potential loss of durability. However, the increased amount of reacted copper suggests a potential improvement in durability. Previous work has found that copper from MBCC-treated wood is resistant to leaching and that the leached wood retained its resistance to decay fungi [30].

EPR has been used previously to study the reaction chemistry of copper-based wood preservatives in wood [15,31]. While several analytical tools are useful for quantifying copper in organic matrices, most techniques cannot provide information on the copper species present or the bonding of the copper to the wood components. One technique that can identify copper–ligand bonding is EPR. However, it too is limited to copper species that are paramagnetic. While this is usually not a problem it could be limiting, in that it cannot detect species for example that are antiferromagnetic or Cu(I) species. Studies by Piesach and Blumberg [32] have shown that the unpaired electron chemical shift in mononuclear *S* = 1/2 Cu(II) species and hyperfine coupling to the *I* = 3/2 copper nucleus along axial direction (the g*z* and A*z* respectively) are most sensitive to electronic and geometric perturbations in the bonding to the copper ion. All the spectra obtained in this study were very similar. They all exhibited strong g*z* values of ≈2.373, and A*z* tensors of ≈131 G which are typical of copper-wood complexes formed with only copper-oxygen bonding. This supported the hypothesis that all of the copper complexes being formed both with, and without peroxide treatment, were identical in the coordination chemistry of the copper. This would sugges<sup>t</sup> that the peroxide treatment is forming carboxylic acid functional groups which are then able to mobilize copper from the remaining basic copper carbonate. The high solubility of the copper compounds formed suggests that, unlike the natural carboxylic acid functional groups present in wood, those formed by the peroxide post-treatment may be wood-degradation products. In the basic copper carbonate present in the MBCC, the copper is antiferromagnetically coupled in the solid state, and so is EPR silent.
