**5. Conclusions**

Peroxide post-treatment of wood pressure impregnated with MBCC may help to minimize color change due to photo-degradation. Erosion due to photo-degradation and colonization by black-stain fungi were similar in samples impregnated with MBCC, regardless of the peroxide post-treatment. The peroxide post-treatment was associated with increased copper leaching, but also with increased reacted copper.

**Author Contributions:** Conceptualization and design, R.S., G.B., and A.U.; photostability testing, G.B.; black-stain testing, A.U. and S.K.; EPR analysis, J.N.R.R.; draft manuscript preparation, R.S. and G.B.; review and editing, R.S., G.B., A.U., S.K., and J.N.R.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project was financial supported by Natural Resources Canada under a contribution agreemen<sup>t</sup> with FPInnovations.

**Acknowledgments:** The authors thank Timber Specialties Co. for providing the micronized basic copper carbonate used in this experiment and Pierre Kennepohl (University of Calgary, formerly University of British Columbia) for providing EPR access. The technical support and guidance from Mathieu Gosselin, Ashley Hook, Paul Morris, Stéphane Thibeault and Daniel Wong is gratefully acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **Appendix A. Peroxide Treatment Optimization**

White spruce (*Picea glauca*) was cut into 1 × 10 × 50 mm<sup>3</sup> strips from sapwood and dip treated with a 1% solution of MBCC intended to simulate the surface of lumber pressure impregnated with a micronized copper containing preservative. These treated strips were used to examine the color change associated with oxidizing the copper when exposed to hydrogen peroxide. Initially a solution of 30% hydrogen peroxide was applied to dried copper treated spruce test strips, and a very dark brown color was produced, confirming that oxidation was possible. To produce the desired brown hue, testing was conducted to optimize hydrogen peroxide concentration and pH. Higher pH was obtained by addition of dilute sodium hydroxide. Lower pH was obtained by addition of dilute hydrochloric acid. Reaction time, drying method as well as influences from species and substrates were also explored.

The initial screening test looked at the effect of hydrogen peroxide concentration on color (Figure A1). A 10% solution (pH 6) resulted in an uneven light brown color. Treatments with 20% (pH 5) and 30% (pH 4) solutions resulted in a more uniform medium brown color.

**Figure A1.** The effect of hydrogen peroxide concentration on the color of spruce treated with MBCC (left to right: untreated, MBCC treated (no peroxide), MBCC-treated 10% peroxide at pH 6, MBCC treated 20% peroxide at pH 5, 30% peroxide at pH 4).

When treated with a 20% solution of hydrogen peroxide, pH 6 was found to best yield a medium brown color (Figure A2). Treatment with a 10% solution at varying pH levels resulted in only light brown coloration at all pH levels and an uneven tone across the samples.

**Figure A2.** The effect of peroxide solution pH of the color of spruce treated with MBCC (top 20% peroxide, left to right pH 4, pH 6 and pH 8; bottom 10% peroxide, left to right pH 4, pH 6, pH 8).

The effect of storage time was assessed on MBCC-treated samples exposed to 10% hydrogen peroxide (pH 6) (Figure A3). Samples were held wet at different time frames then air dried, left wet or oven dried at 100◦C before the hydrogen peroxide was applied. For hold time, a moderate browning was observed in all samples, suggesting that there was no improved color development with a longer storage time.

**Figure A3.** The effect of wet storage time on the color of spruce treated with MBCC and 10% peroxide (left to right 0 h, 1 h and 22 h hold before oven drying).

There was an impact in color development when different drying methods were employed. Little difference was observed between air drying and oven drying the samples before treating them with hydrogen peroxide, as similar colors were produced with the secondary treatment. However, it was noticed that the initial copper treatment washed off when samples were treated with peroxide while still wet. This was evidenced by the peroxide solution turning brown and the treated samples having a slightly muted brown color (Figure A4).

**Figure A4.** The effect of oven drying on the color of spruce treated with MBCC and 15% peroxide.

Samples of mixed sapwood and heartwood were treated with 20% peroxide at pH 6 and 8 (Figure A5). The heartwood/sapwood boundary was visually evident in the basic copper carbonate treated samples and remained visible after exposure to peroxide. However, both wood types turned a similar mid-brown color.

Based on these experiments, the optimum conditions were determined to be 20% hydrogen peroxide at pH 6 on dried copper treated samples, producing a consistent rich brown color across the samples. Wet storage time and substrate did not have major effects, while not drying samples before hydrogen peroxide treatments removed some of the copper treatment applied affecting the color produced.

**Figure A5.** The effect pine sapwood and heartwood on the color of wood treated with MBCC and exposed to 20% hydrogen peroxide at pH 8 (top) and pH 6 (bottom).
