CCA Retention and Disproportioning in White Spruce

Eastern white spruce (Picea glauca (Moench) Voss) heartwood, flat-sawn, and 2 by 6 inches in cross section, at moisture content levels of 54 (green), 25, 20, and 17 percent, was subjected to a dynamic transverse compression treatment prior to impregnation with a type “C” copper-chrome-arsenic solution. Deformation levels were 12.5, 15, and 17.5 percent of the green thickness. Compared to non-compressed joists at 25 percent MC, a combination of drying to 17 percent MC followed by 17.5 percent compression increased total retention by 210 percent (from 0.20 to 0.62 pcf). At a radial depth of 3/16-inch, average retention (based on the metallic oxides) in material at 17 percent MC was increased from 0.23 to 0.63 pcf by the compression treatment. Drying to 17 percent MC followed by 17.5 percent dynamic compression increased minimum depth of tangential penetration from 0.02 inch to 0.09 inch. Drying alone (from 25 to 17 percent MC ) increased the minimum radial penetration from 0.04 inches to only 0.06 inches, whereas drying with compression increased minimum radial penetration to 0.14 inch. Though this increased minimum depth of penetration is still substantially below the requirements of current preservation standards, it suggests that the combined drying-and-compression treatment would be ideal for spruce that requires a heavy, shallow impregnation of fire retardant chemicals. Analysis of retention at various depths showed a disproportioning of copper, chromium, and arsenic with depth of penetration. The arsenic content decreased in relation to both copper and chromium as depth of penetration increased; at a depth of 1/4 inch, the arsenic portion was only 20 percent compared to about 35 percent at the surface.

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