Bleaching of Mineral-Stained Maple–II

Mineral-stained hard maple was treated with bleaching agents under various conditions in an attempt to blend the colors of the stained areas, the darker inner wood (heartwood) of the tree and the light-colored clear sapwood. Hypochlorite solutions at pH values of 4 to 7 and sodium dichloroisocyanurate at pH around 6 darkened the light-colored wood and lightened the color of some mineral stain so that it was barely visible against the darkened sapwood. At pH 9 hypochlorites bleached the darker- and lighter-colored wood to a greyish color. However, in all these cases the very heavy black mineral stain still remained clearly visible. Reducing agents, in general, were not very effective in masking or bleaching mineral stain. However, sodium hydrosulfite did reduce the color intensity of the lighter-colored mineral stain and the heartwood without changing the clear sapwood to any noticeable extent. Sodium chlorite solutions (5-10 percent), applied to wood which had been acidified with dilute acetic acid, preferentially bleached the darker wood. The degree of bleaching could be varied depending on the number of treatments and on the concentration of the chlorite solutions. Even dark mineral stain could often be bleached to the stage at which it was barely visible. Usually one hour was sufficient for the treatments. Combinations of treatments with sodium chlorite, sodium dichloroisocyanurate, and hypochlorites also gave good results in blending the colors of sapwood, heartwood, and mineral stain of hard maple.

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