![]() ![]() Nickel together with a small amount of cobalt was used for decolorizing of lead glass. Lead crystal with added nickel acquires purplish color. Nickel, depending on the concentration, produces blue, or violet, or even black glass.2 to 3% of copper oxide produces a turquoise color.Very small amounts can be used for decolorizing. The best results are achieved when using glass containing potash. Small concentrations of cobalt (0.025 to 0.1%) yield blue glass.Details of the process and the composition of the glass vary and so do the results, because it is not a simple matter to obtain or produce properly controlled specimens. This process is widely confused with the formation of "desert amethyst glass", in which glass exposed to desert sunshine with a high ultraviolet component develops a delicate violet tint. In New England some houses built more than 300 years ago have window glass which is lightly tinted violet because of this chemical change and such glass panes are prized as antiques. Manganese dioxide, which is black, is used to remove the green color from the glass in a very slow process this is converted to sodium permanganate, a dark purple compound. ![]() Manganese is one of the oldest glass additives, and purple manganese glass was used since early Egyptian history.
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