Cyanide is used in “bombing” to give gold pieces a thin layer of pure gold. Any older jewelry text would detail the process…IDK if it is used that much anymore or not, as I don’t work in gold. Basically you put the jewelry into a solution of potassium cyanide and then pour in some peroxide and this leads to an effusion of cyanide gas and a boilover, hence the name bombing. The process actually removes some gold and leaves a clean, pure gold layer. This should be done under a fume hood or outside, as cyanide gas is released. -royjohn
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