Copper carbonate (often called copper carbonate or cupric carbonate) is a blue-green compound (chemical formula CuCO3) forming part of the grey-green patina one sees on weathered brass, bronze, and copper. The color can vary from bright blue to green, because there may be a mixture of both copper carbonate and basic copper carbonate in various stages of hydration. It was formerly much used as a pigment, and is still in use for artist’s colors.
Copper in moist air slowly acquires a dullish green coating because its top layer has oxidized with the air.
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