The Columbia Encyclopedia, Sixth Edition. 2001-07.
patina
(pt´n) (KEY) , coating of carbonate of copper on articles of copper or bronze, formed after long exposure to a moist atmosphere or burial in the earth. Although commonly green, patina varies in color and consistency; it may be red, brown, black, blue, or gray, or it may be smooth, glossy, or crusty. It may be imitated by a number of oxidation processes. The term has been extended to include the film formed on metals, pottery, marble, and other materials by exposure and to the mellow surface acquired by furniture with time and waxing.
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