Verdigris. A natural green or bluish deposit formed on copper and copper alloys including bronze. A uniform natural verdigris covering the entire surface is extremely desirable on ancient coins, less so if the color is unattractive or covering only part of the coin. In nature this patina, called incrusted patina, is formed by exposure to moist atmosphere and the formation of copper carbonate takes place. Such a surface deposit precludes further oxidation, toning or tarnish from occurring. Buried coins or bronze objects frequently form this patina when buried in moist soil. In modern times similar green or blue color patinas can be formed chemically by a treatment of acids and heat on any copper or bronze object. See finish and finishing.
excerpted with permission from
For Artists, Makers, Collectors and Curators
COMPILED AND WRITTEN BY D. WAYNE JOHNSON
Roger W. Burdette, Editor