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• Firestain •

"Paul Revere, Jr.: Hot-water urn (1990.226a-d)"
In Timeline of Art History
New York: The Metropolitan Museum of Art
http://www.metmuseum.org/toah/hd/rvre/ho_1990.226a-d.htm
(September 2008)

Firestain is oxidized copper (shown as the blotchy areas on the urn above) in solid silver alloys that can be found on many pre-colonial through early twentieth century pieces. If, after cleaning your silver (not silverplate) piece, a purplish stain remains, do not mistake this stain (cuprous oxide) for tarnish! Attempting to remove it will only damage your prized piece. It is not generally seen on pieces that have been produced by the large American silver companies after the 1930s, but many one-person silversmithing shops still use this technique. I will not get into the technicalities of firestain here, except to say that the stain is usually obscured with fine silver either by silver plating the object or through a process called depletion. This is accomplished with heat and acid that dissolves the upper layer of copper in the alloy. The firestain under this fine silver layer, which may be a few thousandths of an inch thick, may not show up until after many years of polishing.

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