Definition

• Sulfur Spot.  A small discoloration of tarnish on the surface of a coin or medal, the contaminate is sulfur. The spot is usually darker than the base metal of bronze or silver, sometimes with a black or green center. The sulfur contaminate combines with the surface metal forming a sulfate of the base metal (copper sulfate, silver sulfate) which may continue to spread. The spot should be removed professionally as quickly as possible by chasing (see refinishing). The term was formerly called carbon spot, but this was a misnomer.

  excerpted with permission from

  An Encyclopedia of Coin and Medal Technology

  For Artists, Makers, Collectors and Curators

  COMPILED AND WRITTEN BY D. WAYNE JOHNSON

  Roger W. Burdette, Editor

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