Carbon Spot. A small discoloration of tarnish on the surface of a coin or medal; actually a misnomer, since the contaminate is sulfur rather than carbon. 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). A more accurate term would be sulfur spot.