Cleaning tarnished copper. A 1981-minted US penny (one cent) coin has been half-submerged into saturated citric acid in a Petri dish. After several hours, the coin was removed from the acid. Before 1982, US pennies were minted from brass (95% copper, 5% zinc). Brass coins tarnish or corrode over time. Tarnish appears as a dull, dense film on the surface. It protects the underlying metal from further corrosion. Tarnish consists of various oxides, sulfides, sulfates, etc. that form due to reactions with gases in the atmosphere as well as oils from human hands. A common way to clean copper (i.e. remove tarnish) is to treat it with an acid, as demonstrated here.