A thin, greenish film or discoloration that forms, after long exposure to the atmosphere, on bronze and copper, sometimes on jewelry of such metals, adding a prized artistic effect. The natural patinais a carbonate of copper that forms to protect the metal from further oxidation. By extension of the term, it has been applied to a reddish patina-effect on ancient gold jewelry and to the so-calledpatina on silver that is a surface blur resulting from numerous shallow scratches. An artificial patina can be produced with acids.