rare coins,copper piping, whatever else?
Exposure to chlorine, even if in the form of chlorides (like salt) causes coper to develop to develop a green tarnish. The Statue of Liberty has such a patina, for instance.
Copper is attacked by weak acids (verdigris) and sulfur compounds (tarnish), but after lengthy intervals the copper patina forms a second coating considered to be a mark of genuineness of age, which should not be removed in the normal state.
