Silver is the most reactive element among the noble metals and it tarnishes readily on exposure to indoor and outdoor atmospheres containing sulphur compounds. Tarnishing can be defined as "the occurrence of one or more chemical processes at the surface of silver causing darkening of the metal and rendering it aesthetically unacceptable". The main source of aggressive sulphur compounds are the combustion products of sulphur bearing fuels. These prevail in industrial atmospheres but also in urban atmospheres where they are emitted by automobiles. As little as one volume of hydrogen sulphide in 600 million volumes of air will produce visible tarnishing on silver.

Author: L. Gal-Or