Silver is one of the least reactive elements in the periodic table. At moderate temperatures, neither water or oxygen will cause silver to tarnish. Silver oxide may be formed at extreme temperatures, or from exposure to low levels of ozone.
The black finish that appears on silver items is a result of common atmospheric pollutants. Commonly called tarnish, this coating is silver sulfide, which is formed slowly as the silver is exposed to airborne sulfur compounds. These atmospheric pollution components are typically emitted by industrial processes and fossil fuel ignition.
Silver sulfide is one of the most insoluble salts, when exposed to solutions containing water, but common table salt is an element that will corrode silver and copper alloys. Alloys containing lower concentrations of silver will tarnish or corrode more easily, since metals mixed with silver may be subject to chemical reactions with oxygen at ordinary temperatures.
Removal of severe tarnish or corrosion may need to be accomplished by a professional, since some polishing compounds or techniques can damage valuable silver items.
