When silver objects begin to darken, this discoloration, commonly known as tarnish, is a natural chemical process occurring on the metal’s surface. Tarnishing is distinct from rusting, which is the oxidation of iron, and it does not destroy the silver itself. Understanding this chemical change explains why silver items lose their luster and provides the knowledge necessary to slow the process.
The Chemistry of Silver Tarnishing
The darkening of silver is caused by sulfidation, not primarily by a reaction with oxygen. Silver reacts readily with trace amounts of sulfur-containing compounds present in the air, such as hydrogen sulfide gas. This chemical interaction causes surface silver atoms to bond with the sulfur, forming a new compound.
The resulting black film is silver sulfide, which adheres tightly to the surface. This extremely thin layer gives the metal a dull, blackened, or yellowish appearance. Since the reaction is continuous, the silver sulfide layer gradually builds up as long as the silver is exposed to sulfur sources. Because this process only affects the surface, the underlying metal remains structurally sound.
Environmental Contributors to Blackening
Several common elements significantly accelerate the sulfidation process. High humidity and moisture speed up the reaction between silver and airborne sulfur compounds. This is why silver stored in damp areas, like bathrooms or basements, tarnishes more quickly than items kept in dry conditions.
Sulfur sources are found in many unexpected places, including air pollution from industrial emissions or vehicle exhaust. Certain household materials and foods also contain sufficient sulfur to accelerate the darkening process.
Common Sulfur Sources
Exposure to the following can cause or accelerate tarnishing:
- Foods high in sulfur, such as eggs, onions, and mustard, especially when in direct contact with serving dishes.
- Household materials like rubber bands, wool, felt, and newspaper ink.
- Natural oils, acids, and salts secreted through human skin, particularly on jewelry.
- Chlorine, found in swimming pool water or household bleach, which forms a black silver chloride layer.
Maintaining Silver and Slowing Tarnish
Since the reaction requires exposure to sulfur and is accelerated by moisture, preventative measures involve minimizing contact with these elements. Storing silver in a cool, dry location, such as interior drawers or lined china cabinets, reduces the rate of tarnishing. Avoid areas with fluctuating temperatures or high moisture, like kitchens or sunlit windows.
For long-term storage, placing items in an airtight container or sealed polyethylene bag limits exposure to ambient air and its sulfur content. Anti-tarnish cloths or strips, which absorb sulfur-containing gases, can be included for extra protection. Adding small packets of silica gel or a piece of chalk helps absorb residual moisture within the sealed environment. Wrap each piece individually using acid-free tissue or anti-tarnish fabric to prevent scratching and reduce surface exposure.