Sterling silver, commonly identified by the “925” stamp, is an alloy composed of 92.5% pure silver and 7.5% other metals, most frequently copper. The silver component typically discolors to a dark hue, but the copper alloy can produce a greenish tint under specific conditions. This discoloration is not a sign of poor quality but the expected result of chemical reactions between the metal and the environment. Understanding the difference between the usual black tarnish and the occasional green discoloration depends on which metal in the alloy is reacting.
Composition and the True Color of Tarnish
The standard composition of sterling silver uses copper as the hardening agent. Pure silver is too soft for practical jewelry, so the addition of copper provides the necessary strength and durability for everyday wear. This small percentage of copper makes the alloy susceptible to various chemical reactions.
The most common form of discoloration is tarnishing, which results in black, brown, or dark gray hues. Tarnish is a chemical reaction called sulfidation, occurring when silver atoms react with sulfur compounds in the air. This forms a thin, surface layer of silver sulfide.
Sulfur compounds are naturally occurring in the atmosphere and in common household items. The resulting silver sulfide layer eventually darkens to the familiar black film. This darkening process is a surface-level change that does not structurally damage the metal, but it does obscure its luster.
The Copper Connection: Why Green Discoloration Occurs
While tarnish results in a dark color, green discoloration stems directly from the copper content in the alloy. This greenish color is the result of oxidation, a separate chemical process accelerated by contact with moisture and certain chemicals. When copper reacts with oxygen, moisture, and acids, it forms copper salts.
These copper salts, such as copper chloride or copper carbonate, are naturally blue-green and are responsible for the patina seen on bronze statues. When sterling silver jewelry, particularly rings, contacts human sweat, the acidity and moisture create an ideal environment for copper oxidation. The resulting green residue can deposit onto the skin, leaving a temporary, harmless stain.
The likelihood of this green discoloration increases with high humidity, excessive sweating, or the use of products like lotions, perfumes, or cleaning agents. The more acidic a person’s sweat is, the faster the copper will oxidize and produce this residue. This greening effect is a superficial reaction of the copper alloy.
Preventing and Reversing Discoloration
Preventing discoloration involves minimizing exposure to reactive agents. Apply perfumes, lotions, and hairspray before putting on jewelry, ensuring the products have fully dried. Removing silver pieces before swimming, showering, or strenuous activity that causes heavy sweating also slows the tarnishing and greening processes.
Proper storage is an effective preventive measure against both sulfidation and oxidation. Storing jewelry in an airtight bag or container reduces exposure to atmospheric sulfur and moisture. Anti-tarnish strips can be placed in the container to absorb residual sulfur compounds.
To reverse discoloration, the method depends on the residue type. Black tarnish (silver sulfide) can often be removed with a soft polishing cloth. For heavier tarnish, a home method uses hot water, baking soda, and aluminum foil to transfer the sulfur from the silver. Green copper residue is removed effectively with mild soap and water, followed by thorough drying.