Tarnish is the common term for the dulling or darkening that occurs on the surface of silver objects, particularly jewelry. This discoloration is the result of a chemical reaction, which is naturally accelerated when the silver is in constant contact with the body. Tarnishing while being worn is a direct consequence of the metal’s inherent chemical reactivity combined with environmental exposure and the wearer’s physiological output.
The Basic Chemistry of Silver Tarnish
Silver tarnish is primarily a process called sulfidation, not oxidation. The dark layer that forms is silver sulfide (\(\text{Ag}_2\text{S}\)), which results from silver reacting with sulfur compounds in the air and on surfaces. The most common reactant is hydrogen sulfide (\(\text{H}_2\text{S}\)), a gas found in trace amounts in the atmosphere. The chemical reaction requires moisture and oxygen to proceed efficiently, forming the black silver sulfide layer.
Most silver jewelry is Sterling Silver, an alloy of 92.5% silver and 7.5% other metals, usually copper. Pure silver is resistant to sulfidation but is too soft for practical use in jewelry. The copper in the alloy is more reactive than the silver and contributes to tarnish by forming copper sulfide (\(\text{Cu}_2\text{S}\)) and undergoing some oxidation.
Because of the copper content, Sterling Silver tarnishes faster than pure silver. The resulting tarnish layer is a very thin surface film that changes how light reflects off the metal, causing the characteristic dull or black appearance.
Environmental Factors That Accelerate Tarnish
The air contains various sulfur-containing gases that constantly initiate tarnishing. Air pollution, particularly in urban or industrial areas, introduces higher concentrations of sulfur dioxide (\(\text{SO}_2\)) and hydrogen sulfide, accelerating the reaction. Higher humidity levels and warmer temperatures also speed up the formation of silver sulfide.
Silver also contacts aggressive sulfur sources during daily activities. Household cleaning products containing ammonia or chlorine bleach can rapidly corrode the metal. Specific foods, such as eggs, onions, and mustard, contain high levels of sulfur compounds that transfer onto the jewelry.
Materials used for storage or wear can also contain accelerating agents. Wool, felt, or common rubber products often contain sulfur compounds that transfer to the silver’s surface.
The Direct Influence of Body Chemistry
When silver jewelry is worn, the human body acts as a powerful catalyst, dramatically intensifying the sulfidation reaction. Sweat contains sodium chloride (salt) and organic acids, creating an electrolyte solution on the skin’s surface. This moisture and salt combination accelerates the degradation of the silver and the copper alloy.
The body also naturally excretes sulfur compounds through perspiration, derived from the breakdown of sulfur-containing amino acids in the diet. These compounds are directly deposited onto the silver, providing a concentrated source of the reactant needed to form silver sulfide.
Body temperature further contributes to the darkening of silver. Increased heat from the body or physical activity raises the temperature of the jewelry, accelerating all chemical reactions, including the formation of silver sulfide.
Individual variations in skin acidity, or pH level, also play a significant role. Some people have more acidic or alkaline sweat than others, which dramatically affects the metal’s reaction. Highly acidic skin tends to react more aggressively with the copper component of Sterling Silver, leading to faster discoloration.
Finally, the localized application of cosmetics, lotions, and perfumes introduces chemicals that directly contact the metal. Many beauty products contain compounds that react with silver, and residues from these products or sulfur-based medications excreted through the skin intensify the corrosive micro-environment.