Sterling silver is an alloy used for its strength and durability in jewelry and tableware. It contains 92.5% pure silver and 7.5% of other metals, typically copper. This composition, often marked as 925, makes the metal hard enough for daily use, which pure silver is not. Tarnish, the dull, dark coating that develops on the surface, is a visible sign of a chemical reaction. This reaction is accelerated because the copper alloy is far more reactive to atmospheric elements than pure silver would be.
The Role of Sulfur and Hydrogen Sulfide
The chemical process that results in tarnish requires sulfur-containing compounds, which are the primary corrosive agents. Silver is vulnerable to a specific reaction called sulfidation, making sulfur the necessary ingredient. The most common form of this reactant is hydrogen sulfide (H2S) gas, which exists ubiquitously in the atmosphere.
Hydrogen sulfide is a colorless gas often associated with the smell of rotten eggs, but it causes tarnishing even at concentrations too low to detect. It is capable of causing tarnish when present in the air at concentrations as minute as parts per billion (ppb). Even in clean environments like museums, hydrogen sulfide is present at 86 to 600 parts per trillion (ppt).
The Formation of Silver Sulfide
When silver atoms encounter sulfur atoms on the metal surface, the tarnish compound is created. This process, known as oxidative sulfidation, is a chemical reaction that occurs even at room temperature. The silver reacts with hydrogen sulfide and oxygen in the air to form silver sulfide and water.
The resulting compound, silver sulfide (Ag2S), is a dense, non-soluble solid. This chemical transformation is distinct from classic oxidation, like the rusting of iron, because the primary driver is sulfur, not oxygen. The silver sulfide forms a thin, crystalline layer on the metal surface.
This layer is characterized by its dark appearance, progressing from a pale yellow or straw color to iridescent purple, and finally to the familiar deep black or dark brown coating. Because silver sulfide is less reflective than the underlying metal, the surface loses its bright, metallic luster. Once formed, this layer is the physical manifestation of tarnish.
Everyday Environmental Accelerants
While the chemical reaction is constant, the rate at which tarnish forms is increased by various common environmental factors. High humidity and moisture provide a medium that facilitates the chemical exchange between the silver and atmospheric sulfur. The presence of water vapor speeds up the sulfidation process.
Sulfur-containing compounds are abundant in many household items and foods that can directly accelerate tarnishing.
- Foods such as onions, mustard, and eggs.
- Materials like wool, felt, and vulcanized rubber (e.g., rubber bands or latex gloves).
Air pollutants, such as sulfur dioxide from industrial emissions and vehicle exhaust, contribute to sulfur compounds. Direct contact with the skin is another accelerator, as natural body oils, sweat, and acidity contain trace sulfur compounds. Exposure to household chemicals, including cleaners (chlorine or ammonia) and personal care products (perfumes and lotions), can also exacerbate the reaction.