Real silver (Ag) does not rust in the way that iron-based metals do. Rusting is a term reserved for the corrosion of iron, involving a reaction with oxygen and moisture to form hydrated iron oxide, a red-brown, flaky material. Silver is a noble metal, meaning it is resistant to oxidation, but it is not immune to environmental attack. The dull, dark coating that appears on silver is the result of a separate chemical process called tarnishing.
Rust vs. Tarnish: The Chemistry of Corrosion
The fundamental difference between rusting and tarnishing lies in the chemical agents involved. Rust is a form of oxidation where iron reacts with oxygen and water to produce iron oxide, which is structurally weak and continues to flake away, exposing fresh metal to the environment. This ongoing process can eventually destroy the entire metal object.
Silver tarnish, however, is not primarily an oxidation reaction with oxygen. Instead, it is a sulfidation reaction, where the silver surface reacts with sulfur-containing compounds in the atmosphere, such as hydrogen sulfide. This reaction creates a thin layer of silver sulfide, which is the dark substance that coats the metal. The resulting layer of silver sulfide is generally non-porous and adheres tightly to the surface, which is why tarnishing only affects the appearance of the metal and does not typically lead to structural damage.
Identifying and Understanding Silver Tarnish
The visual progression of tarnish begins subtly, often appearing as a slight yellowish or golden sheen on the silver surface. As the layer of silver sulfide thickens, the color advances through shades of brown and eventually becomes the characteristic dark gray or black coating. The presence of moisture and airborne pollutants accelerates this chemical reaction.
Tarnishing is caused by common environmental culprits that release trace amounts of sulfur compounds. Hydrogen sulfide gas, a primary agent, is a component of air pollution, but it is also released from everyday items. This includes rubber products, wool, certain paints, and foods like eggs, onions, and mustard. Additionally, natural oils and sweat from human skin promote tarnish, which is why silver jewelry worn frequently often discolors faster.
The composition of the silver itself influences the rate of tarnish. Pure silver (99.9%) is resistant to tarnish but is too soft for most functional items. Sterling silver, the most common alloy, is 92.5% silver and 7.5% other metals, usually copper. Since the copper component is more reactive to sulfur and oxygen than the silver, the alloy is more prone to tarnishing than its pure counterpart.
Protecting Silver from Tarnish
Minimizing exposure to sulfur and moisture is the most effective way to slow the tarnishing process. Silver items should be stored in a cool, dry place, ideally sealed in airtight containers or zip-top plastic bags. Anti-tarnish strips, which are chemically treated papers that absorb airborne sulfur, can be placed inside storage containers for added protection. Reducing the humidity is also beneficial, and desiccants like silica gel packets can help achieve this.
For items that have already begun to tarnish, gentle cleaning methods are preferred to remove the silver sulfide layer without causing damage. Specialized silver polishes are formulated to safely remove tarnish, but they should be used sparingly as they remove a microscopic layer of silver with each application. A less abrasive technique involves a chemical reduction method using a mild solution, such as a bath with baking soda and aluminum foil, which reverses the sulfidation reaction. Always ensure silver is completely dry before storing it to prevent trapped moisture from accelerating future tarnish.