Pure silver (fine silver, 99.9% silver) does not tarnish easily, but it is not entirely immune. The popular misconception that all silver tarnishes rapidly is due to the widespread use of silver alloys, which are purposefully mixed with other metals to increase their hardness. This alloying process, while making the metal suitable for durable objects like jewelry and flatware, introduces a highly reactive component that drastically accelerates the discoloration process.
The Chemistry of Tarnish
Tarnish is a form of corrosion that appears as a yellow, brown, or black film on silver, but it is fundamentally different from rust or simple oxidation. Unlike iron, silver does not readily react with oxygen in the air. Instead, the discoloration is primarily a sulfidation reaction caused by airborne sulfur compounds.
The most common sulfur compound responsible is hydrogen sulfide (\(text{H}_2text{S}\)), a gas naturally present in the atmosphere. When silver (\(text{Ag}\)) reacts with hydrogen sulfide, a thin, dark layer of silver sulfide (\(text{Ag}_2text{S}\)) forms on the surface. This compound is the tarnish, and its presence dulls the metal’s reflective luster. The reaction is accelerated by the presence of moisture and is represented by the formula \(2text{Ag} + text{H}_2text{S} + text{O}_2 rightarrow text{Ag}_2text{S} + text{H}_2text{O}\).
Why Purity Matters
The rate at which silver tarnishes is directly related to its purity, which is why fine silver (99.9%) tarnishes far slower than common alloys. Fine silver is too soft for practical applications, so it is typically alloyed with other metals to improve its strength and rigidity. Sterling silver, for instance, is the most common alloy, consisting of 92.5% pure silver and 7.5% of another metal, most often copper.
Copper is significantly more reactive to sulfur compounds and moisture than silver itself, forming copper sulfide (\(text{Cu}_2text{S}\)) alongside the silver sulfide. This added reaction exponentially increases the speed and severity of the tarnish layer, requiring sterling silver pieces to be cleaned much more frequently than pure silver. The tarnish on fine silver is almost exclusively silver sulfide, which builds up so slowly that it is often imperceptible under normal conditions.
Common Causes of Accelerated Tarnish
Beyond the metal’s composition, several environmental factors can dramatically speed up the sulfidation reaction for all silver types. High humidity and elevated temperatures are two major atmospheric accelerators, as moisture is a necessary component for the chemical reaction to proceed efficiently. Industrial air pollution, which releases higher concentrations of sulfur-containing gases, can also cause silver to tarnish rapidly.
Contact with common household items containing sulfur is another frequent cause of discoloration, including materials like rubber, latex paint, wool, and certain types of paper products. Even human body chemistry plays a role, as acidic perspiration and natural skin oils can contain sulfur compounds that react directly with the silver when worn as jewelry. Foods like eggs, onions, and mustard also release sulfur as they degrade, making silver flatware and serving dishes particularly susceptible to rapid tarnish.
Preventing and Removing Tarnish
The most effective way to prevent tarnish is to limit the silver’s exposure to airborne sulfur and moisture. Proper storage involves keeping silver items in a cool, dry place, ideally in airtight plastic bags or specialized anti-tarnish cloth pouches. These pouches are often treated with chemicals that absorb sulfur gases, creating a protective micro-environment around the metal.
When tarnish does form, it can be removed using one of two primary methods. The least abrasive method is the electrochemical process, which uses aluminum foil, baking soda, and hot water. In this reaction, the aluminum acts as a sacrificial metal, chemically reversing the sulfidation by transferring the sulfur from the silver sulfide back to the aluminum. A simpler, more common technique is using a gentle silver polishing cloth, which employs a mild abrasive to physically buff away the layer of silver sulfide without chemically altering the tarnish.