The dark discoloration that appears on silver, commonly known as tarnish, is a chemical compound that forms on the metal’s surface. This black coating is often mistaken for rust or corrosion, but it is a natural process that occurs when silver reacts with certain substances in the atmosphere. This signals a chemical change rather than a physical breakdown of the metal. The reaction is primarily driven by sulfur compounds, which creates the dull, darkened appearance.
The Chemical Reaction Behind Tarnish
The foundation of silver tarnish is a reaction between silver atoms and sulfur-containing gases in the air, most notably hydrogen sulfide. This chemical interaction forms a new substance called silver sulfide, which is black and adheres tightly to the metal’s surface. The resulting layer is extremely thin, but its dark color dulls the silver’s reflective shine.
Pure silver (99.9% silver) tarnishes at a much slower rate than common sterling silver. Sterling silver is an alloy typically composed of 92.5% silver and 7.5% other metals, usually copper. This copper content is the main accelerator of tarnishing, as copper is more reactive with sulfur and moisture than pure silver. The presence of copper leads to the formation of both silver sulfide and copper sulfide, making the alloy significantly more susceptible to discoloration.
Everyday Sources of Sulfur
The sulfur compounds responsible for this reaction are present in many common household and environmental sources. Hydrogen sulfide is a gas released by industrial emissions, vehicle exhaust, and the natural breakdown of organic matter. Indoors, certain materials and foods are major contributors to these tarnish-causing gases.
Items made of rubber, such as latex gloves, rubber bands, and foam padding, release sulfur compounds that can quickly darken silver. Certain foods, including eggs, onions, mustard, and mayonnaise, are also high in sulfur and can cause rapid tarnishing upon contact. Personal care products like wool, perfumes, lotions, and cosmetics contain trace sulfur that transfers to the metal during handling. The natural oils, salts, and acids from human skin accelerate the reaction when wearing silver jewelry.
Preventing Silver from Tarnishing
Slowing the rate of tarnish requires minimizing the silver’s exposure to both sulfur and moisture. The most effective method is to store silver items in an air-tight barrier, such as using flannel bags or sealed plastic containers, which limits contact with airborne pollutants.
Incorporating anti-tarnish strips or cloths into the storage area can further protect the metal. These materials are chemically treated to absorb sulfur compounds before they can react with the silver. Reducing humidity is also beneficial, as moisture acts as a catalyst for the chemical reaction. Silver should be removed before using cleaning products, applying makeup, or exercising, as these involve contact with reactive substances.
Safe Ways to Remove Black Tarnish
Once the black silver sulfide layer has formed, it can be safely removed using methods that either physically abrade the compound or chemically reverse the reaction. For light tarnish, a soft cloth treated with a mild silver polish can gently remove the surface layer. Polishing cloths are effective because they contain fine abrasives that physically buff away the silver sulfide.
A highly effective, non-abrasive method utilizes an electrochemical reaction to convert the tarnish back into silver metal. This process involves placing the silver in a container lined with aluminum foil and submerging it in a hot water solution containing baking soda and salt. The aluminum acts as a sacrificial metal, drawing the sulfur atoms away from the silver sulfide and restoring the silver’s luster. This chemical reduction method is useful for pieces with intricate detailing that are difficult to clean with polishing cloths.