Sterling silver is a metal alloy composed of 92.5% pure silver and 7.5% other metals, typically copper, which is added to increase the hardness and durability. The presence of copper gives the alloy better strength than fine silver, making it ideal for jewelry and tableware. Sterling silver does not rust; instead, the surface of the metal undergoes a chemical change known as tarnishing.
Understanding Tarnish Versus Rust
The fundamental difference between rust and tarnish lies in the chemical process and the elements involved. Rust is the result of iron oxidation, a reaction requiring iron, oxygen, and moisture to form reddish-brown iron oxide. Since sterling silver contains no iron, it cannot rust.
Tarnish, conversely, is the formation of a thin, dark surface layer primarily composed of silver sulfide (Ag₂S). This reaction, known as sulfidation, occurs when silver atoms react with airborne sulfur compounds, such as hydrogen sulfide gas. The copper in the sterling silver alloy is actually more reactive to sulfur than the silver itself, often initiating the process by forming copper sulfide.
The resulting dark film, silver sulfide, is distinct from the flaking caused by rust. Tarnish is purely a surface phenomenon and does not cause the structural degradation or internal breakdown associated with iron oxidation. It is a cosmetic issue that can be reversed without permanently damaging the metal.
Factors That Accelerate Silver Tarnish
The rate at which sterling silver tarnishes depends heavily on its surrounding environment and exposure to specific sulfur-containing agents. High humidity increases the likelihood of sulfidation by providing a medium for chemical reactions on the metal surface. Air pollution, especially in urban or industrial environments, often contains high concentrations of sulfur dioxide and hydrogen sulfide gases that fuel the tarnishing process.
Direct contact with certain household items can also accelerate the darkening of the metal. Materials like wool, felt, latex paint, and rubber bands contain sulfur compounds that quickly transfer to the silver. Personal chemistry plays a role, as acidic skin oils, makeup, and perspiration contain sulfur and chloride compounds that readily react with the alloy’s copper content.
Exposure to household cleaning products, such as those containing bleach or chlorine, will also speed up the formation of silver sulfide. These substances create an aggressive chemical environment that significantly lowers the energy needed for the sulfidation reaction to proceed rapidly. Even brief contact can lead to noticeable tarnish.
Best Practices for Preventing Tarnish
Preventing tarnish involves minimizing the exposure of sterling silver to the sulfur compounds and moisture that drive the reaction. Proper storage is the most effective proactive measure, meaning items should be kept in an airtight container or zip-top plastic bag when not being worn. Reducing air circulation limits the availability of airborne sulfur gases needed for sulfidation.
Placing anti-tarnish strips or specialized silver cloth inside the storage container helps by absorbing sulfur gases before they reach the metal surface. These strips typically contain activated carbon or other compounds that chemically scavenge the reactants from the air. Before storing, gently wipe the silver with a soft cloth to remove any residual skin oils or perspiration.
It is advisable to remove silver jewelry before engaging in activities like showering, swimming in chlorinated water, or using household cleaning chemicals. Allowing lotions, perfumes, or hairspray to dry completely before putting on silver also reduces the transfer of reactive chemicals to the metal surface. This routine care significantly slows the rate of tarnish formation, minimizing the need for aggressive cleaning later.
Methods for Cleaning Tarnished Silver
Once the dark silver sulfide layer has formed, several methods can be used to remove it and restore the metal’s luster. For light tarnish, a professional silver polishing cloth, often infused with a mild chemical cleaner, can physically rub off the surface layer. Silver dips offer a chemical solution that dissolves the silver sulfide, but they should be used sparingly as they can be harsh and may remove small amounts of the underlying silver metal over time.
A highly effective, non-abrasive method uses a simple electrochemical reaction involving aluminum foil and baking soda. This process requires lining a glass dish with aluminum foil, adding hot water and baking soda, and then immersing the silver item so it touches the foil. The aluminum acts as a sacrificial anode, meaning it is more reactive to the sulfur than the silver sulfide.
In the presence of the baking soda electrolyte, the sulfur atoms transfer from the silver sulfide to the aluminum, reducing the silver sulfide back into metallic silver. This method removes only the tarnish layer without physically abrading the silver surface, making it gentler than using polishing compounds. After treatment, the item should be rinsed in clean water and dried thoroughly with a soft cloth to prevent water spots.