Silver is a noble metal, prized for its brilliant white luster and resistance to chemical attack under most conditions. However, it is not immune to tarnishing, a visible change over time. Silver objects, such as flatware and jewelry, eventually lose their shine and develop a darkened surface when exposed to the environment. This process is not simple oxidation but results from the metal’s unique chemical vulnerabilities.
The Chemical Process of Tarnishing
Silver’s darkening is sulfidation, not oxidation. This chemical reaction occurs when silver (Ag) reacts with airborne sulfur compounds, primarily hydrogen sulfide (H2S). The reaction forms a thin layer of silver sulfide (Ag2S) on the metal’s surface. Silver sulfide is a dense solid compound that appears dark gray or black, dulling the silver’s reflective surface. The initial tarnish may appear as a subtle yellow or brown film before progressing to the familiar black coating. This transformation happens even at very low concentrations of sulfur gas.
The speed of tarnishing is heavily influenced by the silver’s purity. Fine silver (99.9% pure) tarnishes slowly, but it is too soft for most functional items. Sterling silver, the standard for jewelry and tableware, is an alloy of 92.5% silver and 7.5% other metals, usually copper. The copper component readily reacts with sulfur, accelerating the formation of the visible tarnish layer.
Environmental Factors Driving Silver Degradation
The rate of silver sulfide formation is accelerated by several environmental and household factors. High relative humidity acts as a catalyst, increasing the speed of the chemical reaction between silver and sulfur compounds. Areas with high air pollution, such as industrial regions, contain higher concentrations of sulfur dioxide and hydrogen sulfide, which significantly hasten the process.
Common household materials and products are a primary source of the sulfur compounds that cause tarnishing. These include vulcanized rubber items, wool and felt fabrics, newsprint, and certain paints. Foods rich in sulfur, such as eggs, onions, and garlic, can also cause rapid tarnishing when contacting silver cutlery or serving dishes.
Direct contact with the skin and cosmetics also contributes to degradation. Natural oils, acids, and salts in human sweat transfer to the metal, creating a localized environment that encourages tarnishing. Perfumes, lotions, and hairsprays often contain sulfur compounds that speed up the darkening process, making jewelry worn daily particularly susceptible.
Strategies for Long-Term Preservation
Preserving silver involves creating a protective barrier against atmospheric sulfur compounds. Proper storage is a primary measure, starting with controlling the immediate environment. Silver should be stored in an area with stable, low humidity, ideally below 50% relative humidity, to slow the reaction rate.
Specialized anti-tarnish storage materials neutralize the airborne threat. Anti-tarnish bags and cloths are often impregnated with compounds like activated charcoal or zinc oxide that absorb sulfur gases. Wrapping items in acid-free tissue paper also helps, as this paper is pH neutral and free of chemicals that can initiate a reaction.
For display pieces, a clear protective coating or lacquer can be applied to the clean silver as a physical barrier. While effective at preventing contact with air, lacquers can yellow or crack over many years, requiring professional removal and reapplication. This method is generally avoided for silver used frequently, such as flatware.
Safe Restoration Techniques
When tarnish forms, two safe and effective restoration methods exist, each with different mechanisms.
Abrasive Polishing
The abrasive method involves using a gentle silver polish, a fine slurry containing a mild abrasive like calcium carbonate. This polish physically rubs away the layer of silver sulfide, revealing the bright silver underneath. Use only non-abrasive cloths and polishes specifically designed for silver. Overly aggressive scrubbing removes a tiny amount of the underlying silver metal with each cleaning. While short-term material loss is negligible, repeated polishing over decades can thin silver plating and wear down fine details.
Electrolytic Reduction
The second method is an electrolytic process using aluminum foil, baking soda, and hot water. When the tarnished silver touches the aluminum foil in this solution, a galvanic reaction occurs. The aluminum acts as a sacrificial metal, giving up electrons to reverse the chemical reaction. This converts the silver sulfide (Ag2S) back into metallic silver (Ag). This method is useful for intricately detailed pieces and does not remove any of the original silver metal.