Sterling silver is an alloy, a mixture of metals created to enhance durability. It is defined by a standard composition containing 92.5% pure silver and 7.5% of another metal, most often copper. Pure silver is too soft for practical items like jewelry or flatware, so the addition of copper provides the necessary strength and hardness.
Sterling silver cannot rust because it lacks the necessary chemical element. Rusting is a unique type of corrosion reserved exclusively for iron and its alloys, such as steel. This process involves iron reacting with oxygen and water to form hydrated iron(III) oxide, the familiar flaky, reddish-brown substance. Since sterling silver contains no iron, it is chemically exempt from this specific corrosion process.
While sterling silver does not rust, it is susceptible to other chemical reactions involving oxidation. Corrosion is the general term for a metal’s degradation back to a more stable chemical state. Silver is a relatively noble metal, meaning it is less reactive than many others, but it is not completely inert like gold or platinum. The 7.5% copper in the alloy is more chemically active and can slowly react with oxygen to form copper oxide.
The Chemical Cause of Sterling Silver Tarnish
The degradation process that affects sterling silver is known as tarnishing, which produces a chemically distinct compound from rust. Tarnishing occurs when the metal surface reacts with trace amounts of sulfur-containing gases in the air, rather than directly with oxygen and water. The primary culprit is hydrogen sulfide, which is present everywhere in the atmosphere. Sources include air pollution, industrial emissions, and common household materials.
The reaction between silver and atmospheric sulfur compounds creates a thin layer of silver sulfide on the metal’s surface. This compound is black, causing the silver to gradually darken, appearing yellow, then brown, and eventually black as the layer thickens. This process is a chemical attack where silver atoms bond with sulfur atoms to form a new, less reflective substance. Crucially, this chemical coating is purely superficial and does not penetrate the underlying metal.
Although the copper in the alloy can also react with sulfur to form copper sulfide, the silver sulfide layer causes the overwhelming majority of the discoloration. The rate at which this coating forms depends directly on the concentration of sulfur gases and the presence of moisture. High humidity and exposure to certain chemicals can significantly accelerate the formation of the silver sulfide layer.
Practical Care and Tarnish Prevention
Because tarnishing is a surface chemical reaction driven by sulfur compounds, prevention focuses on limiting exposure to these environmental factors. Proper storage in a cool, dry location with limited air circulation is essential. Storing pieces in airtight bags or containers with anti-tarnish strips or cloths can absorb sulfur and moisture. It is also advisable to avoid storing sterling silver near materials that off-gas sulfur, such as rubber, wool, or painted surfaces.
Certain personal care and household items can also contribute to tarnishing. It is best practice to put silver jewelry on last, after applying cosmetics, perfumes, or lotions. Since moisture is a catalyst for the chemical reaction, remove silver before swimming, showering, or engaging in activities that cause heavy perspiration. Regular wear can sometimes slow the process, as the gentle friction from skin and clothing polishes away nascent silver sulfide before it becomes noticeable.
When the dark silver sulfide layer forms, there are two primary methods for reversal. The simplest approach uses a polishing cloth, which contains fine abrasives and chemicals designed to mechanically rub off the silver sulfide coating. A non-abrasive method involves an ionic cleaning process using aluminum foil, hot water, and baking soda. This method utilizes a chemical reduction reaction to convert the black silver sulfide back into metallic silver, reversing the tarnishing without removing any underlying metal.