Tarnish is a surface phenomenon appearing as a dull, dark film on metal surfaces, such as jewelry or silverware. This discoloration is a thin layer of corrosion that forms when the metal reacts with elements in the surrounding environment. Unlike rust, which affects iron and causes the metal to flake away, tarnish is typically a self-limiting chemical change that only affects the outermost layers of the metal. Although tarnish reduces the jewelry’s luster, it is a natural and often reversible process.
The Chemical Process of Tarnish Formation
Tarnish formation is a chemical reaction distinct from the deep oxidation that causes rust, though it is sometimes broadly referred to as oxidation. For most jewelry metals, the primary culprit is not oxygen alone but airborne sulfur compounds, a process known as sulfidation. These sulfur compounds, such as hydrogen sulfide, react with the metal atoms on the surface. The reaction creates a new chemical compound, a metal sulfide, which is the dark film seen on the jewelry. For example, when silver reacts with hydrogen sulfide, the resulting black layer is silver sulfide (Ag2S). Sulfur is the far more reactive element that triggers rapid discoloration on silver and many alloys, differentiating the dark tarnish on jewelry from the reddish rust on iron.
Susceptibility of Common Jewelry Metals
The speed of tarnishing depends heavily on the chemical properties and composition of the metal. Silver is notorious for tarnishing because it has a strong affinity for sulfur, making it highly reactive to airborne sulfur compounds. Sterling silver (92.5% silver and 7.5% copper) tarnishes faster than pure silver because the copper component readily reacts with sulfur and oxygen.
Metals like copper and brass (an alloy of copper and zinc) also tarnish rapidly due to their high reactivity. Copper reacts with oxygen and moisture to form copper oxide, often developing a greenish patina.
In contrast, gold is highly resistant to tarnishing because it is one of the least reactive, or “noble,” metals. Pure 24-karat gold will not tarnish. However, gold jewelry is typically alloyed with metals like silver and copper to increase hardness. The lower the karat weight (e.g., 10K or 14K), the higher the percentage of reactive base metals, making the piece more susceptible to tarnish. Plating, such as rhodium, is often used on white gold and silver to provide a tarnish-resistant barrier, though this layer can wear off.
Environmental Factors That Accelerate Tarnish
Several external factors accelerate the chemical reactions that cause tarnish. High humidity and moisture provide an ideal medium for sulfur compounds and other corrosive agents to adhere to and react with the metal surface. Moisture encourages the necessary chemical exchange to occur.
Body chemistry is a significant accelerant, particularly perspiration. Sweat contains chlorides (salt) and sulfur compounds that react directly with the metal surface, causing rapid discoloration. Jewelry worn daily often tarnishes faster than pieces kept in storage.
Household chemicals and personal care products also contribute to surface breakdown. Direct contact with cleaning agents, especially those containing bleach or ammonia, can cause immediate tarnishing. Sulfur compounds in cosmetics like perfumes, lotions, and hairsprays can transfer to the jewelry and speed up the sulfidation process.
Practical Steps for Tarnish Prevention
Understanding the causes of tarnish allows for the implementation of specific prevention strategies. Since airborne sulfur and moisture are the main drivers of the reaction, proper storage is the first line of defense. Jewelry should be kept in a cool, dry place, ideally in airtight containers or sealed plastic bags to limit exposure to atmospheric compounds.
Anti-tarnish strips or cloths can be incorporated into storage containers to further slow the process. These strips absorb sulfur gases, scavenging corrosive elements in the enclosed space before they react with the metal.
A simple routine is to wipe jewelry with a soft, clean cloth after each wear. This removes skin oils, sweat, and residue from cosmetics, preventing corrosive compounds from initiating the chemical reaction.
It is also recommended to remove jewelry before engaging in activities like swimming, showering, house cleaning, or applying personal care products, as these actions introduce primary accelerants directly to the metal.