Tarnish is a chemical transformation that occurs on the surface of metals, causing them to darken and lose their reflective shine. It is a distinct form of corrosion that results in a dull, discolored film. Unlike rust, which is often porous and can flake away to expose more metal, tarnish is typically a thin, self-limiting layer of a metallic compound. It is a natural reaction that happens when certain metals are exposed to non-metal compounds in the environment.
The Chemistry Behind Tarnish
The core of jewelry tarnish is a chemical reaction involving a metal and a non-metal compound, most frequently sulfur or oxygen. This reaction involves the transfer of electrons from the metal atoms to the non-metal atoms, a process known as oxidation.
For most common jewelry metals, the primary culprit is not oxygen but sulfur, a process called sulfidation. Sulfur-containing gases, like hydrogen sulfide in the air, react with the metal atoms on the surface to form a metal sulfide. This sulfide compound settles on the jewelry as a thin layer, dulling the metal’s natural luster and often appearing as a dark film. Because tarnish only affects the very top layers of the metal, it is considered a surface phenomenon that can be removed without significant damage.
Common Metals and Their Specific Reactions
Silver is the metal most famously associated with tarnish. Sterling silver, which is 92.5% silver and 7.5% copper, is particularly vulnerable to sulfidation. The reaction forms silver sulfide, which manifests as a distinct black surface discoloration.
Copper and brass, a copper-zinc alloy, tarnish through a combination of oxidation and sulfidation. Copper initially reacts with oxygen to form a black or reddish-brown copper oxide. Over longer periods, in the presence of moisture and carbon dioxide, this oxide transforms into copper carbonates, creating the familiar blue-green layer known as patina or verdigris.
Pure gold, specifically 24 karat gold, does not tarnish because it is a noble metal that is chemically inert and does not readily react with oxygen or sulfur compounds. However, gold jewelry is almost always an alloy, mixed with less noble metals like silver and copper to increase hardness and durability. These base metals within the alloy are the parts that react with the environment, causing lower-karat gold, such as 10k or 14k, to exhibit tarnish over time.
Environmental Factors That Accelerate Tarnish
Certain environmental elements dramatically speed up the chemical reactions that lead to tarnish. High humidity and moisture provide an electrolyte solution on the metal’s surface, which facilitates the electron transfer necessary for chemical changes to occur. Air pollution, especially in urban or industrial areas, contains elevated levels of sulfur dioxide and hydrogen sulfide gas that are highly reactive with silver and copper.
Common household and personal products also act as accelerants due to their chemical content. Substances such as perfumes, hairsprays, lotions, and cosmetics contain chemicals that can initiate or hasten the tarnishing process. Cleaning agents, particularly those containing bleach or ammonia, will rapidly corrode the surface of reactive metals.
The natural oils and perspiration from the skin also contribute to the dulling of jewelry. Sweat contains various chemical compounds, including sulfur and chlorides, which are corrosive to metal alloys. The acidity level of an individual’s skin can influence the rate at which these compounds react with the metal.
Practical Steps for Prevention
Understanding the chemistry of tarnish allows for targeted prevention strategies to protect jewelry. Because tarnish is caused by exposure to air and sulfur, proper storage is an effective defense.
- Keeping pieces in airtight containers, small zip-top bags, or boxes with a tight seal limits contact with atmospheric gases.
- Incorporating anti-tarnish strips into storage containers can further slow the process by absorbing reactive gases.
- Routine, gentle cleaning with mild soap and water helps remove residues of sweat, oils, and cosmetics.
- Avoiding exposure to aggressive accelerants, such as removing jewelry before swimming in chlorinated pools or applying perfumes.
Rhodium Plating
A professional solution for resistant tarnish is rhodium plating, often applied to white gold and sterling silver. Rhodium is a highly noble, non-tarnishing metal that forms a protective, thin barrier over the alloy underneath. This plating acts as a sacrificial layer, shielding the copper and silver from environmental exposure and maintains a bright, white finish.