Tarnish is a common surface discoloration that results in a dull or darkened appearance on many metals. This change is a chemical reaction on the metal’s outermost layer, not rust (iron oxide). While pure gold remains unaffected, the gold used in most jewelry is an alloy that often tarnishes. Understanding this process explains why some gold items remain pristine while others require regular cleaning.
The Chemical Inertness of Pure Gold
Pure gold (24-karat gold) is classified as a noble metal, possessing a remarkable resistance to chemical change. Metals in this group, which also includes platinum, have a stable electron configuration. Gold’s electrons are tightly bound, meaning it does not readily form chemical bonds with other elements like oxygen or sulfur compounds found in the air and water. This chemical inertness allows ancient gold artifacts to retain their luster after millennia.
The Role of Alloying Metals in Tarnish
Gold used in jewelry is almost always an alloy, a blend of gold and other metals added to increase hardness and durability, since 24-karat gold is too soft for everyday wear. These alloying metals—specifically silver, copper, and sometimes nickel or zinc—are the actual culprits in the tarnishing process. Tarnish occurs when these reactive metals encounter sulfur compounds or oxygen in the environment, leading to the formation of dark, visible metal sulfides or oxides on the surface.
The primary tarnish-causing agents are sulfur compounds (sulfides), which are present in air pollution, certain foods, and chemicals like those found in cosmetics and perfumes. When a copper atom in the alloy reacts with sulfur, it creates a dark layer of copper sulfide. Even natural skin oils and perspiration contain trace amounts of sulfur and moisture that accelerate this surface reaction. The resulting discoloration is a film of chemical compounds forming on the reactive metal components, not the gold itself changing color.
How Karat Weight Affects Tarnish Risk
The risk of gold jewelry tarnishing is directly tied to its karat weight, which measures purity out of 24 parts. For instance, 18-karat gold is 75% pure gold, 14-karat gold contains 58.3% gold, and 10-karat gold is 41.7% pure gold. A lower karat number indicates a higher concentration of reactive alloying metals. Consequently, 10-karat gold is far more susceptible to tarnish than 18-karat gold.
Since the tarnish reaction occurs only with the non-gold metals, jewelry with a higher percentage of these reactive components will tarnish more quickly and noticeably. The higher the karat, the greater the proportion of chemically inert gold, which provides a protective effect. For this reason, 14-karat gold is a popular choice for daily wear, offering a balance of durability and a relatively low tarnish risk.
Preventing and Removing Gold Tarnish
Preventing tarnish involves minimizing the jewelry’s exposure to the reactive agents that trigger the chemical process. It is advisable to remove gold jewelry before swimming (especially in chlorinated water) or before applying personal care products like lotions and perfumes. Storing pieces in a cool, dry place, ideally in individual soft cloth pouches or a lined jewelry box, helps limit exposure to humidity and atmospheric sulfur compounds. Regularly wiping the jewelry with a soft, non-abrasive cloth also removes surface oils and residues that accelerate discoloration.
If tarnish does form, it is a surface-level phenomenon that can be removed with gentle cleaning methods. A simple approach is to soak the item in a solution of warm water and a few drops of mild dish soap for 10 to 15 minutes. Using a soft-bristled toothbrush can help gently scrub away any accumulated grime or dark film. After cleaning, the jewelry must be thoroughly rinsed with clean water and patted completely dry with a lint-free cloth before being worn or stored.