Tarnish is a chemical reaction that dulls or darkens a metal’s surface through exposure to air and moisture. While many metals are prone to this surface corrosion, pure gold (24-karat gold) is almost entirely resistant to the process. The discoloration commonly seen on gold jewelry is not the gold itself reacting, but rather the other metals mixed with the gold to create a stronger, more durable alloy. Since almost all gold items people own are alloys, the familiar darkening is a reaction of the non-gold components.
The Chemical Stability of Pure Gold
Pure gold is classified as a noble metal, prized for its lack of reactivity. This chemical inertness stems from its unique atomic structure, where the outer electrons are tightly bound. Because of this stability, pure 24-karat gold has virtually no affinity for atmospheric oxygen or moisture.
The metal will not oxidize, which is the process that creates rust on iron, nor will it react with common sulfur compounds found in the air. Gold’s resistance means it remains bright and untarnished even when exposed to water, air, and most common acids. This inherent stability is the reason that pure gold artifacts can remain brilliant and unchanged for thousands of years.
Why Gold Jewelry Discolors: The Role of Alloys
The vast majority of gold jewelry is crafted from alloys, which are mixtures of gold and other base metals like copper, silver, or nickel. Since pure gold is extremely soft and easily scratched, these other metals are added to increase the item’s hardness and durability. The purity of gold jewelry is measured in karats: 18-karat gold contains 75% pure gold, 14-karat contains 58.3%, and 10-karat is 41.7% pure gold.
The discoloration that appears on jewelry is a reaction of the base metals mixed into the alloy, not the gold atoms themselves. These base metals, particularly copper and silver, readily react with external substances through sulfidation or oxidation. For example, silver reacts with sulfur compounds in the air to form black silver sulfide, while copper can oxidize to create a dark or greenish film.
The tarnishing process is accelerated by common substances that contact the jewelry surface. Human sweat contains chlorides and urea, which react with the copper and silver in the alloy to form dark chemical compounds. Household chemicals, cosmetics, perfumes, and air pollutants containing sulfur can also hasten the corrosion of the base metals. The lower the karat of the gold, the higher the percentage of reactive base metals it contains, making lower-karat items more susceptible to discoloration.
Maintaining Luster and Preventing Surface Reaction
Preventing the discoloration of gold alloys relies on minimizing exposure to reactive chemicals and moisture. It is advisable to remove gold jewelry before swimming, especially in chlorinated water, as chlorine can weaken the structure of the base metals. Applying perfumes, lotions, and hairspray should be done before putting on jewelry to ensure these cosmetic chemicals do not coat the metal surface.
Regular cleaning helps remove the buildup of skin oils, sweat residue, and grime that can initiate the tarnishing reaction. A simple method involves soaking the piece in a solution of mild dish soap and warm water for several minutes. After soaking, a soft-bristled brush can gently clean crevices, followed by a thorough rinse and drying with a soft, lint-free cloth. When storing jewelry, use soft-lined pouches or separate compartments to prevent scratching and keep the items in a cool, dry place.