Whether gold tarnishes depends entirely on its purity. Pure 24-karat gold (99.9% pure) does not tarnish. Tarnish is a form of surface corrosion—a chemical reaction that dulls the metal’s shine and changes its color. Pure gold’s unique atomic structure prevents this reaction from occurring under normal conditions, distinguishing it from the gold alloys commonly used in jewelry.
The Chemical Process of Tarnish
Tarnish is a thin layer of chemical compounds that forms on a metal’s surface when it reacts with substances in the environment. The reaction typically involves the metal losing electrons to elements like sulfur or oxygen, forming a compound on the surface.
The most common cause of tarnish on metals like silver and copper is a reaction with sulfur compounds, such as hydrogen sulfide, which is naturally present in the air. This reaction results in the formation of a metal sulfide layer, which appears as a dark, dull discoloration. Tarnish is an electrochemical process often accelerated by humidity, moisture, and pollutants in the atmosphere.
Why Pure Gold Remains Inert
Pure gold’s resistance to tarnish is explained by its classification as a noble metal, a group that includes platinum and rhodium. This classification signifies its low chemical reactivity and strong resistance to corrosion and oxidation. Gold atoms are fundamentally stable and do not readily give up electrons to form compounds with other elements, such as oxygen or sulfur, under normal atmospheric conditions.
The stability of gold is rooted in its unique atomic structure, where its outermost electrons are tightly bound to the nucleus. This tight binding makes it difficult for environmental elements to strip away the electrons needed to initiate a chemical reaction like oxidation or sulfidation. Because the gold atoms do not react, they do not form the surface film of corrosion that characterizes tarnish, allowing the metal to maintain its luster.
How Alloying Metals Lead to Tarnish
The confusion regarding gold tarnishing arises because almost all gold jewelry is made from gold alloys, not pure 24-karat gold. These alloys, such as 18-karat (75% gold) or 14-karat (58.3% gold), are created by mixing pure gold with other base metals to increase hardness and durability. It is these added metals that are responsible for the tarnishing seen on lower-karat gold pieces.
The most common alloying metals are silver and copper, both of which are significantly more reactive than gold. Copper readily reacts with oxygen to form copper oxides, and silver reacts with airborne sulfur compounds to form silver sulfide. When a gold alloy piece tarnishes, the chemical reaction is happening only to the reactive base metals on the surface, not to the gold atoms themselves. The resulting discoloration (black, dark gray, or green) is the corrosion of the copper or silver component. The higher the proportion of these reactive metals—the lower the karat number—the more susceptible the piece is to tarnishing over time.