Gold is a precious metal prized for its enduring luster and resistance to decay. The definitive answer to whether pure gold rusts or tarnishes is no. Elemental gold is chemically stable, meaning it does not break down or discolor when exposed to normal environmental conditions. Confusion about gold discoloring stems from the fact that most consumer gold items are not pure. This article explores the scientific reasons behind gold’s stability and explains what causes discoloration in jewelry and other gold objects.
The Chemistry of Noble Metals and Oxidation
The reason gold resists rusting lies in its classification as a noble metal. Noble metals are elements that exhibit exceptional resistance to chemical reaction, particularly oxidation. Rust is a specific form of corrosion known chemically as iron oxide, which forms when iron reacts with oxygen and moisture. Since gold is not iron, it cannot form iron oxide and does not rust.
Gold’s atomic structure makes it highly unreactive with oxygen and water at room temperature, the primary components necessary for rust formation. This chemical inertness allows pure gold to maintain its metallic shine for centuries, explaining why ancient gold artifacts still look brilliant today.
Understanding Tarnish vs. Corrosion
It is important to distinguish between rust, a form of deep corrosion, and tarnish, a thin surface-level chemical reaction. Tarnish typically appears as a dull, gray, or black film on a metal’s surface, often caused by a reaction with sulfur compounds. Corrosion is the gradual destruction of a metal due to reaction with its environment; rust is a specific type of corrosion that flakes away and causes deep damage.
Tarnish is most often the result of sulfidation, where the metal reacts with hydrogen sulfide or sulfur dioxide found in air pollution. While pure gold is highly resistant to these sulfur compounds, other metals like silver and copper are susceptible. Unlike rust, tarnish is often self-limiting, meaning the thin surface layer that forms can protect the underlying metal from further reaction.
Why Gold Alloys Discolor
The discoloration people notice on gold jewelry is almost always due to the presence of other metals mixed with the gold. Pure gold (24-karat) is too soft for everyday use, so it is combined with stronger, less expensive metals to create an alloy. These alloying metals include silver, copper, nickel, and zinc, which increase the metal’s durability and alter its color.
The karat system indicates the proportion of pure gold in the alloy: 24K is 100% gold, 18K is 75% gold, and 14K is 58.3% gold. The lower the karat number, the higher the percentage of reactive non-gold metals present. These non-gold metals, particularly copper and silver, are susceptible to chemical reactions that cause discoloration.
Copper in the alloy can oxidize, while silver reacts readily with sulfur compounds in the air to form silver sulfide, a dark film. This surface reaction creates the visible tarnish, dark spots, or dullness on the jewelry, not the gold itself. An 18K piece, with its lower concentration of base metals, is significantly more resistant to tarnishing than a 10K or 14K piece. The appearance of tarnish is a sign that the alloying metals are reacting with environmental agents like air, moisture, and chemicals.
Preventing and Cleaning Discoloration
Controlling the exposure of the reactive alloy metals to environmental compounds is the most effective way to prevent discoloration. Remove gold alloy items before swimming, especially in chlorinated water, or before using household cleaning products. Avoiding contact with cosmetics, perfumes, and lotions before they have dried fully also helps reduce chemical interactions that accelerate tarnishing.
Proper storage is important for preservation. Items should be kept in a dry environment, such as a jewelry box or an anti-tarnish pouch, to limit contact with moisture and airborne sulfur compounds. For cleaning, a mild solution of warm water and gentle dish soap is effective for removing surface contaminants. The piece can be soaked briefly, then gently scrubbed with a soft cloth or brush, followed by thorough rinsing and drying with a lint-free cloth.