Tarnish is a common issue for jewelry, appearing as a dulling, discoloration, or darkening of the metal surface. Unlike rust, which is a form of deep corrosion, tarnish is a superficial chemical reaction confined to the outermost layer of the metal. This discoloration happens when the metal reacts with compounds in its surrounding environment.
Tarnishing: The Chemical Mechanism
The darkening known as tarnish is primarily the result of two types of chemical reactions: oxidation and sulfidation. Oxidation occurs when a metal reacts with oxygen in the air, a process common in copper and the base metals alloyed with gold. The resulting film is typically a metal oxide, which can appear as a subtle discoloration or a dull patina on the surface.
Sulfidation, however, is the main culprit behind the distinct blackening of silver jewelry, such as sterling silver. This reaction happens when silver—and the copper alloyed with it—comes into contact with sulfur-containing compounds. The sulfur reacts with the metal to form a compound called silver sulfide, which is a dark, non-reflective layer that adheres to the surface.
These reactions are accelerated by moisture and heat, which act as catalysts to speed up the chemical conversion. Since pure gold (24 karat) is highly resistant, tarnish is mostly seen in alloys like sterling silver (which contains copper) and lower-karat gold. This occurs because the reactive alloy metals are exposed to the environment.
Environmental Contaminants That Trigger Tarnish
Tarnishing can occur even when jewelry is stored and not being worn, due to atmospheric contaminants constantly present in the air. The most significant environmental trigger is the presence of sulfur compounds, particularly hydrogen sulfide, a gas that readily reacts with silver. This gas is often a byproduct of industrial emissions and air pollution, making jewelry in urban areas more susceptible to tarnishing.
High humidity and moisture in the air significantly accelerate the chemical reactions that cause tarnish. When moisture clings to the metal surface, it provides an ideal medium for the reactive sulfur and oxygen compounds to interact with the metal. This is why jewelry stored in high-humidity areas, such as bathrooms, often tarnishes much faster than pieces kept in dry, airtight containers.
Other sources of sulfur are common, including certain foods like eggs and onions, and materials used in storage. Rubber bands, wool, and some types of paper contain sulfur compounds that off-gas and cause tarnish. Display cases made from synthetic woods like particle board can also release gases that accelerate the reaction, especially when new.
Personal Care Products and Skin Chemistry
Beyond the environment, the most direct causes of tarnish are related to the wearer’s body and the products applied to the skin. Human sweat contains chlorides and salts that are corrosive to many jewelry metals, especially silver and the copper alloys used in gold jewelry. The acidity of the skin, measured by its pH level, also plays a role; more acidic skin can accelerate the oxidation process, causing the metal to tarnish more quickly.
Cosmetics, lotions, and perfumes contain chemical ingredients that can react directly with the metals in jewelry. For instance, certain chemicals in moisturizers or deodorants can speed up the oxidation of sterling silver or gold alloys, leaving behind residues that create discoloration. Jewelry should always be the last item put on after applying any beauty products, allowing the products to dry and absorb fully before contact.
Harsh household cleaning agents pose a threat due to the presence of chlorine and ammonia. Chlorine, found in pools and many common cleaners, is particularly damaging because it attacks the alloyed metals in gold. This can cause the piece to become brittle and weaken its structure. Prolonged exposure to household cleaners containing ammonia can also dull the surface of gold jewelry.