tarnish

Tarnish is a discoloration or dulling of a metal surface caused by a chemical or electrochemical reaction with substances in the environment. It forms a thin layer of corrosion products rather than a deep, structural decay, and in many cases it can be removed to restore the metal’s original appearance. Tarnish is most commonly discussed in relation to precious metals and lightly reactive metals such as silver, copper, and their alloys.

Common examples include silver tarnish, which results from reaction with sulfur-containing compounds in the air to form silver sulfide (Ag2S) that appears dark gray to black. Copper and its alloys tarn when exposed to oxygen, moisture, and pollutants, forming copper oxide, copper sulfide, or copper carbonate (creating a green patina known as verdigris). Aluminum tarnishes by forming a thin, protective oxide layer (aluminum oxide). Gold and platinum are relatively resistant to tarnish, though extreme environments can cause surface changes.

Causes and factors include environmental pollutants (such as sulfur compounds and chlorides), humidity, temperature, and exposure time. Mechanical abrasion can remove protective surface layers and accelerate tarnishing by exposing fresh metal. Some finishes and coatings slow tarnish, while sealed storage or controlled environments reduce its rate.

Prevention and removal strategies vary by metal. Protective coatings, waxes, lacquers, or passivation can inhibit tarnish; anti-tarnish packaging and humidity control help for sensitive items. Cleaning methods range from gentle polishing with appropriate cloths to chemical cleaners formulated for specific metals; harsh abrasives or inappropriate acids can damage the object or remove desirable patina.