Tarnish is the dull, darkened film that appears on the surface of certain metals over time, diminishing their original luster. This discoloration is not dirt but the visible result of a chemical reaction between the metal and compounds in the surrounding environment. Tarnish forms a thin layer of corrosion on the outermost surface, typically appearing as a gray, black, or yellowish coating on items like silverware or brass fixtures. Understanding this chemical transformation helps in caring for and protecting susceptible metal objects.
The Chemical Reaction Behind Tarnish
The fundamental process behind tarnish involves a chemical exchange known as sulfidation, where the metal reacts with sulfur compounds present in the atmosphere. The primary culprit is often hydrogen sulfide gas, which exists in the air even at very low concentrations. When a metal like silver is exposed to this gas, the atoms on the metal surface bond with the sulfur atoms to form a new compound. For silver, this reaction creates silver sulfide, which is the compound responsible for the characteristic black or dark brown tarnish. This sulfide layer adheres tightly to the surface, and unlike some forms of corrosion, tarnish is a self-limiting surface phenomenon that protects the underlying metal from further reaction.
Metals Susceptible to Tarnish
Certain metals are inherently more reactive to sulfur and oxygen compounds, making them more vulnerable to tarnish than others. Silver is the most widely recognized metal that tarnishes readily, particularly when alloyed with copper to create sterling silver. The copper content, typically 7.5% in sterling silver, is highly reactive to sulfur, accelerating the discoloration process for the entire alloy. Copper and its alloys, such as brass and bronze, are also highly susceptible, often developing dark brown or black surface films. In contrast, metals like pure gold, platinum, and rhodium are significantly more resistant because they are chemically inactive.
Environmental Factors That Accelerate Tarnish
The rate at which tarnish forms is significantly influenced by external environmental conditions that increase the presence or concentration of reactive compounds. High humidity introduces moisture that facilitates the chemical reactions between the metal and atmospheric gases, accelerating the process. Similarly, elevated temperatures can speed up the chemical kinetics of the tarnishing reaction. Air pollution is a major source of sulfur compounds, as industrial emissions and car exhaust introduce sulfur dioxide and hydrogen sulfide into the air. Beyond atmospheric sources, common household items can also contribute to tarnish, including foods with high sulfur content like eggs, onions, and garlic, and materials such as wool, felt, and rubber bands.
Tarnish Versus Rust and Patina
Tarnish is often confused with other forms of metal surface alteration, but it is chemically distinct from both rust and patina. Rust is the specific corrosion of iron or steel, resulting in the formation of hydrated iron oxide. Rust is typically reddish-brown, flakes off, and is undesirable because it continuously deteriorates the metal’s structural integrity. Patina, on the other hand, is a stable, protective layer that forms on copper and bronze, often appearing green or blue-green. While tarnish is a metal sulfide layer that is usually considered undesirable and easily polished away, a patina is a stable surface that is often intentionally preserved.