why do coins tarnish?

The Deep Dive

The tarnishing of coins is a classic example of oxidation and corrosion, driven by chemical reactions. Most coins are alloys, meaning they are mixtures of metals. Common coinage metals like copper, nickel, and zinc are susceptible to reacting with their surroundings. Copper, a primary component in many older coins and pennies, readily reacts with sulfur compounds present in the air, such as hydrogen sulfide (H2S), and with oxygen. This reaction forms copper sulfides and copper oxides, which are typically dark in color, leading to the familiar greenish-black or brown tarnish. Silver, used in many older circulating coins and some modern commemorative issues, also tarnishes. Silver reacts with hydrogen sulfide in the air to form silver sulfide (Ag2S), a black compound that appears as a dark film on the coin's surface. Even seemingly inert metals can react over time, especially in humid or polluted environments. The rate of tarnishing depends on the specific metals involved, the concentration of reactive substances in the air, temperature, and humidity. The tarnish is essentially a thin layer of corrosion products that alters the original luster of the coin.

Why It Matters

Understanding coin tarnishing is important for coin collectors (numismatists) who value the original condition and appearance of coins. Tarnishing can affect a coin's aesthetic appeal and, consequently, its monetary value. While some collectors embrace natural toning, aggressive cleaning to remove tarnish can permanently damage a coin's surface, significantly reducing its worth. For historians, the patinas and tarnish on ancient coins can sometimes provide clues about their environment and how they were preserved or stored over centuries.

Common Misconceptions

A common misconception is that all tarnish is the same and equally detrimental to a coin. In reality, different metals tarnish in different ways and at different rates, forming various compounds. For example, the dark toning on silver coins is silver sulfide, while the greenish patina on copper coins can be copper oxides, chlorides, or sulfates. Another myth is that tarnished coins are worthless or should always be cleaned. For collectors, a natural, attractive toning can actually enhance a coin's desirability and value, whereas aggressive cleaning can destroy it.

Fun Facts

• The greenish patina on old copper coins is often a result of reactions with sulfur compounds and can be a sign of age and history.
• Silver tarnishes by forming a black layer of silver sulfide, which is why old silverware and jewelry can turn dark.