why do coins tarnish when wet?

The Deep Dive

The tarnishing of coins, particularly those made from copper alloys like brass or bronze, is a classic example of oxidation. When a coin is exposed to moisture, the water acts as an electrolyte, facilitating a chemical reaction between the metal and oxygen. This process is an electrochemical one. For copper, the primary component in many coins, the reaction begins with the loss of electrons from copper atoms, forming copper ions. These ions then react with oxygen and water to create various copper oxides and hydroxides, which are often green or black. For example, copper can react with oxygen and water to form copper(I) oxide (Cu2O) or copper(II) oxide (CuO). In the presence of sulfur compounds, which are common pollutants in the air, copper can also form copper sulfides, contributing to the dark tarnish. The rate of tarnishing is influenced by factors like humidity, temperature, and the presence of other chemicals in the environment. Pure silver coins also tarnish, forming silver sulfide (Ag2S) when exposed to hydrogen sulfide in the air, which is also accelerated by moisture.

Why It Matters

Understanding coin tarnishing is important for collectors who value the original appearance of their coins. Tarnished coins may be worth less than their pristine counterparts. For everyday currency, while minor tarnishing doesn't affect usability, it's a reminder of the continuous interaction between materials and their environment. This chemical process also highlights the durability and degradation of metals, informing the design and selection of materials for various applications, from jewelry to structural components, where resistance to corrosion is crucial.

Common Misconceptions

A common misconception is that coins 'rot' or 'decay' like organic matter. Tarnishing is not biological decay but a chemical reaction called oxidation or corrosion. Another myth is that only old coins tarnish; even new coins made of susceptible metals will tarnish if exposed to the right environmental conditions, especially moisture and air pollutants. The color of the tarnish, often green (verdigris on copper alloys) or black, is sometimes mistakenly thought to indicate the coin's purity or value, when it simply reflects the specific chemical compounds formed.

Fun Facts

• The green patina seen on ancient bronze or copper coins is a form of tarnish called verdigris, which is a mixture of copper carbonates and acetates.
• Silver tarnishes by reacting with sulfur compounds in the air to form silver sulfide, which is black.