People tend to think of metals as strong, stable materials. Like other substances, metals are affected by the environment. Some metals rust while others may tarnish or turn green. Many people mistakenly believe that copper statues such as the Statue of Liberty turn green due to acid rain. Any acid in rain does not affect the copper. What causes copper to turn green?
Occasionally, a person may see a copper penny that is covered in more green than the expected color of copper. Pennies, statues, pipes, roofs, and other copper exposed to air and condensation is often partially green. When copper is exposed to air and water, a thin layer of a green substance forms on the surface of the copper.
This green substance is called copper carbonate. The process that causes copper carbonate to form is called oxidation. When iron becomes oxidized, it rusts causing it to become weaker. The oxidation of copper does not weaken the metal. Some people believe the oxidation and subsequent buildup of copper carbonate protects the metal. The greenish tint on the copper is often referred to as patina.
When copper is first oxidized, it does not immediately turn green. Oxidation is a process. The copper may turn black or dark brown. Gradually, shades of green may appear. Many times, the layer of copper patina is thin. The patina is considered by some to be waterproof and a desired effect.
Not everyone wants their copper fixtures to turn green. One way to prevent copper pots or dishes from turning green is to keep the copper dishes stored out of direct sunlight. Vinegar and salt can be used to clean copper and remove patina. People can clean pennies by putting them in a jar with vinegar and salt. Often, the removal of the patina occurs rather quickly, but the person may need to wait an hour or more depending on the amount of copper carbonate.
Another green copper fact is that when people’s blonde hair turns green from spending time in swimming pools, it’s the copper in the water that is responsible for the green color. Most people think it is caused by chlorine. However, the truth is that copper tends to build up in swimming pools from some of the algaecides used, wear from pipes and pool machinery, and the trace amounts of copper in the water when water is added to the pool. Since the copper does not breakdown, it continues to accumulate in the water in the form of copper sulfate.