If you’re a coin collector or just happen to handle change a lot, you’ve probably seen old copper coins in various colors. From white to green to blue! Why do these coins take on so many different colors? One may think it’s because of something that got stuck to it, maybe a candy wrapper or some sticky food but these colors are actually naturally occurring!
There are two common forms of attack upon these older metals. In the milder case, a metal may tarnish. “Tarnish” is a thin coating on the surface of a metal and is usually very uniform and does not often destroy the intended purpose of the metal. “Corrosion,” on the other hand, is often not uniform, but may cause pits and may reach such proportions as to destroy the metallic object so that it cannot be used for its intended purpose.
In dry air, even tarnishing takes place quite slowly; however, with the usual atmosphere around us, the humidity accelerates the tarnishing process. The lowest oxide level of copper is cuprous oxide, or cuprite. Its color is pink. Barely noticeable at first, a penny becomes darker over time due to the tarnish layer thickening, as well as the continued oxidation to the black cupric oxide, tenorite.
Over time, and upon repeated or prolonged exposure to moisture in the presence of dissolved acidic substances, such as carbon dioxide and the polluting substances found in acid rain, tarnished copper turns green. Among these acid substances are the oxides of sulfur and the oxides of nitrogen. Reacting with moisture, they form dilute solutions of strong acids.
Copper that is exposed to open air will corrode and undergo a series of chemical reactions that lead to the development of a patina – a coating of copper oxide molecules which actually protects the metal beneath. Over time, copper transitions from its shiny brown color to a darker brown shade.
After many years it transitions into blues. At an even later stage the formation of copper sulfate, carbonate and chloride salts in varying concentrations turns the surface green. There are several factors which affect the amount of time these processes take including moisture, temperature, and the level of pollution. The formation of the natural green patina seen on copper roofs and statues takes a very long time, but methods have been developed to speed the process up using chemical reactions.
Coins aren’t the only place we often see this chemical reaction take place; we’ve all seen the greenish blue Statue of Liberty, but did you know Lady Liberty was once a copper color? That’s right, the famous statue was once covered in a thin layer of copper and was bronze when she first arrived in the United States from France. Acid in rain covers the Statue of Liberty whenever storms hit New York, and her exposure to oxygen from being in the middle of the ocean gradually turned her blue over the years.