A very observant trainee in the Color Lab once came to me with a concern. He was batching up a color called “Aged Copper” but was concerned because the formula had produced a green color rather than the copper that he had expected. I told the trainee that it was a good catch but copper turns green as it ages. It turns out that copper will actually turn into several different shades as it ages, from the brilliant metallic to a seafoam blue.
The ability of copper to change colors is known as patina and is caused by exposure to weather. Patina is a natural reaction of the metal to protect itself from the environment it is in. Patina prevents the metal from decaying past a certain point unlike iron which will rust entirely through. Patina in urban environments occurs much faster and is caused by exposure to pollutants in the air such as, chlorides, sulfides, sulfates and carbonates. In rural environments patina occurs much slower and is caused by exposure to carbon dioxide and water.
The varying color of patina is determined by the types of pollutants in the atmosphere and the length of time the copper has been exposed to the elements. The Statue of Liberty, one of the most famous copper structures in the world, has taken a century to turn to the iconic shade of pale green we see today. The patina layer on the statute only measures .005 of an inch deep and will protect the statue for centuries to come.