Why Does Copper Oxidize and Turn Green?
Understanding why copper turns green requires at least a basic knowledge of oxidation and of the word “patina.” Patina is commonly defined as a film or thin coating that forms on some metals, such as bronze and copper. But it can also be the same type of thin layer that forms on wood furniture after years of use and exposure.
Oxidation causes the patina, which is responsible for the green color. In the case of copper statues, such as the Statue of Liberty and others, the copper molecules react to the outside air, eventually turning almost black. This slightly altered form of copper reacts with certain chemicals in the air to form the patina layer. Much of the resulting chemical layer is made up of green/blue forms of copper that give the surface its distinct color.
It normally takes a few years for obvious color changes on bronze and copper. We first see the brown or gray/black hue, which may last for three years or a bit more. After this time the greens and greenish/blue colors begin to show. It’s interesting to note that with some art objects and structures people speed up the oxidation process to achieve the desired appearance. The oxidation process can be slowed or stopped with the correct treatment at the proper time. Sealing the surface before the copper begins to oxidize will halt further change for some time.
To better understand oxidation, think of the changes that take place in other metals, such as iron. This type of metal rusts when subject to oxidation. The combination of moisture and exposure to air turn iron reddish-brown. In most cases the rusted material seems dry and brittle. Copper and bronze don’t rust as iron does. They are a bit more resistant to the elements, so the obvious change is in the color of the surface (and sometimes the feel/texture). In fact, the patina that forms on copper serves as a protective layer.
Throughout the years, scientists, art enthusiasts and others have found that bronze and copper statutes react differently depending on their location. The differences come from various levels of moisture in the air (humidity) and from the ingredients of air pollution. The rate and color of change in copper roofs, for example, might be different in Europe because of coal and other fuels while in another country the statues might be slightly darker green, lighter green or even more blue/gray.
Close observation of copper and bronze objects that are constantly exposed to the elements shows that the color changes are very gradual in the first few years. But the oxidation rate increases significantly after the first decade, give or take a year or two. Again, the rate of change and the color depend on the composition of the air and the air pollution of the region. Low humidity can make quite a difference in oxidation as well. To see a quicker example of oxidation, slice an apple and leave the inside exposed to air overnight. This is the same type of change that takes place with metal. It just happens more slowly.