1. A thin greenish layer, usually basic copper sulfate, that forms
on copper or copper alloys, such as bronze, as a result of corrosion.
2. The sheen on any surface, produced by age and use.
3. A change in appearance produced by long-standing behavior, practice, or use: a face etched with a patina of fine lines and tiny wrinkles.
As a general term, patina refers to the change in an object's surface resulting from natural aging. In the context of bronze sculpture, patina specifically refers to the surface of the bronze and the alterations made by the sculptor with acid or the application of other chemicals in order to give the sculpture a specific color. The purpose of applying a patina is not merely to give the piece a different color, but rather to highlight the contours and enhance the specific modeling of the bronze.
A patina is created by applying chemicals to a sculpture's surface under heat. The artist will seal the surface with a coating of wax in order to protect such a deliberately applied patina. This will prevent the changes which can take place...and which lead to naturally-occurring patinas...caused by exposure to natural atmospheric elements, such as heat and humidity.