The patina is a thin coating of various chemical compounds (e.g. oxides, carbonates, sulfides, or sulfates) that form on the surface of a metal when exposed to the air, the most common example would be rust. When casting bronze there are a number of patinas that can be used to either provide a unique surface effect or colour. These are generally used to enhance a particular feature on a sculpture (e.g. black, polished or green), or to create an overall colour that suits a particular sculpture. Changing the colour of a sculpture can have a dramatic effect on its look.
The most common patinas are natural bronze, various shades of brown through to black and green. There are also a number of special finishes that can create a number of textured looks.