Patina is a chemical compound formed on the surface of metal. Patinas form on metal from exposure to the elements. They are often deliberately added by artists and metalworkers. Patinas may be used to 'antique' objects, as a part of the design or decoration of art and furniture.
Patinas are restricted to exposed surfaces and can flake off. One reason bronze is so highly valued in statuary is that its patina protects it against further corrosion. This natural patina seldom shows a tendency to flake. Brass is also resistant to corrosion, but it is, in the long run, not as attractive since local pitting shows against the shiny background.
Figuratively, patina can refer to any fading, darkening or other signs of age, which are felt to be natural and/or unavoidable.
The chemical process by which a patina forms is called patination, and a work of art coated by a patina is said to be patinated.
One example of a patina is a green surface texture created by slow chemical alteration of copper, producing a basic carbonate. It can form on pure copper objects as well as alloys which contain copper, such as bronze or brass.