Patinas on metal are caused by the corrosive action of chemicals. Sculptures like statues are exposed to the elements, buried, or immersed in salt water for an extended period of time. The metal acquires a patina, frequently greenish in color, that can be visually pleasing. Similar effects can be made by applying chemicals to the surface of the metal.

Pat"ina (?), n. [It., fr. L. patina a dish, a pan, a kind of cake. Cf. Paten.]

1.

A dish or plate of metal or earthenware; a patella.

2. Fine Arts

The color or incrustation which age gives to works of art; especially, the green rust which covers ancient bronzes, coins, and medals.

Fairholt.

© Webster 1913.