Patina
pa·ti·na | pə-ˈtē-nə
noun
The discoloration that forms on metals such as silver and bronze, but is often planned for in the artist's design and can be introduced artificially through the use of chemicals
Patina is a thin layer that appears on certain metals that oxidize and tarnish, resulting in pronounced discoloration. It commonly occurs in copper, brass, bronze and silver - which are impacted by oxidation and other processes influenced by certain chemicals. The process can also be forcibly attained through the use of certain chemicals when the artist wants to introduce a patina to a piece. Common types of acquired patina include the green coloration of copper roofs and domes on municipal buildings. There are several instances in architecture as well as jewelry and other fineries that patina is applied intentionally for aesthetic purposes.