'Patina' means an oxidation layer on metal objects. Think, for example, of the gray-green layer that you often see on copper and bronze objects. The formation of a beautiful natural patina layer can take decades.
Patina layers can be formed naturally, but metal objects can also be artificially patina coated by the use of chemicals. This is called with ‘Patination’. For example, this technique is often used in bronze sculptures, where artists use patina layers mainly because of the beautiful shiny structure. Various colors can be obtained by using different chemicals. For example, ‘liver of Sulphur’ or ‘potassium-polysulphide’ is used for a gray to jetty black color, iron nitrate for brown and copper nitrate for green, but numerous applications of chemicals and colors are possible. This technique can also be used for 'discoloration' of, for example, a dial.