Patina

n. green incrustation on bronze (due to age); gloss on object (due to age)

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.
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Noun
1. a fine coating of oxide on the surface of a metal
(hypernym) coating, coat
(hyponym) verdigris
(derivation) patinate, patinize, patinise

n. aerugo, verdigris; green coating on metal

v. skate, ski