Oxidation results when metal combines with oxygen and produces an oxide or antique finish. This is not tarnish but a coating of oxide such as what results when sterling silver (a copper and silver alloy) is heated and a copper oxide coats the object.

A double sided silver lorgnette with extravagant open pierced work and plentiful French style scrolling. The silver is imbued with natural black oxidation amassed over ten decades of gentle aging.
Image courtesy of Lang Antiques