The result of combining an element or metal with oxygen, producing an oxide that presents an 'antique' finish. It is not related, as often incorrectly stated, to the process by which silver acquires a tarnish, that being due to contact with sulphurous fumes in the air. The process can be effected artificially by immersion of the metal in heated potassium sulphide solution. When silver is alloyed with copper, the latter is oxidized by the heating, so that the alloy must be immersed in pickle to remove the copper oxide.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson