Gold-filled jewelry is made from components that have a layer of gold that is mechanically bonded with heat and pressure to another material, usually a base metal which is most commonly brass. Gold-filled products are resistant to tarnishing because the outside layer is actually gold and is thick enough to prevent oxidation of the inner material. The gold content must be at least 1/20th of the total weight, where the gold itself is no less than 10 karat in purity. Under FTC regulations, the purity (karat) of the gold content must be part of the marking or designation. As an example, a piece of gold-filled jewelry made with 14 karat gold filled could be stamped 14/20 GF.