Sometimes, an antique piece of jewellery will be made of a metal that looks like gold, tests as if it is gold but which is a another metal with a gold coating. The amount of gold coating can vary depending on the method used.
Gilding, the overlaying of a metal or wood with gold or gold alloy, was a technique used by early man. There are a number of ways by which it can be achieved. One way is through the application of gold leaf using an adhesive called a mordant. Another way was very dangerous for the workers who did it as it involved applying an amalgam of gold and mercury with a brush, then heating the object until the mercury vaporised, leaving a gold film. The mercury fumes were very toxic. Other methods involved electrogilding, electroforming and electroplating.
Starting from about 1817, rolled gold was made when a layer of gold was fused over a base metal, with the composite piece then being rolled out mechanically into a sheet from which the jewellery item could be stamped. Sometimes the gold could be fused on both sides of the metal. It may be marked as RGP with the gold carat included, for example, 10ct RGP but this is not common. Generally, antique pieces may show some wear, with the base metal appearing in some places.
Gold filling is a later version of rolled gold and has a higher amount of gold content in the layer. In the US, gold filled items must have a layer of gold which is at least 1/20th of the total weight of the metal and must be marked ‘gold filled’ and the carat of gold used must be included. The piece should be marked, for example, ’10 carat gold filled’ or ’10kt GF’. Much of the antique gold fill jewellery still retains its gold coating, with little wear.
Electro goldplating was patented in 1849. Gold plating is when a thin layer of gold is applied to a base metal using a chemical bath and an electric current. The base metal has an electrical current through it, and the karat gold has the opposite electrical current running through it, which causes the particles to be drawn onto the base metal and coat the piece . This method of applying gold results in the thinnest layer of gold and it wears through very quickly.
Two other terms which you may see to describe gold metals are vermeil (also known as silver gilt) and gilt. Vermeil is when gold is laid over silver and gilt is wood or metal painted with gold paint or gold leaf. In the US, vermeil pieces must have the same gold content as gold fill to be labelled as vermeil and must be marked accordingly. Also, in Australia, auction houses often use the term ‘gold lined’, which seems to be a catch all to cover unmarked rolled, filled or plated gold jewellery.