Depletion and enrichment of an alloy surface

In studying the ancient objects made of noble metal alloys, such as gold with copper or silver, the phenomenon of surface enrichment is often encountered [1]. This phenomenon is regarding the composition changes of the surfaces, changes that happen because objects had been buried for a long time. In fact, this alteration of surfaces is not due to their enrichment, that is, to an addition of gold, but it is due to a depletion process, during which the less chemically stable elements leach out slowly, causing the surface composition to change. It is the same for gilding; we can have an enrichment of the surface when gold is added to it, or a depletion of the surface when other elements of the alloy are removed and the local percentage of gold is increased as a consequence. Therefore, an ancient object can have a surface rich of gold because of time or because of an ancient depletion gilding treatment. This depletion gilding was used by pre-Columbian populations for the tumbaga, a gold and copper alloy, to give it the luster of gold.