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.