Tumbaga
Tumbaga was the name given by Spaniards to a non-specific alloy of gold and copper which they found in widespread use in Pre-columbian Mesoamerica.
Composition and properties
Tumbaga is an alloy composed mostly of gold and copper. It has a significantly lower melting point than gold or copper alone. It is harder than copper, but maintains malleability after being pounded.
Tumbaga can be treated with a simple acid, like citric acid, to dissolve copper off the surface. What remains is a shiny layer of 23-karat gold on top of a harder, more durable copper-gold alloy sheet.
Use in the Americas
Tumbaga was widely used by the pre-Columbian cultures of central America to make religious objects. Like most gold alloys, tumbaga was versatile and could be cast, drawn, hammered, gilded, soldered, welded, plated, hardened, annealed, polished, engraved, embossed, and inlaid.
The proportion of gold to copper in artifacts varies wildly; items have been found with as much as 97% gold while others instead contain 97% copper. Some tumbaga has also been found to be comprised of metals besides gold and copper, up to 18% of the total mass of the tumbaga.
In 1992, approximately 200 tumbaga bars were recovered in wreckage off Grand Bahama Island. They were composed of gold, copper, and silver plundered by the Spaniards during the conquests of Cortez and Pizarro and hastily melted into bars of tumbaga for transport across the Atlantic. Because all the metals that reached Europe were melted back into their constituent metals in Spain, the bars found in the shipwreck are the only known bars of tumbaga that remain.[citation needed]
Some Mormon scholars suggest that the Golden Plates from which the Book of Mormon was allegedly translated may have been made from tumbaga. http://farms.byu.edu/display.php?table=jbms&id=393
Orichalcum, the legendary metal of the island of Atlantis, is commonly held to have been a gold-copper alloy, thus fitting the same description.