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.

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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 nearly pure gold on top of a harder, more durable copper-gold alloy sheet. This process is referred to as depletion gilding.  

Use in America

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 composed 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.^[1]

Orichalcum, the legendary metal of the island of Atlantis, is commonly held to have been a gold-copper alloy, thus fitting the same description.

Tumbaga objects were many times made by using the so called lost wax technique and the alloy used was a mixture of copper (80%), silver (15%), and gold (5%). The indicated concentrations varied from object to object. Once the object was taken out of the cast, it was burned and as a consequence, copper from the surface of the object was oxidized to copper oxide and was then removed mechanically. The object was then placed in an oxidizing solution containing, it is believed, sodium chloride (salt), and ferric sulfate. This process removed through oxidation the siver from the surface of the object leaving only gold. When looking through a microscope, one may clearly see the empty spots from where the original elements copper and silver were removed.

References

1. ^ "Of What Material Were the Plates?" article from FARMS hosted by BYU

See also

• Tombac
• Corinthian bronze
• Shakudo