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Tumbaga is the name given by Spanish Conquistadors for a non-specific alloy of gold and copper, and metals composed of these elements. Pieces made of tumbaga were widely found in pre-Columbian Mesoamerica in North America and South America.
The term is a borrowing from the Tagalog language tumbaga. This came from Malay language tembaga, meaning 'copper' or 'brass'. It has also been spelled tumbago in literature.
Tumbaga can be treated with a carboxylic acid, such as oxalic 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.
The proportion of gold to copper in artifacts varies widely; some items have been found with as much as 97% gold while others 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.
Tumbaga objects were often made using a combination of the Lost-wax casting and depletion gilding. An alloy of varying proportions of copper, silver, and gold (typically in a percentage ratio of 80:15:5) was cast. It was burned after removal, turning surface copper into copper oxide, which was mechanically removed. The object was placed in an oxidizing solution, likely composed of sodium chloride (salt) and ferric sulfate. This dissolved the silver from the surface, leaving only gold. When viewed through a microscope, voids appear where the copper and silver had been.
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