Tumbaga was used in pre-Columbian times from Mesoamerica to Peru and Chile as a generic term for any combination of gold and copper. It could range from 95% copper to 95% gold, although tumbaga or guanin gold was usually made by adding 10 to 30% copper to gold. Tumbaga usually contains 5 to 10% silver as well, which occurred naturally in the gold and wasn’t intentionally added.
There were several reasons tumbaga was popular. A primary one is that 70% gold/30%copper will melt at around 800 C., much lower than gold or copper separately. That’s important because melts were done in large clay pots using a team of men huffing on blowpipes. Molten metal then flowed from a hole in the bottom of the vessel into open molds made from stone or clay. These molds have been found archaeologically from Mexico to Chile.
Depletion gilding was routinely used to decorate the surfaces of objects made from low-gold alloys.Tumbaga had another interesting use as well. Some cultures like the Moche in Peru placed small tumbaga ingots in the mouths of their high-ranking dead prior to burial.
The Moche were astonishingly skilled goldsmiths and lapidarists, able to drill consistent straight holes in quartz crystal beads and ultra-tiny holes in turquoise disks. They used only stone tools but were able to accomplish amazing things.