The scientific tourist #169 — it’s gold! Or is it?

Imagine the surprise and disappointment experienced by the Spanish conquistadors of the 16th century. They went halfway round the world, conquered kingdom after kingdom in search of gold — only to find that much of the loot they stole (let’s be frank here) wasn’t actually gold, but a mix of gold, silver, and copper which came to be known as tumbaga.

Tumbaga had a number of advantages for pre-Columbian metalsmiths. It had a much lower melting point than did either copper or gold alone, was considerably harder (and thus more durable) than pure gold, and yet could be made to look and shine just like gold via a process called depletion gilding.

It's gold! Or is it?

In depletion gilding, the surface is first burned to convert copper near the surface to copper oxide — the resulting scaly black crust was then removed by hammering. Next, the object was bathed in a series of acidic solutions to dissolve the rest of the copper oxide as well as the silver. The remaining porous surface is pure gold, which was smoothed into a shiny surface by heating and polishing.

This little tumbaga figurine is on display at the Denver Art Museum in Denver, Colorado.

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