By Richard Skelton

In 1532, with just 168 men, a cannon, and mounts for fewer than 1 in 5 of his men, the Spanish Conquistador Francisco Pizarro defeated the Incan Empire. The Incas possessed, at the time, the largest and wealthiest empire in the Americas, encompassing an area nearly four times greater than that of modern Spain, and with a population estimated at anywhere between 4 and 37 million people. For the men involved in this expedition, as with those who had accompanied Hernan Cortez a few years earlier in his conquest of the Aztecs, the primary motivation can only have been plunder. The Incan empire was fabulously rich, with temples and palaces virtually overflowing with ornaments and statues made from gold and silver.

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Machu Picchu: the “Lost City of the Incas” (machupicchu.org)

However, looking back from the vantage point of the 21st Century, there is something even more remarkable than the quantity of gold and silver objects that the Spanish found in the Andes. For the ancient Andes represent something quite extraordinary: an independent metallurgical tradition that appeared separately from that of the Old World.

The Andes were uplifted during the subduction (still ongoing) of the Nazca plate beneath the South American plate, which also resulted in the emplacement of extensive copper deposits throughout the region. Indeed, of the five largest producers of copper today, two (Chile and Peru) are to be found with the borders of the ancient Incan empire. Magmatic gold and silver deposits are likewise found across the mountain range. While copper and silver were mined extensively by the Incas, most of their gold was obtained from rivers (or their neighbours…).

But the Andean civilisations did more than simply mine for metals. They also worked them into fabulous structures, exploiting the ductility of gold and silver to create intricate patterns. Sometimes, this was done by hammering the metal into a thin sheet, which could be wrapped around a wooden mould and then beaten to create the desired shape. Imperfections in the metal introduced by this mistreatment decrease the malleability of the metal, making it harder to work. There is evidence that the Incas fired (ie. annealed) the gold as it was shaped, restoring its malleability by healing defects in the crystal structure. Gold and silver were also worked into larger pieces, even including life-sized llama statues.

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Golden statue of a llama, *not produced to scale (nga.gov.au/exhibition/INCAS)

Unfortunately for the Spanish, the Inca often alloyed their gold with silver or copper, creating what was called tumbaga. Much as the alloying of iron with carbon creates a harder material, steel, so too was tumbaga is harder than native gold. It could even be fashioned into practical implements. Alloying of gold with other metals was also used for more whimsical purposes, as in the case of “red gold,” which the Chimu produced by alloying Cu with Au in the ratio 7:3 (in what must be considered an extremely primitive example of band-gap engineering).

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The conquistador’s nightmare: a “pectoral” made of gilded copper (nga.gov.au/exhibition/INCAS)

Much to the disappointment of the conquistadors, some of the apparently golden objects were actually made of copper, with thin layers of gold deposited on the surface by electroplating. This process involved the dipping the copper object (not infrequently a llama) in a highly corrosive solution containing dissolved gold. The gold ions are deposited through an exchange reaction with copper on the surface of the object (Lechtman 1979). This process allowed the deposition of very fine layers of metal; the metallurgists of the Vicus culture were reportedly able to produce copper ornaments with gilt layers as thin as 1 micrometre (one-millionth of a metre).

But was Andean mining and metallurgy a late development, one that arose only in the final centuries before the colonial period? The answer can be found buried beneath in lakes all across the central Andes.

Silver can be mined as a native metal, but it is often mined from ore minerals, of which a few are: acanthite (Ag2S) andorite (PbAgSb3S6), and pyrargyrite (Ag3SbS3). To extract the silver, these ores must be smelted, leaving behind waste materials (heavy metals, in this case), which may enter the environment as pollutants. It appears that the prehistoric Andean mining industry had a sometimes cavalier attitude towards environmental protection. Sediment cores taken from lake beds show concentrations of heavy metals – like bismuth, silver, lead, and mercury – far above the levels that occur naturally in these environments. This is precisely what happened in the Andes, and spikes in heavy metal concentrations can be seen in the lake sediments, corresponding to heavy increases in mining activity. In some places, these extend all the way back into the dark ages, with the largest peaks corresponding to the rise (and fall) of sophisticated cultures, such as the Wari, Tiwanaku, and Chimu.

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Heavy metal concentrations in Laguna Loboto, from the rise of Islam to the present day. The spike in concentrations at 1100 AD corresponds to the Tiwanaku period (Abbott and Wolfe 2003).

However, sediments in some lakes, such as Laguna Loboto in modern-day Bolivia, show that even in the interludes separating the fall of one civilisation and the rise of another, heavy metal pollutants were still being produced (Abbott and Wolfe 2003). Once the Pandora’s box of industrial metal production had been opened, it could not be easily shut.

Further reading:

MB Abbott and  AP Wolfe (2003) Intensive Pre-Incan Metallurgy Recorded by Lake Sediments from the Bolivian Andes. Science 301:1893–1895.

H Lechtman (1979) A Pre-Columbian Technique for Electrochemical Replacement Plating of Gold and Silver on Copper Objects. Journal of Metals.

W Bray W (1978) Gold-working in Ancient America. Gold Bulletin