Origins of Smelting: Lake yields core of pre-Inca silver making

According to 16th-century Spanish accounts, an Incan ruler who reigned nearly 600 years ago discovered Cerro Rico, a major silver deposit in southern Bolivia’s Andes Mountains. More recently, archaeological discoveries have documented the Incas’ extensive efforts to mine silver ore and extract the precious metal in smelting operations.

It now appears, however, that the Incas were latecomers to silver production. A thriving silver industry existed in southern Bolivia about 1,000 years ago, according to a new study. Four centuries before comparable Incan operations, Bolivia’s Tiwanaku culture probably launched silver mining at Cerro Rico and the large-scale smelting of silver ore, say Mark B. Abbott of the University of Pittsburgh and Alexander P. Wolfe of the University of Alberta in Edmonton.

The two geologists extracted a sediment core from a lake near Cerro Rico and measured concentrations of five metals associated with smelting. “Our data imply that several thousand tons of silver were produced in pre-Incan times,” Abbott says.

He attributes the scarcity of silver artifacts in Bolivia dating back a millennium to a combination of looting at archaeological sites, transport of silver elsewhere in the Americas as a trade item before the Spanish conquest, and export of silver overseas by the Spanish after conquest.

The ratio of metals such as silver, lead, and antimony in the lake core indicates whether smelting occurred, and radiocarbon analyses provided age estimates for soil layers in the core.

Cerro Rico ores were smelted from A.D. 1000 to A.D. 1200, a period ending close to the time that Tiwanaku society collapsed, the researchers conclude in the Sept. 26 Science.

Silver smelting declined markedly for the next 2 centuries, they say. From A.D. 1400 to A.D. 1650, the practice again flourished, first among the Incas and then their Spanish conquerors.

The new findings on early silver production support the view that Tiwanaku society grew considerably around 1,000 years ago, shortly before severe drought contributed to its breakup, remarks archaeologist Alan L. Kolata of the University of Chicago. Kolata directs an ongoing excavation of the Tiwanaku capital on Bolivia’s western border.

That project has unearthed many silver artifacts, including ceremonial masks, rings, and tubes packed with pigment. Kolata finds it “somewhat surprising but plausible” that Tiwanaku society produced thousands of tons of silver, as Abbott and Wolfe contend.

“The [new] estimate of the tonnage of silver produced prior to Inca involvement . . . is remarkable,” comments archaeologist Heather Lechtman of the Massachusetts Institute of Technology.

However, the latest archaeological evidence indicates that Tiwanaku society collapsed between A.D. 900 and A.D. 950, before the proposed start of silver production, she says.

Archaeological investigations under way in central and southern Bolivia need to identify other early cultures that may have initiated silver mining and smelting, Lechtman holds.

Analyses of metal concentrations in other Bolivian lakes will also prove crucial in disentangling the origins of silver production, Abbott says.


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Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.

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