After excavating ancient coin molds (dated to the 11th century) in Mali in 2005, an archaeologist wanted to find out how medieval Africans purified the gold they were using for their currency. Sam Nixon discovered little droplets of highly refined gold left over in the molds and fragments of glass.

Scientists, through a collaboration between Northwestern University and the Art Institute of Chicago, re-created the process behind the purification method they used. Europeans in the 10th and 11th centuries purified their gold through cupellation, a process in which lead is mixed with gold laced with impurities, and then heated in a furnace until the droplets of purer gold can be skimmed off. Medieval West Africans, however, “were taking the ore and other raw materials from the river and mixing it with glass.”

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