Sometimes a story that appears too good to be true, actually turns out to be real. Accounts from the 11th century of West African gold coins of unsurpassed purity were dismissed as exaggerations by modern historians. However, after evidence was found in 2014 to support the tales, material scientists have reconstructed their method of purification and shown it works, without the hazardous mercury other civilizations used.

Humanity has a funny way of inventing and believing improbable tales about gold, from lost cities made entirely of it, to streets paved with the metal. So historians' skepticism regarding stories of the purity of unstamped “blank dinar” ingots from Mali wasn't simply a racist refusal to believe African technology surpassed that of Europe at the time, although that may have been a factor.

However, when University of East Anglia archaeologist Dr Sam Nixon found crucibles and coin molds containing gold-flecked glass slag at Tadmekka, Mali he wondered if there might be something to these tales.

Together with Professor Thilo Rehren, a specialist in ancient technologies, Nixon published a paper in the Journal of Archaeological Science setting out how the Tadmekkans may have heated gold, sand, and glass to “Separate panned gold particles from mineral contamination...by melting the gold and floating the mineral particles off in a light slag melt.” In this way, they achieved purity contemporary civilizations could only match with mercury.

Dr Marc Walton of Northwestern University has now tested Nixon and Rehren's proposed method and found it works. “These medieval Africans, at a confluence of trade routes in the Sahara, were sophisticated in their use of available materials,” Walton said in a statement. “Their technique of percolating raw materials through molten glass had not been seen before. It is unique to the archaeological record.” The oldest record of a similar technique elsewhere comes from an Australian gold panner in 1904.

The Tadmekkan approach to gold purification didn't rely on any specific local features. Presumably, they used sand from the Sahara, whose southern boundary borders the city, but Walton took his from the shores of Lake Michigan.

West African civilizations had not used gold as currency until they came in contact with Arab traders, with glass beads being their closest equivalent. When they discovered how much their new trading partners valued the metal, which they had previously used for decorative purposes, they took advantage of the rich mines located in some of their territories.

It seems the Tadmekkans quickly worked out how to refine gold with greater purity than civilizations that had been using it as a means of exchange for centuries.

Nixon also found the first evidence for the African production of crucible steel in the Tadmekkan metal-works, along with flecks of copper slag, revealing their innovations in metallurgy were not limited to gold.