Medieval Africans had a great way to purify gold

Scientists have successfully used a gold purifying process from medieval West Africa.

Humble fragments of clay crucibles and coin molds flecked with gold that a team of British and Malian archaeologists excavated in 2005 led archaeologist Sam Nixon, in consultation with Thilo Rehren, a specialist on ancient materials and technologies, to theorize how West Africans used them to purify gold and cast unmarked coins during the 10th and 11th centuries in Tadmekka, Mali.

“Their technique … is unique to the archaeological record.”

Writings from the time—largely thought in recent times to be exaggeration—that praised Tadmekka’s pure gold “blank dinar” [coins] support the theory.

Now, materials scientists at Northwestern University have experimentally replicated the medieval gold purification method that Nixon and Rehren outlined in a 2014 paper using the same material resources and found the process works incredibly well. The unusual method involves heating a mixture of gold, sand, and glass to high temperatures and separating out the gold.

“These medieval Africans, at a confluence of trade routes in the Sahara, were sophisticated in their use of available materials,” says Marc Walton, who led the analytical team. “Their technique of percolating raw materials through molten glass had not been seen before. It is unique to the archaeological record.”

gold mold fragments - gold purifying
These coin mold fragments, excavated at the site of Tadmekka, Mali, are archaeological remnants of medieval gold processing. Institut des sciences humaines, Mali. (Credit: Clare Britt/Block Museum of Art)

Gold dust, glass, and sand

The team used sand from nearby Lake Michigan, gold dust, and recycled glass to conduct a reduced version of the original process. They melted the gold dust and filtered it through crushed glass to purify it. They then used copies of the original clay thumbprint molds to cast replica blank coins in bronze instead of gold, due to gold’s high cost.

Walton is co-director of the Center for Scientific Studies in the Arts, a collaboration between Northwestern and the Art Institute of Chicago. He and former postdoctoral fellow Gianluca Pastorelli conducted the replication experiments after Kathleen Bickford Berzock, the curator of the “Caravans of Gold” exhibition and associate director of curatorial affairs at Northwestern’s Block Museum of Art, put them in touch with Nixon about the excavation.

“Experiments like these allow us to envision life in medieval Saharan Africa with new detail and depth,” Berzock says.

Changing perceptions

Two of the molds used to produce gold coins in Tadmekka and three replica coins made of wax appear in Berzock’s exhibition, “Caravans of Gold, Fragments in Time: Art, Culture, and Exchange Across Medieval Saharan Africa,” currently at the Block Museum. The show on the movement of things, people, and ideas across the Sahara Desert in the medieval period aims to change perceptions of Africa’s role in the global economy in the 8th to 16th centuries.

“Archaeologists are bound by what they find,” says Walton, a research professor of materials science and engineering at Northwestern’s McCormick School of Engineering. “These fragments speak loudly about ancient civilizations and human history. Now we have proved the process used to refine gold in Tadmekka is real.”

Walton, Pastorelli, and Nixon, curator of Africa in the department of Africa, Oceania, and the Americas at the British Museum, are authors of a chapter in the exhibition catalog (Princeton University Press, 2019) about the preliminary results from the gold processing replication.

Source: Northwestern University