Mali was the heart of a wealthy empire in medieval times, with vast natural resources and traders traveling through the Sahara. Emperor Mansa Musa, who ruled in the 13th and 14th centuries, was one of the richest people ever. An archeological discovery in 2005 now reveals more about the way Mali handled gold. Ancient coin molds were found at Tadmekka, Mali, that contain residual drops of gold and glass shards that were used in the gold purification process.
"This is the first time in the archaeological record that we saw glass being used to be able to refine gold,” says Marc Walton, codirector of the Center for Scientific Studies in the Arts, a collaboration between Northwestern University and the Art Institute of Chicago. “The glass appeared to be material that was [actually] recycled glass materials … so it really shows the industriousness and creativity of the craftsmen, who understood the properties of gold and glass enough to [use them for] this process of refining gold.” The recycled glass materials were remnants of broken vessels. Tadmekka was a town right in the middle of the trans-Saharan caravan route, so Nixon uncovered several types of material culture that had to do with trade, namely molds for “bald dinar,” or coins that hadn’t been stamped with the name of a mint (or a 10th-century equivalent of one).
Walton's team tested the glass purification theory by doing it themselves, on a small scale, and it worked. Read about the Malian glass purification process at Atlas Obscura.