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Precious metals in medieval West Africa

About this Project

The archaeological materials investigated are from the site of Tadmekka in northern Mali, an early Islamic trans-Saharan trading site excavated and studied by Dr. Sam Nixon, an archaeologist and senior researcher at the University of East Anglia (UK), since 2005, and include the only known archaeological examples of molds for producing gold coins, as well as the only physical evidence for pre-modern silver coins found in West Africa. An experimental archaeology study of these gold-coin molds and their associated gold-working crucibles was undertaken, to understand their original form and function, and the chaîne opératoire of the goldsmithing process they related to, in turn enabling the reconstruction of replicas. The 3D surface shape of the silver coins, currently hidden under layers of encrustation and corrosion, was also non-destructively captured, as a preliminary step in the characterization of the coins prior to cleaning by selective and irreversible removal of the corrosion. The combination of these studies, together with previous chemical analysis on gold droplets affixed to the molds, will provide major advances in our research understanding of precious metals extraction, use and trade, and monetary cultures of pre-modern West Africa, as well as forming a crucial element of the interpretation, long-term preservation and display of this material in Mali. The site of Tadmekka is to form a central element of the exhibition Caravans of Gold, Fragments in Time, to be shown at Northwestern’s Block Museum in 2019.

The Investigation

The Process of Refinement

The gold refining process evidenced by the crucibles found in Tadmekka involved first mixing a panned gold sediment with glass cullet, likely recycled from broken glass vessels or beads, then melting the mixture at high temperatures. The crushed glass used by the metalworkers in the refining process acted as a molten “bath,” leading to the mineral particles being separated from the gold dust, thereby leaving the refined gold. This process of refinement has been replicated in our lab, using gold powder, sand, and finely crushed glass, to demonstrate its efficacy. After the firing process, a gold button formed at the bottom of a crucible. Succeeding in reconstructing this gold extraction procedure through experimental archaeological work represents a major contribution to the study of early gold metallurgy within the trans-Saharan trade and within gold metallurgy on a global scale.

The Coin Casting

Using 3D-imaging/printing techniques, we replicated the original molds and tested their fabrication process, providing a fuller understanding of how the form of the molds shaped the appearance of the coins made from them. Through this process, we aim at producing the closest evidence possible of the actual Tadmekka gold coins, which, unfortunately, have never been recovered. The available fragments of coin molds make it possible to infer the original form of the molds, as one of the pieces is clearly a section of a larger, circular plate-like object. This allowed casters to make up to thirty blanks in a single mold. These blanks could be easily hammered into the form of Islamic gold coins (dinars) to meet the expectations of Islamic traders.

The Silver Coins

The materials from Tadmekka give insight into early trans-Saharan trade and the arrival of Islamic culture. The pre-modern silver coins from the site have layers of encrustation and corrosion which prevents their characterization and the display of Arabic symbols that would potentially add to this insight. In collaboration with the Center for Advanced Molecular Imaging, we succeeded in imaging five silver coins from Tadmekka. Thanks to its ability to “see beneath the surface”, non-destructive X-ray computed tomography (CT) was used to characterize these coins, which are of particular interest/value because they are the only physical evidence for pre-modern coins in West Africa.

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