Efficiency of ancient iron production: A case study from the Kaniasso district, Ivory Coast (11th-19th century)

• Novel comparison of iron smelting techniques between KAN1 and KAN3 in Kaniasso, Côte d'Ivoire. • Developed a novel method to compare iron smelting techniques through efficiency. • KAN1 and KAN3 reflect distinct ore-sourcing strategies and shifts to mass production. • Temperature management in the furnace is crucial to optimizing iron output based on ore quality. This study explores the management of two iron smelting sites, KAN1 and KAN3, in the Kaniasso district of northwest Ivory Coast. These two technical traditions present quite different archaeological remains, including furnaces, tuyères, and slag. The KAN1 technique (11th-13th century CE.) used smaller furnaces and produced tapped slag with a consistent chemical composition, indicating a selective use of iron ores. In contrast, the KAN3 technique, from a later phase (17th-19th century CE.), shows a clear evolution towards mass production with larger furnaces and a more variable slag composition, reflecting the use of a wider range of ore sources. Bulk chemical analyses conducted using X-ray Fluorescence (XRF) on slag and ore samples revealed significant differences in ore types and slag compositions between the two techniques. Mass balance calculations were performed to estimate metal production and its evolution according to the techniques. The study introduces a new comparative approach that integrates the technical, economic, and social dimensions in order to question the efficiency of the processes. This theoretical reflection, based on archaeological examples, underscores the importance of considering the socio-economic context and the smelters expertise and know-how to better understand ancient metallurgical practices.