A selection of archaeometallurgical remains from the 3rd/4th century A.D., found in Aksum, Ethiopia, were analysed in order to determine the nature of the process by which they were produced, i.e. copper or iron smelting. Chemical and mineralogical analyses excluded a relationship to copper smelting; instead, all samples are consistent with a highly efficient iron smelting operation using the bloomery process and slag tapping furnaces. A lateritic iron ore containing at least 80 wt% FeO was smelted, resulting in an estimated one unit by weight of iron metal produced for every unit by weight of slag left behind, and little erosion of furnace wall material. The zoning of spinels, with chromium- and aluminium-rich inner parts and mixed hercynitic-ulvitic outer rims, reflects the evolution of the melt phase under strongly reducing conditions.