Geochemical Link Between Slag Inclusion and Hammerscale, a Potential New Tool to Better Understand the Iron Supply of an Archaeological Workshop

Abstract

Hammerscale is the waste formed during hammering of hot iron on the anvil. It is mainly composed of iron oxides (wüstite, magnetite, hematite) with a variable amount of silica-rich component. This research focuses on the potential geochemical link between hammerscale and smelting derived slag inclusion contained in the metallic iron. In the frame of previous research, the smithing practice of African Dogon craftsmen (Mali) has been recorded in details. Different samples were collected including, slag, hammerscale, metal. Two different groups of iron bars produced by different bloomery process and more than one hundred experimental pieces of hammerscale were chemically investigated in situ by Energy Dispersive X-ray Spectroscopy (SEM–EDS) and by X-ray Fluorescence spectrometry (WD-XRF) in Fribourg / Switzerland. A small part of them was also analyzed by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) in Orléans / France. The results show a strong contamination of the hammerscale by the working environment (flux, ashes, sand, etc.) impacting the concentrations of most of non-reducible elements, in particular the most abundant ones (Si, Al, K, Ca). However, several in situ analyses revealed abnormal high contents of Ti, Mn, Zr in contrast to the chemical composition of the forging environment. The concentrations of these elements are affected by contamination but the ratios, in this context, remain unchanged. TiO2/Zr ratios measured in slag inclusion are consistent with the ratios measured in hammerscale produced from the same iron type. Those results support that hammerscale could still carry the chemical signature of the smelting system and thus the geochemical fingerprint of the iron worked. Applying this methodology on archeological hammerscale would make it possible to assess the chemical diversity of worked irons and to provide information about the primary ore sources. Improving our current knowledge of the workshop iron supplies and therefore the circulation of iron in the past times.