Hammerscale and slag inclusions: New insights into metal supply during the early iron Age in Western Europe

Understanding the nature and origin of iron processed in ancient smithing workshops is essential for unraveling procurement strategies and, more broadly, the circulation of iron. Studying smithing slags traditionally proves complex due to their diverse compositions influenced by hearth substances. Another technique, analyzing slag inclusions in metallic fragments, offers direct insights into the iron's chemical nature. However, the scarcity or absence of these fragments poses challenges in assessing sample representativeness. An alternative approach studies hammerscale, hot iron waste from hammering, providing insights into iron's chemical composition and source. At Weyersheim, an early Iron Age site in France, extensive analysis of 300 hammerscale samples and 77 slag inclusions in 6 iron scraps unveiled a unique geochemical signature. This signature showcases enrichment in Vanadium (V > 800 ppm), Chromium (Cr > 400 ppm), Rare Earth Elements (REE) (∑REE + Y > 1500 ppm), especially Heavy Rare Earth Elements like Yb (>100 ppm), and High Field Strength Elements (HFSE) (e.g., U > 30 ppm). The remarkable chemical uniformity observed in various hammerscale samples, and the slag inclusions indicates that the majority of the processed iron originates from a singular source. Comparison with known iron artifacts, particularly the bi-pyramid iron bars revealed similarities. This source exhibited uncommon enrichment and partitioning patterns of REEs, hinting at a rare geological deposit or a closely associated group of deposits. Plausibly sourced from weathered peralkaline rocks, the iron ore's geological origins potentially link to regions like Vosges and the Black Forest massifs. Nevertheless, identifying the geological formations accountable for these distinctive iron traits remains challenging, as there have been no findings of deposits or iron slag with similar compositions to aid in identification.