Characterizing the nature and origin of iron processed in an archaeological smithing workshop is crucial for understanding iron circulation and procurement strategies. However, this task presents challenges. The traditional approach involves studying smithing slags, but their chemical compositions are complex and influenced by various substances accumulated in the smithing hearth. Another method involves analyzing slag inclusions in metallic fragments, which can provide a direct information about the chemical nature of the iron worked. Nevertheless, these fragments are frequently scarce or even absent, rendering it challenging to evaluate the representativeness of the samples. A new approach focuses on studying hammerscale, which is the waste generated during the hammering of hot iron. Unlike smithing slag, each hammerscale is produced from a single piece of iron. Unlike metal objects, hammerscale is often found in large quantities. Previous studies on experimental materials have demonstrated that slag inclusions in the worked iron contribute to the formation of hammerscale. The analysis of hammerscale can therefore provide valuable information about the chemical composition and source of the iron that was processed in the smithing workshop. Studying a hammerscale assemblage would allow to assess the diversity of the iron worked in the forge.The Weyersheim archaeological site in northeastern France, dating back to the early iron age (HAD3-LTA, 5e BC) includes various structures, with a notable forge being of particular importance. Extensive chemical analysis of 300 hammerscale samples and 77 slag inclusions from 6 iron waste has provided valuable insights. The findings reveal a distinctive geochemical signature characterized by enrichment in elements like Vanadium (V>800 ppm), Chromium (Cr>400 ppm), Rare Earth Elements (REE) (∑REE+Y>1500 ppm), particularly Heavy Rare Earth Elements such as Y (>500 ppm) and Yb (>100 ppm), High Field Strength Elements HFSE (eg. U>30 ppm). The similarity between the hammerscale and slag inclusions measured in iron waste samples points a direct connection between the slag inclusions and hammerscale. The hammerscale samples exhibited a remarkable consistency in their chemical nature, indicating the utilization of one single chemically homogeneous iron source in the numerous forging operations performed by the blacksmith. This scenario implies either a continuous supply from a single producer or a brief and intensive work utilizing a steady iron stock. This iron source displayed specific characteristics, particularly the enrichment and partitioning pattern of REE, that are uncommon and suggest a rare and distinct geological deposit or a closely associated group of deposits. Furthermore, comparison with known iron artifacts from the surrounding regions indicated potential connections with the bi-pyramid iron bars, specifically the iron bar recovered from Marsal (France). The elevated levels of V, Cr, Al, REE and HFSE found in the slag inclusions in iron waste and hammerscale samples from Weyersheim, as well as in the majority of slag inclusions from bi-pyramid iron bars, indicate that the iron ore used underwent substantial enrichment in these elements. Plausibly, the iron ore could correspond to a weathered formation derived from peralkaline rocks. The Vosges and the Black Forest massifs are identified as potential sources for the iron ore, considering their geological history. However, no deposits or iron sl
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