Bloomery slags from Puisaye (France): reconstructing technological changes in a major iron production area from the Iron Age to the Late Middle Ages

Archaeological research conducted as part of the PCR and TerriFer project in the Bourgogne-Franche-Comté region has identified Puisaye as a major center of primary iron production, active from the Iron Age through to the end of the Middle Ages. A total of 110 slag heaps were investigated and sampled through surface surveys, excavations and waste collection. The bloomery slags were classified based on their morphology, chemical composition (by ICP-MS-AES), and mineralogical (by optical microscopy, SEM-EDS and XRD) characteristics. These data were cross-referenced with 120 radiocarbon dates, allowing us to associate specific slag types with distinct periods of activity and to track technological changes over time. For the early periods, changes are mostly characterized by an evolution of the architecture of the furnaces. The late Iron Age and Antiquity are periods of experimentation with new practices, and a time of major innovation in the way that techniques were conducted. For the Middle Ages, a new uniformity in the smelting production is documented. This makes Puisaye a key region for studying the long-term evolution of iron smelting techniques in Western Europe.