Cast a different iron: Grey and mottled cast iron production in early China

Cast iron smelting has been the dominant technique for iron production in Central Plains China since the 5th century BC. Objects made of grey or mottled cast iron have been identified in metallographic studies, yet the techniques employed to make them or the underlying motivations for producing such materials have not been studied in detail. Here we present analyses of mold-cast objects excavated at two civilian cemeteries in north-eastern Xi’an, Shaanxi Province, which are dated to the late Warring States period (ca. 3rd century BC). Metallographic examinations show that the majority of these products are made of mottled or grey cast iron, with flake graphite on the pearlitic matrix, indicating a systematic and purposeful production of materials that clearly required production techniques different from those involved in the making of white cast iron. Based on a comprehensive discussion, we argue that the production of such materials was an intentional process based on controlling the solidification process during casting, which allowed a good balance between acceptable quality and low cost in the manufacture of certain types of artefacts.