Rock magnetism for investigating the firing temperature of ancient ceramic artifacts: The case of the Sada Nishizuka coffin, Japan

The Kofun Period is an important epoch of the Japanese protohistory, spanning from the early 3^rd to the early 7^th century AD. It takes its name from the large megalithic mound tombs, called kofuns (“ancient graves”), which characterize and define this period. Such monumental burial mounds comprised a central stone funeral chamber, inside which a wood, clay or stone coffin was positioned together with offerings to the dead, such as jewelry, ornaments, weapons and pottery. Kofuns are of great importance for better understanding the Japanese archaeology and the social evolution that took place during the Kofun period in Japan. At the same time, the study of the pottery style and the technology of the baked clay coffins offers information about the ancient ceramic technology and the evolution of the funeral practices over time. In this study, we use rock magnetic analysis to investigate the firing temperatures and production technology of a clay coffin excavated at the Sada Nishizuka archaeological site, in Okayama prefecture, Japan. Thermal demagnetization, magnetic susceptibility, thermomagnetic curves and hysteresis loops performed before and after laboratory heating at increasing temperatures were used to determine the ancient firing temperatures. The results showed that all samples, even if belonging to different fragments coming from different parts of the coffin, experienced important enhancement on their magnetic properties at temperatures around 300-400 ^oC. Such important changes suggest that the coffin was fired at relative low temperatures, which were not enough to cause complete magnetic mineralogy transformations. The magnetic results were further compared with independent data obtained from X-ray powder diffraction analysis (XRPD). The clay mineral pattern obtained from the XRPD data indicates firing temperatures around 400 °C, based on the presence of kaolinite, while in some samples clay minerals characterized by thermal stability up to higher temperatures were also observed. Such results suggest that the coffin, probably due to its large dimensions, has not experienced homogeneous heating during its production, and thermal equilibrium was reached at relatively low temperatures, much lower than those applied in the pottery production. Nevertheless, such firing was sufficient to harder the coffin and make it suitable for its use. This study aims to highlight the potential of magnetic analyses to investigate ancient baked clays and the importance of multidisciplinary archaeometric analysis to better understand our past.