Determination of burning environment and temperature by colour and magnetic susceptibility based on heating simulation experiments and its application in Sanxingdui site in Sichuan, China

Magnetic susceptibility and colour are essential physical indicators for identifying traces of ancient fire use and are widely applied in archaeological research to reconstruct fire-related activities. The Sanxingdui site (4.8–2.8 ka) in Southwest China stands as one of the most significant archaeological discoveries of the 20th century. In this study, heating simulation experiments were conducted on Guanghan clay–widely distributed across the site–under both air and argon atmospheres, to investigate how its colour and magnetic parameters changed with temperature. Burnt soil from Pit K4 was used as a case study to infer the burning temperature and environment. The results indicated that changes in magnetic susceptibility and colour were closely tied to the phase transitions of magnetic minerals. Elevated magnetic susceptibility results from the formation of ferromagnetic minerals during high-temperature heating, whereas increased redness was attributed to the formation of hematite. The bluish-gray colour observed in an anaerobic environment was associated with the abundant formation of ferrous oxide. By comparing the colour and magnetic data of burnt soil from Pit K4 with those of Guanghan clay, combined with magnetic analysis and previous research, we inferred that fire rituals at the Sanxingdui site reached temperatures of approximately 850 °C and were likely conducted in open, well-ventilated settings. The method developed in this study, based on heating simulation experiments, for determining burning temperatures and environments using colour and magnetic susceptibility, can provide a reference for preliminary estimation of firing temperature ranges of burnt soil formed by human fire use or wildfires at numerous ancient Shu sites distributed on the Guanghan clay layer. It also provides insights into studying ancient fire-related behaviours at other archaeological sites.