Phytolith analysis reveals plant tempering in bronze-casting moulds from Bronze Age China (1250 -221 BC) as a technological choice

Ceramic bronze-casting moulds played an important role in determining the quality of ancient Chinese bronze artefacts. While the addition of vegetal temper to ceramic moulds is considered an important factor in enhancing mould quality, detailed studies on this practice have been limited. This study conducts phytolith analysis of ceramic moulds from four Bronze Age foundry sites: Yinxu (Late Shang), Yaoheyuan (Western Zhou), Houma (Eastern Zhou), and Guanzhuang (Eastern Zhou). A comparative analysis of these samples with local loesses revealed significant differences in phytolith compositions and charcoal concentration, indicating intentional use of plant materials in the moulds rather than a result of natural inclusions. The primary vegetal tempers were various species of Poaceae grasses, with notable differences in selection across sites, reflecting various technological choices. Although foxtail millet (Setaria italica), broomcorn millet (Panicum miliaceum) and rice (Oryza sativa) husks were identified in some moulds used to make more exquisite artefacts, they were not the dominant vegetal temper used during the Bronze Age, in contrast to the widespread use of rice husks as the main temper material in the later periods. These findings provide preliminary insights into the technological choices of Bronze Age craftsmen, and highlight temporal variations in temper material selection and its impact on mould performance.