Abstract
Intricate ceramic bronze-casting moulds are among the most significant archaeological remains found at Bronze Age metallurgical workshops in China. Firing temperature was presumably one of the most important technical factors in mould making. However, it has proven difficult to determine the firing temperatures of excavated moulds using existing analytical methods. This study establishes an innovative new method for using Fourier-transform infrared spectroscopy (FTIR) to estimate the firing temperature of clay-containing remains. The method is based on the finding that the infrared absorptivity of fired clay minerals, measured at the Si–O–Si stretching resonance band, is negatively correlated with firing temperature. Moulds and mould cores dating to the Early Shang period (sixteenth to fourteenth century BCE) are found to have been fired at extremely low temperatures—as low as 200–300 °C in many instances. These results provide critical new data for understanding the metallurgical technology of ancient China.
Highlights
The production of bronze vessels was the most prominent handicraft industry in Bronze Age China (c. twentieth to fifth century BCE) and was deeply embedded in the ritual and political systems of the Shang and Zhou dynasties
Research has shown that the thermal expansion method used in these initial analyses may significantly overestimate the firing temperatures of low-fired ceramics—i.e., of ceramics fired below the sintering temperature of c lay[10]
Irreversible transformations that occur during the firing of clays and related silicate minerals can be identified in Fourier-transform infrared spectroscopy (FTIR) absorption spectra, making it possible to estimate the firing temperatures of ancient ceramics[16,17,18] and of heated sediments[19,20,21]
Summary
The production of bronze vessels was the most prominent handicraft industry in Bronze Age China (c. twentieth to fifth century BCE) and was deeply embedded in the ritual and political systems of the Shang and Zhou dynasties. Thermal alteration appears most clearly in three regions of clays’ FTIR absorption spectra: the main Si–O–Si stretching vibration band (at wavenumbers around 1000–1100 cm−1), the Si–O–Al bending vibration band (510–580 cm−1), and the stretching vibration band of bound hydroxyl groups (3600–3700 cm−1) Changes to these three absorption bands—e.g., shifts in peak position or changes in peak shape—have served in past FTIR-based studies as qualitative criteria used to estimate firing temperature[19,23]. The position of this peak (initially around 1030 cm−1) remains unaltered in the range of 500–800 °C but shifts abruptly to higher wavenumbers (1080–1090 cm−1) once heated over 800 °C24 Due to this behavior, established FTIR methods may in many cases produce firing temperature estimates too imprecise to be meaningful for addressing archaeological questions
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