Analysis of the potential for coal utilization in metallurgy at the Jirentaigoukou Site

Archaeometallurgical research in China has substantially progressed since the 1970 s, yielding important insights into the origins of copper smelting, the development of bronze-casting systems, and the evolution of copper production technologies. The present study aims to assess whether coal was used as a metallurgical fuel at the Jirentaigoukou (JRTGK) site in the Ili River Valley, Xinjiang.

Coal samples from JRTGK were classified following the Chinese national standard GB/T 5751-2009, supported by proximate analysis. Fourier transform infrared spectroscopy (FTIR) was performed on unburned coal samples mixed with minor amounts of soil, as well as on combusted soil residues collected from JRTGK. The temperature range potentially reached during coal combustion was inferred from changes in the peak intensities of characteristic infrared absorption bands corresponding to specific chemical structures. Additionally, furnace lining samples from JRTGK and slag samples from the Nulasai copper mine were analyzed by X-ray diffraction (XRD), X-ray fluorescence (XRF), and thermogravimetric analysis–differential scanning calorimetry (TGA–DSC).

Radiocarbon dating of animal bones from JRTGK indicates an occupation period of approximately 3400 years before the present. The coal sample was identified as 41CY long-flame bituminous coal. FTIR analysis shows that soil burnt with coal at JRTGK was heated to at least 1100–1200 ℃, meeting the thermal requirements for metal production. The major components of the furnace lining and slag samples require processing temperatures of 1500 ℃ or higher to enable effective smelting.

The JRTGK culture is considered an extension and local development of the Andronovo culture in the Ili River Valley, with a continuous cultural sequence linking the two. However, these cultures cannot be considered identical. Human populations in the upper Ili River Valley (UIRV) during this period cultivated multiple crops, domesticated various poultry and livestock, and cast bronze artifacts—reflecting a relatively advanced level of socioeconomic development. The transition from the mid-Holocene to the late Holocene witnessed remarkable progress in human cultural development, largely attributable to major discoveries and breakthroughs achieved through continuous human–environment interaction. During this interval, a cold and dry climate reduced the extent of coniferous forest on the Tianshan Mountains, leading to a shortage of timber for domestic heating and daily use. Thus, local populations explored alternative survival strategies, eventually discovering that coal could partially replace wood as a fuel with a longer burning duration. Given the abundant outcrop coal resources in the JRTGK region and the present experimental results, it is inferred that coal was probably used as an auxiliary fuel for preliminary ore processing at the Nulasai copper smelting site. The inhabitants of the Ili River Valley may therefore have pioneered coal-fueled copper smelting technology as early as the late Xia Dynasty or early Western Zhou Dynasty, with subsequent transmission of this technology to the Central Plains. These findings have the potential to refine current understandings of the history of human energy use and provide an early case study of how multiple factors, including societal demand, can drive technological innovation in energy utilization.