Unveiling diverse copper trace element profiles from a single smelting site through laser ablation–inductively coupled plasma–mass spectrometric analysis

As an important provenance indicator, trace element profile has been widely used to reveal the origin and mixing history of archaeological bronzes. However, due to the lack of chemical characterization of raw copper from smelting sites, there has been little understanding about the variation of trace element concentration within one copper source. This has significantly confined discussion on copper circulation in prehistoric societies. This research developed a method to analyze copper prills embedded in smelting slag based on laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS). A series of tests have proved that a relative error of less than 20% can be achieved for most elements with appropriate choice of analytical parameters and reference materials. This method is then used to analyze the Early–Middle Shang (1500–1200 BC) period copper smelting slag from the site of Tongling in northern Jiangxi province, China. The results show that the copper smelted from different ores of the same site has systematically different trace element characteristics, and copper prills of this site are highly varied in terms of Ag, Ni, Co, As, and Bi content. When plotting Tongling prills together with the bronze artifacts from the Panlongcheng site, also in the Middle Yangtze River valley, it is found that the trace element concentration of these two sites correspond well with each other, and the Panlongcheng bronzes with highly varied trace element profile could have been manufactured with copper from one source. This analysis demonstrates the potentially heterogeneous nature of the copper trace element profiles produced at one smelting site, and calls for more analysis of copper prills using LA-ICP-MS to establish a new foundation for future discussions on copper provenance based on trace element data.