Surface chromium on Terracotta Army bronze weapons is neither an ancient anti-rust treatment nor the reason for their good preservation

Title	Surface chromium on Terracotta Army bronze weapons is neither an ancient anti-rust treatment nor the reason for their good preservation
Authors	Marcos Martinón-Torres, Xiuzhen Li, Yin Xia, Agnese Benzonelli, Andrew Bevan, Shengtao Ma, Jianhua Huang, Liang Wang, Desheng Lan, Jiangwei Liu, Siran Liu, Zhen Zhao, Kun Zhao, Thilo Rehren
Magazine	Scientific Reports
Date	04/04/2019
DOI	10.1038/s41598-019-40613-7
Introduction	For four decades, a prevalent theory posited that the ancient Chinese Qin dynasty, over two millennia ago, employed sophisticated chromate conversion coating technology (CCC) for metal corrosion prevention. This theory stemmed from the presence of chromium traces on bronze weaponry interred with the Terracotta Army and the remarkable state of preservation of these artefacts. Our investigation involved analysing weapons, lacquer, and soil samples from the archaeological site, alongside performing experimental simulations of CCC and accelerated ageing tests. The findings indicate a correlation between surface chromium and artefact type, but no link to the preservation of the bronze itself. Further analysis reveals that the lacquer coating the warriors and specific weapon components contains significant chromium levels. We establish that the chromium found on the metal surfaces originates from contamination by this adjacent lacquer post-burial. Consequently, the theory attributing metal preservation to chromium anti-rust treatment should be reconsidered. The exceptional metal preservation is likely attributable to the moderately alkaline pH and fine particle size of the burial soil, in conjunction with the bronze's inherent composition.
Quote	Marcos Martinón-Torres, Xiuzhen Li and Yin Xia et al. Surface chromium on Terracotta Army bronze weapons is neither an ancient anti-rust treatment nor the reason for their good preservation. Sci Rep. 2019. Vol. 9. DOI: 10.1038/s41598-019-40613-7
Element	Chromium (Cr) , Copper (Cu) , Tin (Sn)
Industry	Defense , Research & Laboratory