Abstract
For forty years, there has been a widely held belief that over 2,000 years ago the Chinese Qin developed an advanced chromate conversion coating technology (CCC) to prevent metal corrosion. This belief was based on the detection of chromium traces on the surface of bronze weapons buried with the Chinese Terracotta Army, and the same weapons’ very good preservation. We analysed weapons, lacquer and soils from the site, and conducted experimental replications of CCC and accelerated ageing. Our results show that surface chromium presence is correlated with artefact typology and uncorrelated with bronze preservation. Furthermore we show that the lacquer used to cover warriors and certain parts of weapons is rich in chromium, and we demonstrate that chromium on the metals is contamination from nearby lacquer after burial. The chromium anti-rust treatment theory should therefore be abandoned. The good metal preservation probably results from the moderately alkaline pH and very small particle size of the burial soil, in addition to bronze composition.
Highlights
The Qin Terracotta Army of Xi’an is an array of life-sized, realistic ceramic figures representing warriors, stationed in three large pits within the mausoleum of Qin Shihuang (259–210 BC), the first emperor of a unified China
Our research set out to revisit this matter by addressing the following questions: What is the frequency of chromium presence on the surface of the weapons? Is there a correlation between surface chromium presence and bronze preservation? What is the source of chromium? Is surface chromium the result of deliberate action? Here we present analyses of a large sample of archaeological weapons, soils and associated materials, experimental replications of chromium surface treatments, and accelerated ageing experiments of bronzes
We propose for the first time that, rather than chromium presence, one key reason for the good preservation of the bronzes may be in the quality of the local soil
Summary
The Qin Terracotta Army of Xi’an is an array of life-sized, realistic ceramic figures representing warriors, stationed in three large pits within the mausoleum of Qin Shihuang (259–210 BC), the first emperor of a unified China. Our large sample size allowed us, for the first time, to observe patterns in the presence/ absence of chromium: this element was frequently detected in pommels and other fittings of swords and lances (88% of the samples in this category), the handles and tumblers of crossbow triggers (75%), and the ferrules placed at the distal end of long weapon shafts (67%). It was only rarely detected in trigger levers or bolts (13–17%), arrow heads (1.5%) and arrow tangs (0.5%), and in none of the blades from swords and lances (Tables 1 and S2; Fig. 2A)
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