The presence of traces of chromium on bronze weapons from the Qin Terracotta Army may not represent evidence of an advanced preservation method, according to research published in Scientific Reports. The research suggests that the presence of chromium may have been the result of contamination from the lacquer used to treat wooden parts of the weapons. This challenges a long-standing hypothesis that the Qin craftspeople had developed a chromium anti-rust system over 2,000 years ago.
The Qin Terracotta Army of Xi’an was created for the emperor Qin Shihuang (259-210 BC). The site in Xi’an, China, was first excavated in the 1970s and so far 2,000 well-preserved ceramic warriors with fully functional weapons have been excavated. Previous research has suggested that the good preservation of the weapons is due to chromium being used as an anti-rust treatment, applied by the Qin craftspeople to prevent them decaying in the afterlife.
Marcos Martinon-Torres and colleagues examined 464 weapons from the site and found the presence of chromium on only 37. They suggest that the presence of chromium on some weapons is due to contamination from lacquer used by the Qin craftspeople. Chromium was detected on 88% of weapon parts near fittings, such as handles, grips, or scabbards, whereas chromium on arrow heads or sword blades was detected in 2% of the samples or less. The authors argue that the pattern of distribution is due to the metal parts' proximity to the lacquer used to treat the wooden elements of the weapons, which have since decayed. They also propose a number of other factors that may have contributed to the preservation of the weapons including the high tin content of some of the bronze and the soil composition of the site.