Chrome plating on Terracotta Army bronze guns – once thought to be the first form of anti-rust technology – derives from a decorative varnish rather than a preservation technique, finds a new study co-led by UCL and researchers from the Terracotta Army Museum.
The study, published today in Scientific Reports, reveals that the chemical composition and characteristics of the surrounding soil, instead of chromium, may be responsible for the famous weapon preservation power.
Professor Marcos Martinón-Torres (University of Cambridge and former UCL Institute of Archeology) commented: "The terracotta warriors and most of the mausoleum's organic materials were coated with protective layers of varnish before being painted with pigments – but curiously , not the brass guns ".
"We found a substantial content of chromium in the lacquer but only a trace of chromium in the pigments and soil nearby – possibly contamination.The highest traces of chromium found in the bronzes are always in the parts of weapons directly associated with today's decomposing organic elements, such as as spear rods and sword loops made of wood and bamboo, which would also have a varnish coating. Clearly, lacquer is the unintended source of chromium in bronzes – not an old anti-rust treatment. "
The world-famous Terracotta Army of Xi'an consists of thousands of full-size ceramic figures representing warriors, stationed in three large wells within the Mausoleum of Qin Shihuang (259-210 BC), the first emperor of a unified China .
These warriors were armed with fully functional bronze weapons; dozens of spears, spears, hooks, swords, beast triggers and up to 40,000 arrowheads were all recovered. Although the original organic components of the weapons, such as wooden rods, shafts and sheaths have declined to a large extent over the past 2,000 years, the bronze components remain in excellent condition.
Since the early excavations of the Terracotta Army in the 1970s, researchers have suggested that the flawless state of preservation seen in bronze weapons should be the result of Qin gun manufacturers developing a unique method of preventing metal corrosion.
Chromium traces detected on the surface of bronze weapons gave rise to the belief that Qin craftsmen invented a precedent for chromate conversion coating technology, a technique patented only in the early 20th century and still in use today. The story was quoted in some books and media.
Now, an international team of researchers shows that the chromium found on bronze surfaces is simply contamination of the lacquer present in adjacent objects, not the result of an old technology. The researchers also suggest that the excellent preservation of bronze weapons may have been aided by the moderately alkaline pH, small particle size and low organic content of the surrounding soil.
Dr. Xiuzhen Li (UCL Institute of Archeology and Museum of the Terracotta Army), co-author of the study, said: "Some of the brass weapons, swords, spears and halberds exhibit almost perfect surfaces and sharp blades after 2000 years buried with The hypothesis for this was that Qin's gun makers could have used some kind of anti-rust technology because of the chrome detected on the surface of the weapons, but the preservation of the weapons continued to intrigue the scientists for more than forty years.
"The high tin composition of the bronze, the quenching technique and the particular nature of the local soil somehow explain its remarkable preservation, but it is still possible that the Qin Dynasty has developed a mysterious technological process and this deserves further investigation."
In examining hundreds of artifacts, the researchers also found that many of the best-preserved brass weapons had no surface chromium. To investigate the reasons for their still excellent preservation, they simulated the weathering of replica bronzes in an environmental chamber. Bronzes buried in Xi'an soil remained almost untouched after four months of extreme temperature and humidity, in contrast to the severe corrosion of buried bronzes for comparison on British soil.
"It is striking how many important and detailed insights can be recovered through the evidence of both the natural materials and the complex artificial recipes found throughout the mausoleum complex – clay, wood, lacquer and pigments, to name a few.These materials provide complementary tangles in a larger tale of craft production strategies at the dawn of China's first empire, "said co-author Professor Andrew Bevan (UCL Institute of Archeology).
Professor Thilo Rehren (Institute of Cyprus and UCL Institute of Archeology) emphasized the importance of long-term collaboration. "We have started this research for more than 10 years between UCL and the museum. Only with the persistence, trusting cooperation and innovative thinking of our colleagues in China and Britain have we been able to solve this mystery that has lasted for a decade."
Notes to editors
For more information or to speak with the researchers involved, contact:
Natasha Downes, UCL Media Relations. T: +44 (0) 20 3108 3844, E: [email protected]
Surface chromium in the Terracotta Army's bronze weapons is neither an old anti-rust treatment nor the reason for its good conservation is published in Scientific Reports.
"The Terracotta Army Museum's Cooperative Project: Terracotta Army's Confection" is a Research Project of the British Academy which is part of the collaboration between the UCL Institute of Archeology and the Mausoleum Museum of Emperor Qin Shihuang. Some of the team members have already moved to Cambridge University and the Cyprus Institute, but continue to discuss further cooperation.
The research was funded by the British Academy and the Institute for Archaeo-Metallurgical Studies.
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