Science Sheds Light on Mystery of Terracotta Army Weapons

View of Pit 1 of the Terracotta Army showing the hundreds of warriors once armed with bronze weapons. (Image: Xia Juxian)

The chrome plating on the Terracotta Army weapons made of bronze — once thought to be the earliest form of anti-rust technology — derives from a decorative varnish rather than a preservation technique, a new study finds co-led by UCL and Terracotta Army Museum researchers. The study, published in Scientific Reports, suggests that the chemical composition and characteristics of the surrounding soil, rather than chromium, may be responsible for the weapons’ famous preservation power.

Lead author Professor Marcos Martinón-Torres, University of Cambridge and formerly of UCL Institute of Archaeology, said:

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The world-famous Terracotta Army of Xi’an consists of thousands of life-sized ceramic figures representing warriors, stationed in three large pits within the mausoleum of Qin Shihuang (259-210 B.C.), the first emperor of a unified China. These warriors were armed with fully functional bronze weapons; dozens of spears, lances, hooks, swords, crossbow triggers, and as many as 40,000 arrowheads have all been recovered.

View of Pit 1 of the Terracotta Army showing the hundreds of warriors once armed withbronze weapons. Image: Xia Juxian)
View of Pit 1 of the Terracotta Army showing the hundreds of warriors once armed with bronze weapons. (Image: Xia Juxian)

Although the original organic components of the weapons, such as the wooden shafts, quivers, and scabbards, have mostly decayed over the past 2,000 years, the bronze components remain in remarkably good condition. Since the first excavations of the Terracotta Army in the 1970s, researchers have suggested that the impeccable state of preservation seen on the bronze weapons must be the result of the Qin weapon makers developing a unique method of preventing metal corrosion.

One of the bronze swords from the Terracotta Army showing an excellent state of preservation, with a shiny and sharp blade. Also shown are the metal fittings from the grip and scabbard, which were made of organic material and have not survived. Credit: Zhao Zhen
One of the bronze swords from the Terracotta Army showing an excellent state of preservation, with a shiny and sharp blade. Also shown are the metal fittings from the grip and scabbard, which were made of organic material and have not survived. (Image: Zhao Zhen)

Traces of chromium detected on the surface of the bronze weapons gave rise to the belief that Qin craftspeople invented a precedent to the chromate conversion coating technology, a technique only patented in the early 20th century and still in use today. The story has been cited in some books and media. Now, an international team of researchers has shown that the chromium found on the bronze surfaces is simply contamination from lacquer present in adjacent objects, and not the result of any ancient technology.

Detail from the grip and blade from one of the Terracotta Army swords. In most of the swords analysed, the highest concentrations of chromium are detected in the guard and other fittings, which would have been in contact with the lacquered organic parts. Image: Zhao Zhen)
Detail from the grip and blade from one of the Terracotta Army swords. In most of the swords analyzed, the highest concentrations of chromium are detected in the guard and other fittings, which would have been in contact with the lacquered organic parts. (Image: Zhao Zhen)

The researchers also suggest that the excellent preservation of the bronze weapons may have been helped by the moderately alkaline pH, small particle size, and low organic content of the surrounding soil. Dr. Xiuzhen Li (UCL Institute of Archaeology andTerracotta Army Museum), co-author of the study, said:

By analyzing hundreds of artifacts, researchers also found that many of the best-preserved bronze weapons did not have any surface chromium. To investigate the reasons for their still-excellent preservation, they simulated the weathering of replica bronzes in an environmental chamber.

View of Pit 1 of the Terracotta Army showing the hundreds of warriors once armed with bronze weapons. Image: Xia Juxian)
View of Pit 1 of the Terracotta Army showing the hundreds of warriors once armed with bronze weapons. (Image: Xia Juxian)

Bronzes buried in Xi’an soil remained almost pristine after four months of extreme temperature and humidity, in contrast to the severe corrosion of the bronzes buried for comparison in British soil. Co-author Professor Andrew Bevan (UCL Institute of Archaeology) said:

Provided by: University College London [Note: Materials may be edited for content and length.]

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