Scientists shed light on preservation mystery of Terracotta Army weapons

Scientists shed light on preservation mystery of Terracotta Army weapons

Terracotta Army weapons Terracotta Army of Xi'an mausoleum of Qin Shihuang First Emperor of ChinaThe chrome plating on the Terracotta Army bronze weapons - once thought to be the earliest form of anti-rust technology - derives from a decorative varnish rather than a preservation technique, finds a new study co-led by UCL and Terracotta Army Museum researchers.

The study, published today in Scientific Reports, reveals 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), commented: "The terracotta warriors and most organic materials of the mausoleum were coated with protective layers of lacquer before being painted with pigments - but interestingly, not the bronze weapons."

"We found a substantial chromium content in the lacquer, but only a trace of chromium in the nearby pigments and soil - possibly contamination. The highest traces of chromium found on bronzes are always on weapon parts directly associated to now-decayed organic elements, such as lance shafts and sword grips made of wood and bamboo, which would also have had a lacquer coating. Clearly, the lacquer is the unintended source of the chromium on the bronzes - and not an ancient anti-rust treatment."

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 BC), 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 arrow heads have all been recovered. 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 as a result of the Qin weapon makers developing a unique method of preventing metal corrosion.

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 show that the chromium found on the bronze surfaces is simply contamination from lacquer present in adjacent objects, and not the result of an ancient technology. 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 and Terracotta Army Museum), co-author of the study, said: "Some of the bronze weapons, particular swords, lances and halberds, display shiny almost pristine surfaces and sharp blades after 2,000 years buried with the Terracotta Army. One hypothesis for this was that Qin weapon-makers could have utilised some kind of anti-rust technology due to chromium detected on the surface of the weapons. However, the preservation of the weapons has continued to perplex scientists for more than forty years.

"The high-tin composition of the bronze, quenching technique, and the particular nature of the local soil go some way to explain their remarkable preservation but it is still possible that the Qin Dynasty developed a mysterious technological process and this deserves further investigation."

By analysing hundreds of artefacts, 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. 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.

"It is striking how many important, detailed insights can be recovered via the evidence of both the natural materials and complex artificial recipes found across the mausoleum complex --bronze, clay, wood, lacquer and pigments to name but a few. These materials provide complementary storylines in a bigger tale of craft production strategies at the dawn of China's first empire," said co-author, Professor Andrew Bevan (UCL Institute of Archaeology).

Professor Thilo Rehren (The Cyprus Institute and UCL Institute of Archaeology), stressed the importance of long-term collaboration. "We started this research more than 10 years ago between UCL and the museum. Only through the persistence, trusting cooperation and out-of-the-box thinking of colleagues in China and Britain were we able to solve this decade-old mystery."

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Tongzi hominids China Late Middle Pleistocene

Tongzi hominids are potentially a new human ancestor in Asia

Tongzi hominids are potentially a new human ancestor in Asia

Tongzi hominids China Late Middle Pleistocene
Tongzi teeth. Credit: Song Xing

The CENIEH has been participating in a comparative research about human teeth discovered in this Southern China site which has revealed that Tongzi's teeth do not fit the morphological pattern of traditional Homo erectus.

The Researchers María Martinón-Torres and José María Bermúdez de Castro have co-authored a research about the hominids from Tongzi which was published in the Journal of Human Evolution from the Centro Nacional de Investigación sobre la Evolución Humana (CENIEH) which reveals that Tongzi teeth do not fit the morphological pattern of traditional Homo erectus and that these teeth could potentially represent the highly targeted "Denisovans".

Between 1972 and 1983, the four teeth were discovered in the Yanhui Cave in Tongzi, Southern China. Their chronology is between 172,000 and 240,000 years old and they were originally identified as late Homo erectus or ancient Homo sapiens.

This research was led by Song Xing from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing (IVVP). The morphology of hominid teeth has been reassessed by geometric morphometrical analysis and Micro-Computed Tomography (micro-CT) as well as through morphological standard comparisons.

Tongzi teeth have been primarily compared with hominids during the same chronological period (Late Middle Pleistocene) and/or in the same geographical region (East Asia). However, the comparative sample includes a wide range of hominids.

More generally, the results indicate the existence of more than one human population in East Asia during this period: one which may be taxonomically classified as Homo erectus (exemplified by fossils such as Zhoukoudian, Hexian and Yiyuan); and a second which may be characterized by the existence of derived traits more commonly observed in recent species of the Homo genus, such as crown symmetry, tongue thrusting and the simplified dentine surface of the third premolar.

"More genetic and fossil discoveries would be necessary to evaluate the taxonomy of the "non-erectus" populations of the Middle Pleistocene, such as the Tongzi hominids, which could be good candidates for the Denisovan ancestry," says María Martinón-Torres.

The Denisovans

These populations are related to the Neanderthals who lived in Asia during the Late Middle Pleistocene and the Upper Pleistocene period which was discovered in 2010 from the genetic analysis of a phalanx and a tooth found in the Denisova cave in the Altai massif (Russia). An abundant amount of genetic information has been collected from the Denisovans but there are very few fossil remains. Therefore, both their physical appearance and their identification in the fossil record remain a mystery.


Full bibliographic information

Xing, S., Martinón-Torres, M., & Bermúdez de Castro, J. M. (2019). Late Middle Pleistocene hominin teeth from Tongzi, southern China. Journal of Human Evolution, 130, 96-108. doi: 10.1016/j.jhevol.2019.03.001.


Press release from Centro Nacional de Investigación sobre la Evolución Humana CENIEH/ (ES)