Each of the 8,000 Terracotta Warriors has a different face. That is something that draws much attention to those who have visited this monumental mausoleum, one of the greatest archaeological finds of the twentieth century. Sometimes more even than its size: soldiers of 1.78 meters in height. But without any doubt One of the greatest mysteries surrounding the interior of the three pits is how the Xi'an Army has been preserved - the soldiers, the thousands of weapons, the horses, the cars - discovered in 1974 although it dates from 221 to 206 BC when the emperor unified China.
An international team of researchers from the Archeology Institute of the UCL and the Terracotta Army Museum has discovered that the chromed coating of bronze weapons, once thought to be the oldest form of anti-oxidation technology, is derived from a decorative varnish instead of being a conservation technique.
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 conservation power of the weapons that were made in the Qin dynasty.
"The Terracotta Warriors and most of the organic materials of the mausoleum were covered with protective layers of lacquer before painting them with pigments, but curiously not the bronze weapons", explains the study's author Marcos Martinón-Torres, professor of the Department of Archeology of the University of Cambridge and the Institute of Archeology of the UCL.
"We have found a substantial chromium content in the lacquer, but only a trace of chromium in the pigments and soil nearby, possibly due to contamination. The highest levels of chromium found in bronzes are always in the parts of the weapons associated with organic elements now in decomposition, such as the axes of the spears and the hilts of the wooden and bamboo swords, which would also have a Lacquer coating. Clearly, the lacquer is the unwanted source of chromium in the bronzes, and not an old anti-corrosive treatment ", as was thought.
Terracotta's army of Xi'an consists of thousands of life-size ceramic figures representing warriors, stationed in three large pits inside the mausoleum of Qin Shihuang (259-210 BC), the first emperor of unified China. These warriors were armed with fully functional bronze weapons. In fact, since its discovery, dozens of spears, spears, hooks, swords, crossbow triggers and up to 40,000 arrowheads have been recovered. Although the original organic components of weapons, such as wooden axles, quivers and pods have mostly deteriorated in the last 2,000 years, the bronze components remain in very good condition.
Since the first excavations in the 1970s, researchers have suggested that the impeccable state of conservation observed in bronze weapons should be the result of Qin Dynasty weapon manufacturers developing a unique method to prevent metal corrosion.
Chromium traces detected on the surface of the bronze weapons gave rise to the belief that the Qin artisans invented a precedent to the coating technology by converting chromate, a patented technique at the beginning of the 20th century that still exists today. use today.
Now, an international team of researchers shows that the chromium found in bronze surfaces is simply the contamination of the lacquer present in the adjacent objects, and is not the result of an old technology. The researchers also suggest that the excellent conservation of bronze weapons may have been helped by the moderately alkaline pH, the small particle size and the low organic content of the surrounding soil.
"Some of the bronze weapons, swords, spears and halberds (a type of medieval weapon similar to a spear) in particular, show bright surfaces and sharp blades after 2,000 years buried with the warriors. One hypothesis for this was that the arms manufacturers could have used some type of anticorrosive technology due to the chromium detected on the surface of the weapons. However, the preservation of weapons has perplexed scientists for more than forty years, "said Dr. Xiuzhen Li, co-author of the study.
"The bronze composition of the tin, the technique of extinction and the particular nature of the local soil explain in some way its remarkable conservation, but it is still possible that the Qin Dynasty has developed a mysterious technological process and this deserves more investigation ".
By analyzing hundreds of artifacts, the researchers also found that many of the best-preserved bronze weapons did not have any chromium surface. To investigate the reasons for their excellent preservation, they simulated the weathering of the replicas of the bronzes in an environmental chamber. The bronzes buried in the soil of Xi'an remained almost as in the beginning after four months of extreme temperature and humidity, in contrast to the severe corrosion of buried bronzes for comparison on British soil.
"It is surprising the amount of important and detailed information that can be recovered through the evidence of both natural materials and the complex artificial recipes found in the mausoleum complex: bronze, clay, wood, lacquer and pigments, for These materials provide complementary stories in a broader history of craft production strategies at the dawn of China's first empire, "said co-author Andrew Bevan of the UCL Institute of Archeology.
Professor Thilo Rehren (Institute of Cyprus and Institute of Archeology of the UCL) stressed the importance of long-term collaboration. "We started this research more than 10 years ago between UCL and the museum, and only through persistence, trusting cooperation and the immediate thinking of colleagues in China and Great Britain could we solve this mystery of a decade."