After spending almost 2,200 years buried, many of the swords, spears or arrows of Xian's terracotta army were still sharp when they were unearthed. The dominant idea about its exceptional state of preservation was that the artisans who molded the warriors used a thousand-year-old anticorrosion formula already lost based on chromium, a technique that would not be rediscovered until the 20th century. However, a study now shows that the presence of this metal is accidental and points to the protective mantle of the earth.
Discovered in the 70s of the last century, the warriors of Xian are an imposing collection of soldiers this time of clay and life size. It is believed that there are some 8,000 figures among knights, archers, halberdiers, crossbowmen and dozens of horses, each with a different face and each carrying their bronze weapons. The first Chinese emperor of the Quin dynasty, Qin Shihuang, ordered them to accompany him in the afterlife once he died, in 210 before this era.
One of the first things that amazed the archaeologists was the good state of many of the weapons, some of them retaining their original appearance, even with the marks of their polishing and sharpening. Studies conducted in the 80s pointed to a possible explanation: the Qin craftsmen could apply a treatment against corrosion to bronze. They were based on the presence of chromium in various parts of some of the samples. The problem with this idea is that the Coating by conversion, the supposed technique used, was not patented until the 20th century. Did they use a millennial formula that was lost in time?
Studies conducted in the eighties of the last century found chrome in various weapons of the Terracotta Army
Reality seems to be another but equally fascinating. A group of researchers, including the Spanish archaeologist Marcos Martinón Torres, they have analyzed a good part of the armament of the warriors of Xian, carried out experiments of accelerated aging of the bronze and replicated in the laboratory the supposed anti-rust treatment. Both the results of the work, published in Scientific Reports, as the way to reach them surpass any of the CSI series and are a tribute to science.
Using non-invasive techniques, such as X-ray fluorescence (XRF), they measured the presence of chromium in nearly 500 weapons. They only found it in 37 of them, 8% of the analyzed ones. In addition, the distribution of the metal is not uniform, as might be expected from a coating. But it is that the highest concentrations occur in the handles of spears and swords and the handle of the crossbows while there is hardly any chrome in the bolts of the triggers and arrowheads and no trace in blades of swords, spears and halberds, which are in between the best preserved.
Previous studies had proven that the creation of the terracotta army was the work of many small workshops who worked independently. It was possible that only one of them knew the magical formula of chromium. However, the new work does not find any spatial pattern and the presence of chromium in the weapons of room 1 of the mausoleum is random.
"The chromium present in the bronze was not deliberately put there," says Martinón Torres, now a professor of archeology at the University of Cambridge (United Kingdom) and co-author of the study, which he concluded while at the University College of London (UCL). ), within a cooperation project of the UCL with the Terracotta Army Museum of China. "But the question of whether chrome helped dodge corrosion, albeit accidentally, remained in the air," he adds.
To prove it, they used a climatic chamber as a time machine. They submitted bronze samples to extreme conditions of temperature and humidity for four months in a row to simulate the aging of the bronze. "We did not corrode the samples, nor those of untreated bronze or those treated with chrome plaster buried in the earth brought from the mausoleum, nor adding chrome to the ground," says the Ourense archaeologist. But they did notice a deterioration in those buried in British soil. "That led us to think that there was something on the ground that we had to explain," he adds.
A soil with high pH, few organic components and a small grain that hinders the filtration of water and air could protect the weapons all this time
What is special about this soil? "It is a particular but not specific ground of the terracotta warrior pit," Martinón Torres explains. The whole region, northeast of China, is part of the Loees plateau. It is characterized by a soil shaped by the action of the wind. "It usually has a very fine grain and a very low content of organic material, which means that no organic acid is formed, the elements that corrode the metals of the archaeological sites," explains the Cambridge scientist. With a relatively high pH and a grain that makes the filtration of water and air difficult, the oxidation processes are hindered. "That's what happens in the warrior's grave, but also in other sites in the region where the restored bronze has a very good state of preservation," he concludes.
"More than determining them, soil conditions influence the conservation possibilities," says soil expert Mark Kibblewhite, not related to the study. "When the soil is alkaline and dry, a better preservation of the metals is more probable but it is not assured, since other factors must be considered, such as the concrete nature and the initial conditions of the artifact", adds this expert, professor emeritus at soil science from the University of Cranfield (United Kingdom).
One of those extra factors could be the composition of the bronze. Formed from an alloy of copper and tin, the metallurgists could increase the portion of this to give more hardness to the arms. "The high presence of tin in the bronze, the tempering technique and the particular nature of the soil explain in some way its remarkable conservation, but it is still possible that the Qin dynasty will develop a mysterious technological process that will require more research", comments in a note the researcher of the UCL and the Mausoleum of the Emperor Qin Shihuang and coauthor of the study, Xiuzhen Li.
It remained to know where the chromium came from. The weapons were made in bronze, an alloy of copper and tin in which chrome does not paint anything. The detailed study of the armament shows a particular concentration of chromium in handles and handles and in those parts of the weapon that should have been in contact with pods, quivers (to store the arrows), the wood of these and the halberds … that is to say , organic elements such as wood and bamboo that disappeared consumed by time. The idea that the authors bet is that these materials were treated with a substance that contained chromium and the substance could be lacquer.
The Xian warriors analyzed show traces that they were lacquered before painting them, perhaps in an attempt to preserve them better. The same could do with pods and others. The study of this lacquer shows that it has a significant amount of chromium. But the sap of the lacquer tree does not contain this metal naturally so the questions come back. How does the corm get to the lacquer? "It's a question that we still have to answer, it was an unexpected discovery, after looking for the chromium in the metals themselves, in the soil, in the pigments, in the ceramics, finally, we found it in the lacquer", recalls the Spanish archaeologist . So this time it should have been deliberately put there. It remains to know why.