Rust proof metal in ancient times
(appeared in Mar 2019)

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Ascribing the hardiness of ancient metals to technology is sometimes misplaced says S.Ananthanarayanan.

An astonishing find in recent times is the Qin Terracotta Army, a vast array of ceramic art found in the tomb of Qin Shi Huang, who was Emperor of China in the third century BCE. Among the finds were large numbers of swords, arrows and other implements, mostly in bronze, with portions, made of wood. The wooden parts had decayed, but the metal was found to be well preserved, with even polish and the sharpness of blades quite intact

Researchers had found that the bronze parts had a surface content of chromium, which led to the belief that the metalworkers of the time had mastered a technique of ‘chromate conversion coating’, which gave the swords and arrow-heads their durability. Marcos Martinón-Torres, Xiuzhen Li, Yin Xia3, Agnese Benzonelli, Andrew Bevan, Shengtao Ma, Jianhua Huang, Liang Wang, Desheng Lan, Jiangwei Liu, Siran Liu, Zhen Zhao, Kun Zhao and Thilo Rehren, from the University of Cambridge, the Institute of Archeology, London, the site museum at X’ian, in China, the University of Science and Technology, Beijing and The Cyprus Institute, Nicosia, write in the journal, Scientific Reports, that the traces of chromium are not really a part of the bronze implements and are not the reason for the implements’ resistance to rust.

Qin Shi Huang, ascended the throne when he was 13 and by the time he was 38, he had conquered the warring states of a vast territory and unified China, to become its first Emperor. His achievements were administrative and economic reform and include a vast road system and the beginning of the Great Wall of China. And then, the great mausoleum, as large as a city, that he had built for himself during his lifetime.

The mausoleum consisted of a burial chamber the size of a football field, surrounded by halls ranged with full size terracotta sculptures of soldiers, captains and generals, horses, chariots, supposedly to protect the monarch after he died. Over 2,000 ceramic warriors have been excavated and many thousands are believed to be buried. Historians say the chambers contained replicas of palaces and scenic towers, “rare utensils and wonderful objects”, a hundred rivers of mercury and crossbows rigged to shoot intruders. The figures of the soldiers, in realistic size and ritual dress, carried real life weapons, such as swords, daggers, spears, crossbows and arrows, mostly made of bronze.

Much of the metals may have been looted in later years, but the partial excavations after 1974 revealed over 40,000 articles of weaponry, including a large number of arrowheads in bundles of a hundred each. “In most cases, the organic components of the weapons, such as wooden shafts, quivers, scabbards or crossbow stocks, have largely decayed. However, the preservation of the bronze is remarkably good overall, with many of the weapons displaying shiny, almost pristine surfaces and sharp blades,” the Scientific Reports paper says.

This remarkable state of preservation, for over 2,000 years, suggested some anti-rusting technology that the ancient Chinese had. Research that followed the discovery revealed that some of the metal objects were covered with a chromium oxide layer. Chromate conversion coating is rust-preventing measure that is used today with many common metal objects. It was hence suggested that the artisans of the time of Qin Shi Huang had some version of this process, which the weapon makers may have used. Such a process was found to be feasible, given the materials and methods available at the time, the Scientific Reports paper says, and although the process could not be replicated, the idea that the ancient Chinese had ‘highly developed technology’ has taken root.

The researchers put this idea to test by systematic analyses of the surface of a large sample of the buried weapons. They used an extremely sensitive method, X Ray fluorescence, where X Rays are shone on the surface and traces of specific elements emit secondary X Rays. What they found was that although there were traces of chromium in a large number of cases, these were only 8% of all the cases. The presence of chromium was hence not the reason for the apparent resistance to corrosion

This apart, the team checked whether there was a pattern in the placement of the items that showed traces of chromium, to see if chromium treatment could be linked to some workshop or production location. Here again, the distribution was found to be quite random. Another possibility was that chromium present in the copper or tin ores used in creating the bronze had shown up on the surface. This was also ruled out because the reduction of chromium from its oxides would have called for energy beyond the capacity of the processes used to smelt copper, tin, lead or iron, which were practiced in the region

The chromium traces hence must have got there after the article had been made. Here, there were two possibilities - from the earth or from the pigments used to paint the terracotta warriors. Both these were ruled out, based on the nature of the soil and the pigments used. The final possibility was the lacquer coats that were applied before the pigments were put on. And here, it was found that there was a distinct source of chromium. And then, the lacquer as the source of chromium was confirmed by the greatest traces of chromium being on bronze surfaces where the surfaces were nearest to lacquered components, like handles or scabbards.

Having located the source of the chromium, there was still the need to explain how the bronze was preserved. Here, it was found that higher levels of tin in the bronze, and also traces of arsenic, contributed to preservation. But a more important reason was that the soil in the area was clearly alkaline, as opposed to acidic. Alkaline soil acts to neutralise the effects of salts, acids and even bacteria. Further, the soil was very fine, thus covering metal surfaces and keeping away the air and moisture that are needed for corrosion. A control test, with samples of bronze in the soil from the excavation pits and in soil with organic matter, and mildly acidic, confirmed it was the soil in the tomb that had helped preserve the bronze articles.

The paper says it was long-term, international co-operative and interdisciplinary research that helped dispel the notion that the bronzes of Qin were the result of advanced technology

A similar belief that the ancients had advanced capabilities arose from the anti-rusting property of the Iron Pillar in Delhi, which is made of the same material as Damascus steel, which provided the scimitars during the time of the Crusades. This specially tough and shatterproof steel, which came from southern India and Sri Lanka, was believed to be the mark of metallurgical expertise. Scientific analysis, however, has shown that while the process and some ingredients used in forging the steel have their place, the operative difference lay in trace elements found in a particular strain of ore. When the supply of ore ran out, in the 1700s, so did the manufacture of Damascus steel.

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