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Sep 22

The chrome plated mystery of the Terracotta army’s swords

by Mark | Categories: Uncategorized | (305696 Views)



QIMG_6487in the 1st Emperor of China prepared well for the after-life. Throughout his reign he commissioned and built an eternal army of some 6000 soldiers, charioteers and cavalry. The  warriors stood in formation, buried at the foot of his tomb, there to guard the Emperor for eternity.

But all did not go as planned. Shortly after Qin was entombed chaos descended on his newly united China. Qin’s heirs, wishing to defeat him (even after his death) attacked his after-life defences. History tells that the underground barracks that housed the vast army of terracotta warriors were set alight. Fires smouldered for 90 days, structures around the ornate statues collapsed smashing the exquisite army. The broken soldiers and their bronze weaponry lay buried in ash and rubble. The great mausoleum was forgotten. Two millennia passed. Until in 1974, a peasant farmer, whilst digging a well, found fragments of a crushed warrior. And excavations began.

The thousands of individual Qin dynasty soldiers, have been painstakingly pieced together and placed back in formation. They are an awe inspiring sight. But I marvelled just as much when I saw no less incredible bronze weapons that armed the officers. Their swords are still sharp and largely unaffected by the 2200 years that have passed since they were forged. Instead of the green corrosion you’d expect on bronze artefacts the blades actually appear gun metal grey. Why this is the case is something of a mystery.
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There are reports of an analysis of the artefacts conducted by the Chinese Research Institute of Nonferrous Metals and Chinese Academy of Geological Sciences (although I am unable to find the primary data). The suggestions is that a 10-15 micron coating containing chromium oxide (at up to 2% chromium) was found. The conclusion; for millennia this thin layer protected objects from the ravages of time and chemistry.

So where did the chromium oxide layer come from? Did the ancient Chinese metallurgists, as suggested by curators of the Terracotta army, really have chrome plating technologies thousands of years before it was developed in the west? Over the intervening time did the chromium shine lose its lustre as it slowly oxidised, resulting in the grey we see today? Is a 10 micron, dilute layer of chromium oxide really enough to impart anti-corrosion properties?  Or is there another explanation for the immaculate swords?

This isn’t the first time someone’s asked these questions. Its been discussed on a sword forum where suggestions include forgeries and serendipitous impurities in the alloy. The latter seems to be supported by Prof Frank Walsh, an electrochemist now at Southampton University, when he was interviewed for an ABC documentary back in 2003 where:

Professor Walsh notes that the heat from the fires and the presence of carbon would have provided a reducing environment in which chromium atoms could have migrated to the surface of the weapons. There they’d oxidise and form a protective coating … Metals do diffuse over time, so this ‘natural’ explanation is plausible.

For me this isn’t a totally satisfying explanation. Largely since it appears, from the items on display, that only the blade is free of corrosion. The hilt has clearly corroded. If the slow migration of chromium to the surface of the blade is responsible why didn’t this mechanism occur elsewhere on the swords? But the idea that Qin’s weapon smiths knew how to apply anti-corrosion layers to their creations seems rather fanciful.

Which leaves the above questions unanswered. So chemists, time to reopen discussions. What do you think is going on? Can anyone come up with a way that the ancient Chinese might have deliberately or accidentally protected the weapons?  Or what else might have resulted a corrosion free blade, whilst the rest of the weapon is tarnished?

P.S. Any Chinese chemists/metallurgists out there who might be able to track down the analysis of the blades?

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  1. Mitch

    Loved seeing the Terracotta warriors when I was in Xi’An. I’d bet they probably used a chrome-based paint to decorate the sword and the ensuring fire created that protective layer.

  2. Tom Husband

    “But the idea that Qin’s weapon smiths knew how to apply anti-corrosion layers to their creations seems rather fanciful.”

    Why does it seem so fanciful? I’m sure other chemists will have given this more thought than I have, but is it completely inconceivable that they could have stumbled over a method for this? Presumably they could have mocked up a solution of Cr3+ ions and electrode potential data predicts that such ions could oxidise Fe atoms, thereby reducing themselves to Cr atoms. Obviously you couldn’t chrome-plate a sword simply by marinating it in a solution of Cr3+ ions, but it doesn’t seem totally impossible that the process could be achieved without an electricity supply.

  3. Isotope Labeling Service

    We’d be astonished by the chromium oxide covered around the swords, and curious about how did they know the chemistry at that times. But I did believe that the technology had been developed in China about one thousand years before the western people discovered it. As with about five thousand years cultivation, China was powerful in wisdom in the ancient times.

  4. weber hartwin

    There are many tombs been not destroyed and not burned. So you can discover the influence of heating on to that hidden process.
    bye
    wb

  5. weber hartwin

    chroium on terracotta sars

    Another argument is that burning/Heating was not intended. So it cannot serve as part of common processing .

    My believe is that it served as metallic gray paint in order to simulate bright steel . Steel was very exensive in late bronce age and so only used for neglace or religious art. The chinese craftsmen only wanted to make the swords more precious.

    bye wb

  6. Bob Hay

    The Professor suggested that this could be a heat treatment; that is not requiring electricity. So it is possible it was done by the Chinese. Watching a history of the Qin and their 500 year war to dominance prior to the uniting Emperor, I noted a mention of one of the conquered tribes/areas being known for their very specially good bronze swords etc.Maybe the process was more to strengthen the surface/edge of the swords than for bling. And could have been a black Chrome process.

  7. Qin Zheng

    its has been proven that the chinese developed Chromium Oxide Techniques in the 8th Century BC hundreds of years before the Qin Empire Conquered China

  8. Jeffrey Masters

    According to the 2016 book disclosure John titor cronies. Time travelers from the future traveled back in time to help defeat an alien takeover of our time line by changing history they were defeat by a time trageless who gave the 1st empror 5he crome plating technology along with some other help in defeating a anunak invasion. The story is detailed in the book disclosure John titor cronicles

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