Flowing rivers of mercury

Chemistry World  7 January 2015

Philip Ball investigates claims that the burial chamber of China’s first emperor contains rivers of shimmering mercury. 

 

© Image Source / Alamy

 

The Chinese emperor had done all he could to become immortal, but in vain. His physicians had prepared herbal and alchemical elixirs, but none could stave off his decline. He had sent a minister on a voyage far over the eastern seas in search of a mythical potion of eternal life. But that expedition never returned, and now the quest seemed hopeless. So Qin Shi Huangdi, the First Emperor of a unified China in the third century BC, had begun preparations for the next best thing to an endless life on Earth. He would continue his cosmic rule from the spirit world, and his underground tomb would be a palace for the afterlife, complete with its own army of life-size clay soldiers.

 

Those terracotta warriors lay hidden for two thousand years beneath several metres of sandy soil a mile from the First Emperor’s burial mound at Mount Li, to the northeast of the city of Xi’an in Shaanxi province of north-central China. They were rediscovered in 1974 by farmers digging a well, and Chinese archaeologists were astonished to find over the next decade that there were at least 8000 of them, once brightly painted and equipped with clay horses and wooden chariots. As further excavation revealed the extent of the emperor’s mausoleum, with offices, stables and halls, along with clay figures of officials, acrobats and labourers and life-size bronze animals, it became clear that the Han dynasty historian Sima Qian, writing in second century BC, hadn’t been exaggerating after all. He claimed that 700,000 men had worked on the emperor’s tomb, constructing entire palaces, towers and scenic landscapes through which which the emperor’s spirit might roam.

 

Qin Shi Huangdi was China’s first emperor, and with the help of lifegiving elixirs hoped to be its last © The Art Archive / Alamy

Sealed in

No one knows what other wonders the mausoleum might house, for the main burial chamber – a football-pitch-sized hall beneath a great mound of earth – remains sealed. Most enticing of all is a detail relayed by Qian: ‘Mercury was used to fashion the hundred rivers, the Yellow river and the Yangtze river, and the seas in such a way that they flowed’. This idea that the main chamber contains a kind of microcosm of all of China (as it was then recognised) with rivers, lakes and seas of shimmering mercury had long seemed too fantastic for modern historians to grant it credence. But if Qian had not been inventing stories about other elaborate features of the mausoleum site, might his account of the tomb chamber be reliable too?  

 

In the 1980s Chinese researchers found that the soil in the burial mound above the tomb contains mercury concentrations way above those elsewhere in the vicinity. Now some archaeologists working on the site believe that the body of the First Emperor may indeed lie amidst vast puddles of the liquid metal.  

 

Yet it seems unlikely that anyone will gaze on such a sight in the foreseeable future. ‘We have no current plan to open the chambers,’ explains archaeologist Qingbo Duan of Northwest University in Xi’an, who led the mausoleum excavations from 1998 to 2008. ‘We have no mature technologies and effective measures to protect the relics,’ he says. So can we ever know the truth about Qin Shi Huangdi’s rivers of mercury? 

 

A harsh legacy

The construction of this immense mausoleum started fully 36 years before the emperor’s death in 210BC, when he was merely King Zheng of the kingdom of Qin – a realm occupying the valley of the Wei, a major tributary of the Yellow river, now in Shaanxi. Qin was one of seven states within China at that time, all of which had been vying for supremacy since the fifth century BC in what is known as the Warring States period. By finally defeating the last of the rival states in 221BC, Zheng became Qin Shi Huangdi (meaning the First Qin Emperor), ruler of all China.

 

Some etymologies trace the name China itself to the First Emperor dynasty (pronounced ‘chin’), and so you might imagine that it would have a very special status in Chinese history. But the unified state barely outlasted the death of the First Emperor himself – four years later it succumbed to a rebellion that became the much more durable Han dynasty (206BC–AD220). The Qin dynasty is regarded with little fondness in China today, for the First Emperor was a tyrant who ruled with brutal force.

 

What hides within

Yet there’s reluctant admiration in the way Qian describes the magnificence of the First Emperor’s tomb. ‘In ancient China, people believed the souls of the dead would live forever underground, so they would prepare almost everything from real life to bury for use in the afterlife,’ says Yinglan Zhang, an archaeologist at the Shaanxi History Museum in Xi’an and deputy director of the mausoleum excavations from 1998 to 2007. Given what has already been unearthed, he says ‘there should be many other cultural artefacts or relics still buried in the tomb chamber or other burial pits around the tomb – maybe things beyond our imagination’.

 

 

The pits housing the terracotta army lie outside the 2km by 1km boundary wall of the burial mound. Inside this wall are ritual buildings once containing food and other items that the emperor would need to sustain him. There are chambers full of stone armour that could protect against evil spirits, and it is possible that the emperor himself might not have been interred alone in the main chamber: Qian says that officials were buried there with him, and it’s not clear if they were alive or dead at the time.

 

The First Emperor’s tomb is surrounded by thousands of terracotta warriors to guard him in the afterlife © O Louis Mazzatenta / National Geographic Society / Corbis

 

The mound itself was originally about 0.5km by 0.5km (erosion has shrunk it a little) and the burial chamber lies about 30–40m below the original ground surface. Its shape has been mapped out by measuring gravity anomalies in the ground – an indication of hollow or less dense structures – and by looking for changes in the electrical resistivity of the soil, which result from buried structures or cavities. In this way, Chinese archaeologists have figured out the basic layout of the tomb over the past several decades. The chamber is about 80m east to west by 50m north to south, surrounded by a wall of closely packed earth and – to judge from other ancient Chinese tombs – perhaps water-proofed with stone covered with red lacquer. In 2000 researchers discovered that towards the edge of the mound an underground dam and drainage system helps to keep water away from the chamber. So there’s some reason to believe that the tomb itself might be relatively intact: neither wholly collapsed nor water-filled.

 

Quicksilver courses

Measurements of soil resistivity have revealed another intriguing feature. They show a so-called phase anomaly, which is produced when an electrical current is reflected from a conducting surface, such as a metal. Could this be a sign of mercury?

 

The first detailed study of mercury levels in the mound were conducted in the early 1980s, when researchers from the Institute of Geophysical and Geochemical Exploration of the China Institute of Geo-Environment Monitoring sunk small boreholes into the soil over an area of 12,000m2 in the centre of the mound and extracted soil samples for analysis. Whereas soils outside this central region contained an average of 30ppb of mercury, the average above the chamber was 250ppb, and in some places rose to 1500ppb. A second survey in 2003, by a different team that included Duan, found much the same result: unusually high concentrations of mercury both in the soil itself and in the interstitial vapours between grains.

 

Do mercury levels above Qin’s burial mound (left) resemble this 11th century map (right), hinting at a mercurial microcosm of China’s waterways within? © adapted from Liu Shiyi et al 2005, pp 26 &32. Paul Goodhead / © Trustees of the British museum

 

From the grid of borehole samples in the earlier study one can construct a rough map of how the high levels of mercury are distributed. ‘There is no unusual amount of mercury in the northwest corner of the tomb,’ says Duan, ‘while the mercury level is highest in the northeast and second highest in the south’. If you squint at this distribution, you can persuade yourself that it matches the locations of the two great rivers of China – the Yellow and Yangtze – as seen from the ancient Qin capital of Xianyang, close to modern Xi’an. ‘The distribution of mercury level corresponds to the location of waterways in the Qin empire,’ Duan says. In other words, the tomb might indeed contain a facsimile of the empire, watered by mercury.

 

Zhang, however, isn’t so sure that one can conclude much from the present day mercury distribution. He thinks that the tomb chamber must have collapsed thousands of years ago, just like the pits containing the terracotta army. ‘The mercury will have volatilised into nearby soils during this long time, so it would be impossible to show up detailed information that we can connect with particular rivers or lakes,’ he concludes.

 

Elixirs of immortality 

In any case, just because the mausoleum apparently contains a lot of mercury doesn’t in itself verify Qian’s account. Mercury had other uses too, particularly in alchemy, which has some of its oldest roots in China. In the west this art was commonly associated with attempts to make gold from other metals, and some Chinese alchemists tried that too – in 144BC the Han emperor Jingdi decreed that anyone caught trying to make counterfeit gold should be executed. But Chinese alchemy was more oriented towards medicinal uses, in particular elixirs of immortality. 

 

Cinnabar (HgS) was widely used in ancient China for decoration, medicine and alchemy © Joel Arem / Science Photo Library

And perhaps mercury (in Chinese ‘shui yin’, literally ‘water silver’) was the key to these. Chinese legend tells of one Huang An, who prolonged his life for at least 10,000 years by eating mercury sulfide (the mineral cinnabar).The First Emperor was said to have consumed wine and honey laden with cinnabar thinking it would prolong his life, and some have speculated that he might have hastened his death with these ‘medicines’. 

 

During the Warring States period, mercury was a common ingredient of medicines being used to treat infected sores, scabies, ringworm and (even more alarmingly) as a sedative for mania and insomnia. Because it is bright red, cinnabar was also used for art and decoration in China since around the second millennium BC. Its artificial form, produced in the west since the Roman era, became known as the pigment vermilion. 

 

One of the most important uses of mercury at this time has a particularly alchemical tinge. Gold and silver dissolve in mercury to form amalgams, and such mixtures were used for gilt plating. The amalgam was rubbed on and heated to evaporate the mercury leaving behind a gleaming coat of precious metal. Such mixtures also featured in alchemical elixirs: the Daoist concept of yin and yang, the two fundamental and complementary principles of life, encouraged an idea that cold, watery (yang) mercury and bright, fiery (yin) gold might be blended in ideal proportions to sustain vitality. 

 

Obtaining mercury

Throughout antiquity cinnabar was the source of all mercury metal. There was a lot of this mineral in China, particularly in the west. Shaanxi alone contains almost a fifth of all the cinnabar reserves in the country, and there are very ancient mines in Xunyang county in the south of the province that are a good candidate source of the mercury apparently in the First Emperor’s tomb. 

 

To extract mercury from cinnabar one need only roast it in air, converting the sulfur to sulfur dioxide while the mercury is released as vapour that can then be condensed. Since mercury boils at 357°C, this process needs temperatures well within the capabilities of Qin-era kilns. Of course, anyone trying this method in an unsealed container – closed chambers weren’t used until the Han period – risked serious harm.

 

‘The emperor was said to have consumed cinnabar to prolong his life’

But despite there being a mature mercury-refining technology by the time of the First Emperor, and although Zhang attests that ‘the people of the Qin Dynasty had some basic chemical knowledge’, Duan argues that Chinese alchemy was still in its infancy in that period. In particular, he says, there is no good reason to think that the practice of soaking dead bodies in mercury to prevent their decay, common during the Song dynasty in the 10th to 13th centuries AD, was used as early as the Qin dynasty. So even though mercury, either as cinnabar or as the elemental metal, has been found in tombs dating back as far as the second millennium BC, it’s not clear why it was put there. Might its toxicity have acted as a deterrent to grave-looters? Probably not – the dangers of mercury fumes were not recognised until Han times. So if, as it seems, there’s a lot of mercury in the burial chamber, it’s unlikely to be either a preservative or an anti-theft device. 

 

Yet even if this mercury was indeed used for fantastical landscaping, Duan doubts that there can have been much of it. Based on estimates of mercury production from the Song era and allowing for the imperfections of the earlier refinement process, he thinks the chamber might have contained at most 100 tons of the liquid metal: around 7m3.

 

We might never be able to check that. ‘Right now, our archaeological work is focused on deducing the basic layout of the tomb,’ says Duan. Because any breach in the seal could admit water or air that might damage whatever lies within, even robot-based exploration of the interior is ruled out. ‘If the chamber was opened even using a robot or drilling, the balance of the situation would be broken and the buried objects would deteriorate quickly,’ says Zhang.

 

So if we’re ever going to peek inside, it will have to be with better scientific techniques than are currently available. ‘I dream of a day when technology will shed light on all that is buried there, without disturbing the sleeping emperor and his 2000-year-old underground empire,’ says Yongqi Wu, director of the Qin Shi Huang Mausoleum Museum. Maybe these concerns to preserve the unknown heritage will guarantee the First Emperor a kind of immortality after all.