The mysterious colour purple

[Pangu]

The mysterious colour purple
By Fiona McMillan
Cosmos Magazine
China's Terracotta Army was unearthed in 1974, but the mystery of a pigment on the warriors continues to intrigue archaeologists, chemists and physicists today.

Replicas of two terracotta warriors painted in what experts believe were the original colours. After investigating traces of paint on the statues, conservation experts think no two were painted the same. Credit: British Museum/C Roth

HOW DO YOU MAKE PURPLE? Any kid fresh out of art class will tell you: just mix blue and red.

Sounds simple, but new research reveals a story about purple that spans almost three millennia and involves a buried army, immortality, Einstein, a lost dimension, and your favourite pair of faded blue jeans. It begins, prosaically enough, with the search for water.

In the spring of 1974, Shaanxi province in central China was suffering a drought. Near the city of Lintong, local farmers began digging an irrigation well. They didn’t find water – but what they did find is now widely regarded as the eighth wonder of the world: the Terracotta Army.

The farmers had stumbled upon part of the burial complex of Qin Shi Huang, the First Emperor of China, who died in 210 BC. According to the beliefs of the era, to accompany and protect him in the afterlife, he required a spirit army. More than 8,000 life-sized terracotta warriors, horses and other figures were made just for this purpose.

Today the warriors are a natural terracotta colour, but they weren’t always so. Catharina Blänsdorf is an expert on conservation at the Technical University in Munich, Germany. She is currently in Lintong investigating what the terracotta warriors would have looked like more than 2,000 years ago, using minute traces of paint that remain on their surface.

She explains that the lacquer that binds the pigments to the terracotta was waterlogged, and dried out after excavation, causing the flaking and loss of paint that survived the lengthy burial. So what did the warriors look like originally?

“Extremely colourful and each different,” she says. “I did not find two with the same colours.” Blänsdorf doubts the real-life Qin army was ever so colourful, however, “as the peasant soldiers could have hardly afforded these kinds of clothes”.

ONE OF THE COLOURS worn by the model warriors – especially those representing the highest-ranking officers – was purple. This presented an interesting conundrum to archaeologists, because purple pigments are rarely found on ancient artefacts.

“Blue, and purple too, are not earth colours. You don’t find them in surface soils,” says Heinz Berke, a chemist and expert on ancient pigments at the University of Zurich in Switzerland.

Blue pigments only began to appear in human history when mining began, and even then they remained rare – as was the case with lapis lazuli, a blue-coloured stone. “For the whole ancient world there was only one mine, found in today’s Afghanistan,” says Berke. “It was very rare blue stuff, and very expensive.” For the Egyptians, it rivalled gold in status, so they were often used together, he adds.

There were no easy substitutes, because most other mineral pigments and plant dyes don’t maintain their colour. “They are not stable enough,” explains Berke, citing the way indigo dye fades from blue jeans. He adds that dyes can’t be used as pigments anyway.

Dye molecules adhere well to fibres, but not to hard surfaces such as terracotta. With pigments, on the other hand, the molecules form into tiny grains, and when mixed with a binder such as egg yolk, oil, or lacquer, they can be painted onto solid surfaces.

Dyes were still valuable though. From ancient Lebanon through Roman times, a purple dye extracted from molluscs was so rare and precious it became a status symbol. To this day, purple remains associated with royalty, power and wealth.

In the early 1990s, detailed chemical analysis by researchers at the Bavarian State Department of Historical Monument in Munich, Germany, revealed that the purple found on the terracotta warriors is a barium copper silicate with the chemical formula BaCuSi2O6. It has also been found on artefacts from the Han dynasty (206 BC to 220 AD), and was dubbed ‘Han purple’ – though it has since been detected on artefacts dating back to 700 BC or earlier.

SO WHERE DID HAN PURPLE COME FROM? A bit of chemical detective work reveals that it doesn’t occur in nature, so it must be synthetic. The remarkable implication of this is that Chinese artisans were performing synthetic inorganic chemistry over 2,700 years ago, long before the development of the compass or papermaking.

But peer a little deeper into the constituents of this lustrous shade and a new mystery appears. It turns out that, chemically, Han purple (BaCuSi2O6) differs only slightly from another pigment, Han blue (BaCuSi4O10).

That’s not so unusual, but Han blue in turn holds a striking visual resemblance to Egyptian blue – and an uncannily similar chemical make-up. Swap Han blue’s barium for calcium and you have Egyptian blue: CaCuSi4O10.

With just one exception, these were the only synthetic blue or purple pigments until the late 18th century. The Egyptians had been making Egyptian blue as early as 3,600 BC, probably by mixing a mineral such as malachite (for the copper it contains) with sand (for silicates) and lime from limestone (for calcium) and then heating the mixture to around 900˚C. Naturally abundant salts such as sodium sulphate were used as fluxes – catalysts that allow materials to react at lower temperatures.

All this raises a colourful question: is it possible the Egyptians gave their recipe to the Chinese, who then replaced lime with barite (for barium), leading to Han blue and Han purple? If so, this suggests the two cultures were directly exchanging technologies long before the Silk Road was established around 130 BC.

But Egyptian blue has never been found in China. And why would the Chinese replace calcium with barium? There’s plenty of limestone in China, and using barium would have required higher temperatures. Finally, one other nagging little thing: all the Chinese pigments contain traces of lead. And that has turned out to be the most important clue of all.

TO DISCOVER THE ORIGINS of Han purple, Berke and colleagues used a scanning electron microscope – which uses electrons instead of visible light – to look at a Han purple fragment right down to the micron (one thousandth of a millimetre).

They also used a technique called energy dispersive X-ray analysis (EDX): when subjected to a beam of electrons, atoms in a sample absorb the energy and then give off, or ‘fluoresce’, energy in the form of X-rays. Each element has a unique X-ray signal, so its location within a sample can be mapped.

Using these techniques, the researchers deduced that the materials may have been melted, and that the lead was distributed unevenly. And if lead was separating out from the pigment, this suggests it may have been used as a flux.

Physicists Apurva Mehta and Zhi Liu of Stanford University in California wanted to take a closer look. They decided that to get to something so small, they’d have to think big.

The Advanced Light Source (ALS) at the Lawrence Berkley National Laboratory in the U.S. is a synchrotron facility roughly the size of a football field, and seemed a good place to start. In a synchrotron, electrons travelling at close to the speed of light do laps in a giant ring surrounded by powerful magnets.

As the magnets force the electrons to curve with the ring, the electrons give off electromagnetic radiation in the form of X-rays. These X-rays are one billion times brighter than the Sun and focussed into beams as narrow as one micron.

During a trip home to China, Liu visited the museum that houses the terracotta warriors and began collaboration with researchers there; Bo Rong and Tie Zhou. They gave Liu a five-millimetre sample of Han purple from the apron of a kneeling archer.

So what did he and Mehta do with this precious archaeological sample? “We took a little bit of it and crushed it up,” says Mehta. When the synchrotron X-rays bounced off the molecules in the pigment crystals, they created a diffraction pattern that contained information about their structure and molecular make-up. “We could then figure out how big the grains were and how they were oriented,” says Mehta.

Next they tried X-ray fluorescence. When subjected to X-rays, atoms absorb the energy, and then give off – or ‘fluoresce’ – energy in the form of another X-ray. This is similar to EDX, but the use of synchrotron X-rays rather than electrons makes the process much more sensitive. Since each element has a unique fluorescent signal, the physicists used these signals to map their sample, then overlaid this map with their crystal data.

There, in the middle of a tiny pigment clump, was a small pool of lead with large pigment crystals growing out of it. This is strong evidence, says Mehta, “that lead was intentionally put in the samples … to use as a flux to control the temperature.”

According to Berke, the lead performed another, unintended function: the lead salt was able to catalytically break down barite, enabling the pigment to form. He doubts this was fully understood by the artisans at the time.

These unique materials and their novel use suggest the Chinese pigments were developed independently of Egyptian blue. So how did Chinese pigment-making processes evolve? When Han purple was first described, it was noted that the same constituents – barium, lead, and silicates – had been used in ancient glass-making in China, which began as early as 1,100 BC.

A number of ancient glasses were imitations of jade. ‘Jade’ is a broad term, but the jade of ancient China was usually the mineral nephrite. It was highly valued and has been used in ornaments and ritual objects for more than 7,000 years.

“Everyone in China, so far as we know, associated jade with immortality by the third century BC or even earlier,” explains Nathan Sivin, a historian of science and expert in Chinese culture at the University of Pennsylvania in Philadelphia. Though at that time, he notes, “immortality meant living in the memory of one’s family as an ancestor”.

Liu and Mehta wondered if Han purple was a by-product of an attempt by Taoist alchemists to synthesise jade using glass-making techniques, possibly as a way of understanding immortality. Sivin, however, does not believe there was ever a connection between Taoism and glass-making. And there is an issue of timing.

Berke notes that the earliest known use of Han purple occurred well before the beginnings of Taoism. Still, the evidence does point to a connection between Han purple and glass-making. And a further link with glass imitations of jade can’t be ruled out. “What we need is more investigation,” says Berke, though he believes the connection between pigments and glass most likely occurred much earlier.

“Everything started from glazing,” he says, “and then you have parallel development: on the one hand making glasses, and on the other hand making these blue and purple pigments.” He suggests that early glazing techniques used in Egypt and China may have had a common origin.

Berke also notes that the blue and purple pigments may have earned cultural significance in their own right, as there is evidence that pigment sticks began to replace jade in ancient burial rituals.

For nearly a millennium, Han purple adorned pottery, coins, burials, and even a spirit army. Then, around 200 AD, it disappeared from use. The recipe was lost, possibly during the political turmoil and infighting that marked the end of the Han dynasty.

That’s not the end of the story though. Several thousand years later, in the late 1980s, Han purple reappeared, and in the most unlikely of places.

“When people were trying to synthesise high-temperature superconductors it was found as a by-product, accidentally,” says Suchitra Sebastian, a condensed matter physicist at the University of Cambridge in England.

Researchers were preparing a sample of material containing barium and copper. When they heated the mixture in a container made from silica, a magenta residue formed on the container. It was BaCuSi2O6 – Han purple. The phenomenon has generated much interest among condensed matter physicists ever since.


Then, in 2004, there was another unexpected twist. An international team of researchers working at the Los Alamos National Laboratory (LANL) in New Mexico, USA, subjected Han purple to a high-powered magnetic field as they lowered the temperature to close to absolute zero (-273°C).


This caused the magnetic waves of the Han purple molecule to enter a rarely observed quantum physical state called a Bose-Einstein Condensate (BEC). Physicists are interested in BECs because they offer insight into how waves behave at the quantum level: in the Han purple molecules the magnetic waves that formed between the copper atoms ‘superimposed’ and acted as one large wave.


Sebastian, along with a team of researchers from Stanford and LANL, were collecting new measurements, but when they lowered the temperature even further they noticed something unusual: the magnetic waves of Han purple lost a dimension.


“Magnetically it’s behaving similarly to just the surface of water; so it’s not forming a gigantic wave in three dimensions, but it’s forming a gigantic wave in just two dimensions,” says Sebastian, adding that this behaviour had never been seen before.

In fact, it was thought impossible that this could happen so close to absolute zero, which is known as ‘the quantum limit’ – the point at which all thermal motion ceases and only quantum motion exists.

“This is the startling finding we made – that quantum effects can actually contribute to the loss of a dimension,” she says. Because of the unique physical arrangement of the atoms in Han purple, the molecules interact with each other in a way that makes it possible to enter these unusual quantum states.

The discovery is likely to have potential applications in the development of new superconductor materials and in quantum computing, so the story of Han purple is far from over.

But perhaps it’s not about purple after all. Recently, Berke discovered that Han purple, in its purest form, is actually dark blue. As it breaks down, either with time or with high temperatures, small amounts of red copper oxide form. The blue and red combine to give variations of purple. Just like any kid will tell you.

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Fiona McMillan is a science writer based in Brisbane, Australia.