Blue Dye Thinking

Blue dye thinking Blue pigments are so rare that people will go to the depths of the planet to create new ones, says Joshua Howgego Fade to grey: Van pressures found 500 kilometres Gogh’s The Starry beneath Earth’s surface, he was very Night would once much up for the challenge. have been bluer The colour blue has proved such a problem to recreate that most ancient cultures don’t seem to have had a word for it – Homer famously describes the “wine-dark” sea. Only the ancient Egyptians are known to have had one, and it’s probably no coincidence that they alone were able to produce a blue pigment. Egyptian blue was used widely until the Middle Ages when the recipe was lost and artists had to resort to either azurite or ultramarine (see “True blue”, page 48). Both were made from naturally occurring minerals, the latter from lapis lazuli. This was exorbitantly expensive, explaining why blue tended to be reserved for high-ticket items such as the Virgin Mary’s robes. Dobson, who is an artist as well THE STARRY NIGHT, JUNE1889, GOGH, VINCENT VAN/MUSEUM OF MODERN NEW ART, YORK, USA/BRIDGEMAN IMAGES as a scientist, has a long-running collaboration with Jo Volley at UCL’s Slade School of Fine Art. However, OU have probably seen The issue for artists; there are very few he was unaware of just how rare blue Great Wave off Kanagawa – the natural blue colours,” says materials pigments are until a few years ago. YJapanese woodblock print of a scientist David Dobson at University His epiphany came when he attended a huge, foaming wave about to engulf College London. These days, we have meeting of artists to describe his work a group of small boats. It’s no surprise plenty of blue dyes, which, being with one of Volley’s students hunting that the picture is mostly blue; it is a soluble, are ideal for colouring for new pigments in coal-mine sludge. wave after all. materials uniformly. But the insoluble The conference was abuzz with the However, it is part of a series of blue pigments needed for paints, discovery of a pigment called YInMn images called Thirty-six Views of Mount printing inks, ceramics and plastics blue. Volley explained that everyone Fuji by the artist Hokusai, and if you are still rare. That is why, when Dobson was excited about it because blues are flick through them, you will notice realised that he might be able to so rare. That got Dobson thinking about that nearly every one is predominantly create a new one based on a mineral another blue he had seen – a mineral blue. That might seem strange, until that can exist only at the immense that had been discovered deep in the you realise that in 1830, when Hokusai bowels of our planet. began printing these works, blue was It’s not easy to know exactly rather a new thing. The Prussian blue “ The ancient Greeks what rocks deep inside Earth are like he used had been introduced into Japan didn’t even have a word because they exist under extreme just a few years earlier, giving artists for blue – hence Homer’s pressures and change when brought their first blue pigment that was bright, to the surface as the minerals become attractive and lasting. famous ‘wine-dark’ sea” distorted. However, sometimes “Historically, blue has been a big diamonds are dug up that bear > 23/30 December 2017 | NewScientist | 47 TRUE BLUE AZURITE The first blue pigment. Originally made by were in a tetrahedral configuration, in grinding up the mineral azurite, a copper which each is surrounded by four other carbonate. Synthesised artificially from the atoms. “If you could put an iron ion 17th century. Can dehydrate into malachite, into a tetrahedral coordination, it another copper carbonate, which is green. should end up going blue,” he says. In January 2017, Dobson got a chance ULTRAMARINE to test this idea when he became the Made as early as 7000 BC in Afghanistan first scientist in residence at the Slade from lapis lazuli. Famously used on School. What he needed was a mineral Tutankhamun’s death mask, illuminated that would interact with iron ions to manuscripts and Italian panel paintings. give a crystal with the characteristic In the Renaissance it was more expensive tetrahedral configuration found in than gold. First synthesised in 1826. ringwoodite. First he looked at a series of minerals called spinels, which have PRUSSIAN BLUE a cubic crystalline structure. One The first modern synthetic pigment. of these, magnesium aluminate Discovered by accident in 1706, and (MgAl2O4), seemed perfect because its produced by the oxidation of ferrous aluminium ions have the same charge ferrocyanide salts. Exemplified by as the iron ions in ringwoodite. But Hokusai’s The Great Wave off Kanagawa when he tried baking it in an oven with and Van Gogh’s The Starry Night. It fades a source of iron, he found the iron kept to grey over time. slipping into the wrong size gaps – surrounding itself with eight oxygen COBALT BLUE atoms rather than four. “It ended up A mixture of cobalt, aluminium and AMSTERDAM/BRIDGEMAN IMAGES MILKMAID, VERMEER,THE JAN (JOHANNES) (1632-75)/RIJKSMUSEUM, just brown,” says Dobson. oxygen. Discovered by French chemist Then he hit on two other Louis Jacques Thénard in 1802. Famously Vivianite, the blue compounds, zinc silicate (ZnSiO4) used in Bristol blue glass. The pigment is used by Vermeer in and zinc germanate (ZnGeO4), very stable but costly, and cobalt is toxic The Milkmaid, and which contain zinc in just the when inhaled or ingested. azurite (left) both right configuration. When he tried turn green in time substituting the zinc for iron, lo and YINMN BLUE behold he got two new blues. The Discovered by Mas Subramanian at Oregon silicate is a soft, greenish blue that State University in 2009. An inorganic Dobson describes as “duck egg”. pigment, prepared by heating oxides of The germanate gives a richer yttrium, indium and manganese to around “deep water blue”. 1200°C. It is chemically stable, non-toxic A manufacturer of fine-art and does not fade. AZURITE/UHA/UIG/BRIDGEMAN IMAGES products has already shown interest in commercialising his blues. But there is still a hurdle to overcome. At “inclusions” – minerals within them possible to capture its blueness by the moment, when the compound that are trapped at the pressures they engineering a crystal that mimicked Dobson has created is ground into a experience during formation. In 2014, it at surface pressure. fine powder to suspend into a liquid Graham Pearson at the University of You can think of a crystal a bit like a paint, its colour dims. That’s because it Alberta, Canada, found such a diamond 3D version of the colourful, tessellated contains too little iron. But if he adds containing the mineral ringwoodite. tiling often seen in Islamic buildings. too much, the blue will disappear. Geologists were fascinated because Each atom in the crystal structure is The balance of iron and zinc in the it addressed a long-running debate like a tile that must fit snugly next to its compound needs to be just right. about where Earth’s water came from; neighbours. Dobson made an educated “That’s what I’m working on now: it is thought that ringwoodite in the guess that the key to ringwoodite’s trying to see how much iron I can mantle contains enough water to fill blueness was the iron atoms. These dissolve into these structures and so the surface oceans three times over. how intense I can get the blue,” says But Pearson’s discovery intrigued “ Unfortunately, a student Dobson. “Unfortunately, a student Dobson for an entirely different recently blew up my furnace, so that’s reason: the mineral was blue. recently blew up my hampering progress.” Now, ringwoodite’s structure would furnace, so that’s But then, no one said making a new collapse and lose its intense colour blue would be easy. ■ at normal surface pressures, but hampering progress” Dobson wondered if it might be Joshua Howgego has got the blues 48 | NewScientist | 23/30 December 2017 .

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