杏吧原创

YOU have probably seen The Great Wave off Kanagawa 鈥� the Japanese woodblock print of a huge, foaming wave about to engulf a group of small boats. It鈥檚 no surprise that the picture is mostly blue; it is a wave after all.

However, it is part of a series of images called Thirty-six Views of Mount Fuji by the artist Hokusai, and if you flick through them, you will notice that nearly every one is predominantly blue. That might seem strange, until you realise that in 1830, when Hokusai began printing these works, blue was rather a new thing. The Prussian blue he used had been introduced into Japan just a few years earlier, giving artists their first blue pigment that was bright, attractive and lasting.

鈥淗istorically, blue has been a big issue for artists; there are very few natural blue colours,鈥� says materials scientist . These days, we have plenty of blue dyes, which, being soluble, are ideal for colouring materials uniformly. But the insoluble blue pigments needed for paints, printing inks, ceramics and plastics are still rare. That is why, when Dobson realised that he might be able to create a new one based on a mineral that can exist only at the immense pressures found 500 kilometres beneath Earth鈥檚 surface, he was very much up for the challenge.

鈥淭he ancient Greeks didn鈥檛 even have a word for blue 鈥� hence Homer鈥檚 famous 鈥榳ine-dark鈥� sea鈥�

The colour blue has proved such a problem to recreate that most ancient cultures don鈥檛 seem to have had a word for it 鈥� Homer famously describes the 鈥渨ine-dark鈥� sea. , and it鈥檚 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 鈥�True blue鈥�). 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鈥檚 robes.

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Dobson, who is an artist as well as a scientist, has a long-running collaboration with Jo Volley at UCL鈥檚 Slade School of Fine Art. However, he was unaware of just how rare blue pigments are until a few years ago. His epiphany came when he attended a meeting of artists to describe his work with one of Volley鈥檚 students hunting for new pigments in coal-mine sludge. The conference was abuzz with the discovery of a pigment called YInMn blue. Volley explained that everyone was excited about it because blues are so rare. That got Dobson thinking about another blue he had seen 鈥� a mineral that had been discovered deep in the bowels of our planet.

It鈥檚 not easy to know exactly what rocks deep inside Earth are like because they exist under extreme pressures and change when brought to the surface as the minerals become distorted. However, sometimes diamonds are dug up that bear 鈥渋nclusions鈥� 鈥� minerals within them that are trapped at the pressures they experience during formation. In 2014, Graham Pearson at the University of Alberta, Canada, . Geologists were fascinated because it addressed a long-running debate about where Earth鈥檚 water came from; it is thought that ringwoodite in the mantle contains enough water to fill the surface oceans three times over. But Pearson鈥檚 discovery intrigued Dobson for an entirely different reason: the mineral was blue.

Now, ringwoodite鈥檚 structure would collapse and lose its intense colour at normal surface pressures, but Dobson wondered if it might be possible to capture its blueness by engineering a crystal that mimicked it at surface pressure.

You can think of a crystal a bit like a 3D version of the colourful, tessellated tiling often seen in Islamic buildings. Each atom in the crystal structure is like a tile that must fit snugly next to its neighbours. Dobson made an educated guess that the key to ringwoodite鈥檚 blueness was the iron atoms. These were in a tetrahedral configuration, in which each is surrounded by four other atoms. 鈥淚f you could put an iron ion into a tetrahedral coordination, it should end up going blue,鈥� he says.

鈥淯nfortunately, a student recently blew up my furnace, so that鈥檚 hampering progress鈥�

In January 2017, Dobson got a chance to test this idea when he became the first scientist in residence at the Slade School. What he needed was a mineral that would interact with iron ions to give a crystal with the characteristic tetrahedral configuration found in ringwoodite. First he looked at a series of minerals called spinels, which have a cubic crystalline structure. One of these, magnesium aluminate (MgAl₂O₄), seemed perfect because its aluminium ions have the same charge as the iron ions in ringwoodite. But when he tried baking it in an oven with a source of iron, he found the iron kept slipping into the wrong size gaps 鈥� surrounding itself with eight oxygen atoms rather than four. 鈥淚t ended up just brown,鈥� says Dobson.

Then he hit on two other compounds, zinc silicate (ZnSiO₄) and zinc germanate (ZnGeO₄), which contain zinc in just the right configuration. When he tried substituting the zinc for iron, lo and behold he got two new blues. The silicate is a soft, greenish blue that Dobson describes as 鈥渄uck egg鈥�. The germanate gives a richer 鈥渄eep water blue鈥�.

A manufacturer of fine-art products has already shown interest in commercialising his blues. But there is still a hurdle to overcome. At the moment, when the compound Dobson has created is ground into a fine powder to suspend into a liquid paint, its colour dims. That鈥檚 because it contains too little iron. But if he adds too much, the blue will disappear. The balance of iron and zinc in the compound needs to be just right.

鈥淭hat鈥檚 what I鈥檓 working on now: trying to see how much iron I can dissolve into these structures and so how intense I can get the blue,鈥� says Dobson. 鈥淯nfortunately, a student recently blew up my furnace, so that鈥檚 hampering progress.鈥�

But then, no one said making a new blue would be easy.

This article appeared in print under the headline 鈥淏lue dye thinking鈥�