Tuesday, April 21, 2020

Three colours: Yellow

Jan van Huysum’s Flowers in a Terracotta Vase (1736) is a riot of floral colour, the equal of anything else by the Dutch flower painters of the seventeenth and early eighteenth centuries. But some of it looks decidedly odd. The leaves spilling out from among the bright blooms don’t look at all healthy, or indeed natural: they are more blue than green.


Jan van Huysum, Flowers in a Terracotta Vase (1736).

This is neither by intention nor mistake. Simply, the yellow pigment that Huysum mixed with blue to create his greens has faded. It was a common problem noted even at the time: the English chemist and writer Robert Dossie wrote in his Handmaid to the Arts (1758) that “The greens we are forced at present to compound from blue and yellow are seldom secure from flying or changing.”

Because artists did not then have a particularly vibrant green pigment that approached the colour of fresh vegetation, they often needed to resort to this mixing of primaries. But unless your primary pigments are bright and pure, such a mixture may become a little murky. Among the brightest of yellows were lake colours, meaning that the pigment was made from a water-soluble organic (plant- or animal-based) substance – basically a dye – fixed to the surface of fine particles of a white powder like chalk or ground eggshell. But organic dyes don't last well when exposed to light: the rays break up the colorant molecules, and the colour “flees”. (Even today pigments and dyes that are not colourfast are said to be “fugitive”.)

Technically these yellows were not exactly lakes, but pinks. Yes, it’s confusing: the word “pink” originally referred not to a pale reddish colour but to a class of pigments similar to lakes but made without the need for an alkali in the recipe. In the seventeenth century there were yellow pinks, green pinks, and light rose-coloured pinks. It is only because the last of these stayed in use for the longest that the term today denotes a hue.

The colorant used for yellow pinks was typically an extract of weld, broom or buckthorn berries. But one used these materials – as Huysum discovered – at one’s own risk.

It’s not that artists didn't have alternative, more stable yellows available. But as with any colour, not all yellows are equal. Those that could be made from minerals or inorganic compounds produced artificially might last longer, but some were rather dirty or pale in their tint.

There was, for example, yellow ochre: a yellowish form of the iron oxide mineral that also came in reds and browns. But if ochre today conjures up a brownish earth colour, that’s because yellow ochre was in truth more like that: fine for tawny hair, but not at all the thing for tulips or satin robes.

Then there was Naples yellow, as it was known from the seventeenth century: a pigment of rather variable composition but which was generally made from synthetic compounds of tin, antimony and lead. The ancient Egyptians knew how to combine lead with antimony ore to make a yellow, and in fact a natural mineral form of that compound (lead antimonite) was also used as an artists’ material. It could be found on the volcanic slopes of Mount Vesuvius, which is how it came to be associated with Naples. Other recipes for a yellow of similar appearance specified mixing the oxides of lead and tin. The ingredients weren’t always too clear, actually: when Italian medieval painters refer to giallorino, you can’t be sure if they mean a lead-tin or lead-antimony material, and it is unlikely that the painters recognized much distinction. Before modern chemistry clarified matters from the late eighteenth century, names for pigments might refer to hue regardless of composition or origin, or vice versa. It could all be very confusing, and from a name alone you couldn’t always be sure quite what you were getting – or, for the historian today, quite what a painter of long ago was using or referring to.


The chemise of Jan Vermeer’s The Milkmaid (c.1658-61) is painted with a lead-tin yellow.

In some respects that’s still true now. A tube of modern “Naples yellow” won’t contain lead (rightly shunned for its toxicity) or antimony, but might be a mixture of titanium white and a chromium-based yellow, blended to mimic the colour of the traditional material. There’s no harm in that – on the contrary, the paint is likely to be not only less poisonous but more stable, not to mention cheaper. But examples like this show how wedded artists’ colours are to the traditions from which they emerged. When you’re talking about vermilion, Indian yellow, Vandyke brown, orpiment, the name is part of the allure, hinting at a deep and rich link to the Old Masters.

One thing is for sure: you won’t find the gorgeous orpiment yellow on the modern painter’s palette (unless perhaps they are consciously, and in this case rather hazardously, using archaic materials). It is a deep, golden yellow, finer than Naples and lead-tin yellows. The name simply means “pigment of gold”, and the material goes back to ancient times: the Egyptians made it by grinding up a rare yellow mineral. But at least by the Middle Ages, the dangers of orpiment were well known. The Italian artist Cennino Cennini says in his handbook, written in the late fourteenth century, that it is “really poisonous”, and advises that you should “beware of soiling your mouth with it.” That’s because it contains arsenic: it is the chemical compound arsenic sulphide. (A different form of the same compound, also found as a natural mineral, furnishes the pigment realgar, the only pure orange colour available to painters until the nineteenth century.)


Natural orpiment (arsenic sulphide).

Orpiment was one of those gorgeous but costly pigments imported to Europe from the East, in this case from Asia Minor. (In the early nineteenth century there were also imports from China, so that orpiment was sold in Britain as Chinese Yellow.) Such alluring imports often arrived through the great trading centre of Venice, and orpiment was hard to acquire up in Northern Europe during the Middle Ages and the Renaissance – unless, like the German artist Lucas Cranach, who ran a pharmacy, you had specialist connections to exotic materials. Some orpiment was made not from the natural mineral but artificially by the chemical manipulations of alchemists. This type can be spotted on old paintings today by studying the pigment particles under the microscope: those made artificially tend to be more similar in size and have rounded grains. From the eighteenth century it was common to refer to this artificial orpiment as King’s Yellow. Rembrandt evidently had a supplier of the stuff, which has been identified in his Portrait of a Couple as Isaac and Rebecca (often called The Jewish Bride), painted around 1665.

If Dutch painters wanted a golden yellow like orpiment without the risk of poisoning, the Age of Empire supplied another option. From the seventeenth century, Dutch paintings (including those of Jan Vermeer) begin to feature a pigment known as Indian Yellow, brought from the subcontinent by the trading ships of Holland. It arrived in the form of balls of dirty yellowish-green, although bright and untarnished in the middle, which bore the acrid tang of urine. What could this stuff be? Might it truly be made from urine in some way? Lurid speculation abounded; some said the key ingredient was the urine of snakes or camels, others that it was made from the urine of animals fed on the yellow Indian spice turmeric.

The mystery seemed to be solved in the late nineteenth century, when an Indian investigator making enquiries in Calcutta was directed to a village on the outskirts of the city of Monghyr in Bihar province, allegedly the sole source of the yellow material. Here, he reported, he found that a group of cattle owners would feed their livestock only on mango leaves. They collected the cows’ urine and heated it to precipitate a yellow solid which they pressed and dried into lumps.

The cows (so the story goes) were given no other source of nutrition and so were in poor health. (Mango leaves might also contain mildly toxic substances.) In India such lack of care for cattle was sacrilegious, and legislation effectively banned the production of Indian Yellow from the 1890s.


J. M. W. Turner was one of the nineteenth-century artists who made much use of Indian yellow.

There has been debate about how much of this story is true, but the basic outline seems to stand up – the pigment has a complicated chemical make-up but contains salts of compounds produced from substances in mango leaves when they are metabolized in the kidneys.

While artists were having to rely for brilliant yellows on fugitive plant extracts, deadly arsenic-laden powers and cows’ urine, one might fairly conclude that they would welcome better yellows. So, then, it’s not hard to imagine the excitement of the French chemist Nicolas Louis Vauquelin when at the start of the nineteenth century he found he could make a vibrant yellow material by chemical alteration of a mineral from Siberia called crocoite.

This stuff was itself red – it was popularly called Siberian red lead, since there was truly lead in it. But in 1797 Vauquelin found there was something else too: a metallic element that no one had seen before, and which he named after the Greek word for colour, chrome or chromium.


“Siberian red lead”, a mineral source of chromium.

The name was aptly chosen, because Vauquelin soon discovered that chromium could produce compounds with various bright colours. Crocoite is a natural form of lead chromate, and when Vauquelin reconstituted this compound artificially in the laboratory, he found it could take on a bright yellow form. Depending on exactly how he made it, this material could range from a pale primrose yellow to a deeper hue, all the way through to orange. Vauquelin figured by 1804 that these compounds could be artists’ pigments, and they were being used that way even when the French chemist published his scientific report on them five years later.

The pigment was expensive, and remained so even when deposits of crocoite as a source of chromium were discovered also in France, Scotland and America. Chromium could also supply greens, most notably the pigment that became known as viridian and which was used avidly by the Impressionists and by Paul Cézanne.

The chromium colours play a major role in the explosion of prismatic colour during the nineteenth century – evident not just in Impressionism and its progeny (Neo-Impressionism, Fauvism and the work of van Gogh) but also in the paintings of J. M. W. Turner and the Pre-Raphaelites. After the muted and sometimes downright murky shades of the eighteenth century – think of Joshua Reynolds’ muddy portraits and the brownish foliage of Poussin and Watteau – it was as if the sun had come out and a rainbow arced across the sky. Sunlight itself, the post-Impressionist Georges Seurat declared, held a golden orange-yellow within it.

For their sun-kissed yellows, the Pre-Raphaelites and Impressionists did not need to rely on chromium alone. In 1817 the German chemist Friedrich Stromeyer noticed that zinc smelting produced a by-product with a yellow colour in which he discovered another new metallic element, named after the archaic term for zinc ore, cadmia: he called it cadmium. Two years later, while experimenting on the chemistry of this element, he found that it would combine with sulphur to make a particularly brilliant yellow – or, with some modification to the process, orange. By the mid-century, as zinc smelting expanded and more of the byproduct became available, these materials were offered for sale to artists as cadmium yellow and cadmium orange.


The artificial pigment cadmium yellow.

The cadmium colours have always stayed rather expensive, though. Nothing really beats cadmium red, a variant that went on market only around 1910. But it is typically around twice the price of other comparable reds, and the same goes for cadmium yellow. In that respect things have not changed so much since an artist in the Renaissance had to weigh up the worth of acquiring expensive orpiment as opposed to the drabber but much cheaper Naples yellow.

There’s a lesson in the cadmium pigments that applies to all colours, through all ages: they have often been byproducts of some other chemical process altogether, often discovered serendipitously as chemists and technologists pursue other goals – to make ointments, say, or soap, glass or metals.

It’s no different now. If you buy a tube labeled “Indian Yellow”, you can be sure no mangos or urine went into its making. Chances are, it will contain a yellow pigment that goes by the unromantic name of PY (pigment yellow) 139 – no mineral or metal salt, but a complicated organic molecule, meaning today that it is carbon-based and resembles molecules found in some living organisms. Chemists will say that it is a “derivative of isoindoline”, but the key point is that at its core is a ring of six carbon atoms joined into a so-called benzene ring.

That’s a clue to the true heritage of these modern organic pigments. Pure benzene, as well as other molecules closer still in their shape and structure to those of PY139, was first isolated in the early nineteenth century from a substance called coal tar, the black tarry residue left over from the industrial extraction of natural gas from coal for gas lighting. Coal tar has a pungent smell – think of the traditional coal-tar soap, which contained some disinfectant compounds distilled from coal tar. This is because it is full of molecules with benzene rings at the core, which tend to be aromatic. (Chemists use that word simply to signify that benzene rings are present, irrespective of smell itself.) In the mid-nineteenth century, German chemist August Hofmann, the leading expert on aromatic coal-tar compounds, set his young English student William Perkin the challenge of trying to make the anti-malarial drug quinine from coal-tar extracts. Perkin didn’t succeed, but instead he found he had made a rich purple substance that he called aniline mauve and began to sell as a dye. That was the beginning of the synthetic-dye industry, which gave rise to the modern era of industrial chemistry: by the early twentieth century, dye manufacturers were starting to diversify into pharmaceuticals and then plastics.

This is the world from which PY139 comes, along with a host of other organic pigments that mimic the old traditional colours with safer, cheaper compounds – many of them used also as food colorants, dyes and inks. One of the first offshoots of the aniline dyes was a yellow, simply called aniline yellow and belonging to an important class of colorants called azo dyes; it was sold commercially from 1863. There is a good chance that, when you see yellow plastic products today, they are coloured with azo dyes.


Winsor and Newton’s azo yellow.

It seems a deeply unglamorous way to brighten the world today, compared to the age of King’s Yellow, saffron and Indian Yellow. It could feel that what is saved in the purse is sacrificed in the romance. Maybe so. But artists are typically pragmatic people, as eager for novelty as they are attached to tradition. There has never been a time when they have not avidly seized on new sources of colour as soon as those appear, nor when they have not relied on chemistry to generate them. The collaboration of art and science, craft and commerce, chance and design, is as vibrant as ever.

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