Innovation in art has always been a gamble. While originality may be given lip-serving credit, unfamiliarity has an even chance of breeding contempt. There is no other word to describe the critics' response to the first independent exhibition by the Impressionists in Paris in 1874. These artists, it was claimed, had rejected "good artistic manners, devotion to form, and respect for the masters". Part of the outrage was directed at the choice of subject-ordinary people going about their business, for goodness' sake-and part at the quick-fire style of the brush strokes. But the detractors were also offended by the colours.

The critic E. Cardon said sarcastically, "Soil three quarters of a canvas with black and white, rub the rest with yellow, distribute haphazardly some red and blue spots, and you'll obtain an impression of spring in front of which the adepts will be carried away by ecstasy." The second group exhibition two years later elicited similar complaints: "Try to make M. Pissarro understand that trees are not violet, that the sky is not the colour of fresh butter...". Renoir's "green and violet spots" in areas of flesh were seen to "denote the state of complete putrefaction of a corpse".

Yet these were not new charges. In England, the Pre-Raphaelite painters such as William Holman Hunt and John Millais stood accused in the 1850s of using greens "unripe enough to cause indigestion". J. M. W. Turner, the supreme British colourist of the early nineteenth century, had in 1829 been denounced for producing "a specimen of colouring run mad" in his Ulysses Deriding Polyphemus (Figure 1).

What was striking, and to some eyes offensive, about the works of all these artists was that they were using colours never before seen on canvas. Bold use of colour was not in itself a revelation-Titian and Rubens were amongst those who had delighted in it in previous times. But these new colours were different from the red lakes, the Naples yellow and the ultramarine of the Old Masters. They were the products of the Golden Age of chemistry. More than in any earlier age, chemistry had become the handmaid to the arts.

Art in the second half of the nineteenth century was totally transformed by such developments in practical science, and this influence was felt throughout the modern era. Paul Cézanne and Henri Matisse both began their careers as Impressionists, steeped in the new colours. Matisse brought out their full potential as the leading light in the movement known as Fauvism, which brought brightness to a new pitch (Figure 2). Along with Picasso, these two artists are considered by critic Robert Hughes to be the most important of the twentieth century. Picasso himself said:

If all the great colourist painters of this century could have composed a banner that comprised each one's favourite colours, the result would certainly have been a Matisse.

One might imagine, in view of all this, that an appreciation of the painter's sources of colour would be essential to understanding Western art. But this has scarcely been deemed the case. Art historian John Gage confesses that "One of the least studied aspects of the history of art is art's tools". Anthea Callen, a specialist on the techniques of the Impressionists, makes a stronger criticism:

Ironically, people who write on art frequently overlook the practical side of their craft, often concentrating solely on stylistic, literary or formal qualities in their discussion of painting. As a result, unnecessary errors and misunderstandings have grown up in art history, only to be reiterated by succeeding generations of writers. Any work of art is determined first and foremost by the materials available to the artist, and by the artist's ability to manipulate those materials. Thus only when the limitations imposed by artists' materials and social conditions are taken fully into account can aesthetic preoccupations, and the place of art in history, be adequately understood.

So just how did the nineteenth-century iconoclasts obtain their materials? Where did these bright new colours come from?

Making colour

Paints are a combination of two main elements: a colouring material (generally a powdered pigment) and a clear, liquid binding medium. Between the demise of the "egg tempera" technique favoured by medieval panel painters, in which egg yolk was used as the binder, and the introduction of modern synthetic resins and solvents in the twentieth century, most serious artists used oils as the binding medium. Many of the oldest pigments are either ground-up minerals, such as the iron oxide ochres, or inorganic solids manufactured by primitive chemical technology. Vermilion, for instance-the most prized red of the Middle Ages-is mercury sulphide, made by alchemists from pure sulphur and mercury.

Nature provides few brightly coloured substances suitable for use as pigments-the green copper mineral malachite, for example, and the closely related blue copper salt azurite. Most richly coloured pigments were therefore synthetic. Even in ancient Egypt, chemical technology was sophisticated enough to make a variety of strongly coloured compounds, such as lead antimoniate, later called Naples yellow, and lead tetroxide or "red lead".

This palette was supplemented little by little throughout the ensuing centuries-several innovations in the late Middle Ages, such as the preparation of ultramarine and red lake pigments, helped to fuel the explosion of colour evident in Renaissance art. But by the eighteenth century, artists were still lacking reliable, bright materials for many basic colours. Their greens were never really bright enough, and were usually made instead by mixing blues and yellows. There was no pure orange pigment aside from the deadly poisonous realgar (arsenic sulphide). Blues were either expensive, such as ultramarine and azurite, or slightly dull, such as the organic dye indigo. The yellows had a slightly dull edge too, or were liable to fade (if prepared as lake pigments from vegetable dyes). There was no pure purple pigment at all.

Artists felt these gaps keenly. In the 1670s the Dutch painter Samuel van Hoogstraten lamented "I wish that we had a green pigment as good as a red or yellow. Green earth is too weak, Spanish green [verdigris] too crude and ashes [green verditer, a copper carbonate] not sufficiently durable." The Spanish artist Diego Velazquez seems to have concurred with these complaints: he never used a pure green pigment in his life, but always mixed them from azurite and yellow ochre or lead-tin yellow. Even that was not necessarily an improvement: in his painter's handbook of 1758, Robert Dossie says that "the greens we are forced at present to compound from blue and yellow are seldom secure from flying or changing."

For blues, indigo was for centuries the only alternative with a depth of tone comparable to ultramarine. Yet it is a poor substitute: it has a greenish tinge that compares ill with ultramarine's gorgeous purple, and it tends to fade when exposed to light. When the pigment Prussian blue was discovered in the early eighteenth century, the first report describing it begins thus: "Painters who mix oil with their colours have few that represent blue, and those such that, rightly, they wish for [some] more satisfactory."

Prussian blue (iron ferricyanide) is often cited as heralding the new age of synthetic pigments. It was discovered by accident in 1704 by a Berlin-based colour maker, and the convoluted method of its making (involving animal blood) was kept a jealously guarded secret until 1724, when an Englishman named John Woodward acquired and published a description of the process. Prussian blue was claimed to give a colour "equal to or excelling Ultramarine". But Dossie reported that the lighter, brighter and most attractive varieties of Prussian blue were "extremely subject to fly, or to turn to a greyish green". This has made the skies in several painting by Gainsborough, Watteau and Canaletto pearly and washed out where once they would have been a deeper blue.

The pursuit of brightness

By the end of the eighteenth century, chemistry had finally acquired a consistent conceptual system: Antoine Lavoisier's oxygen theory of combustion, which replaced the last vestiges of alchemy embodies in the phlogiston theory. Lavoisier's new chemistry, laid out in his Traité ƒlémentaire de Chimie (1789), became the norm in France, and was energetically advocated in Germany by Martin Klaproth. (England was slower to acquiesce to the Gallic system.) Lavoisier secured the ascendancy of his ideas by renaming the entire system of elements in accordance with it, in Méthode de Nomenclature Chimique (1787) written with fellow chemists Bernard Guyton de Morveau, Claude Louis Berthollet and Antoine Fran?ois Fourcroy. Even after Lavoisier's execution in 1794, this group dominated French chemistry and helped to make France the world leader in the subject.

It was an era of chemical discovery. Lavoisier's Traité ƒlémentaire lists 33 elements; between 1790 and 1848, 29 more were added to the list. Not only had chemistry a new vocabulary; it had developed a powerful set of experimental techniques for analysing substances into their basic components. One of the finest experimentalists of the age was the Swedish apothecarist Carl Wilhelm Scheele, who was one of the first to isolate (though not to identify) oxygen gas-he called it "fire air". Scheele also discovered chlorine and barium, and in 1775, while experimenting on arsenic compounds, he isolated a bright green compound. This was copper arsenite, which soon became used as a green artist's pigment called Scheele's green.

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