Contrasting reds: Detail of "Saints Jerome and John the Baptist", c.1428-9, by Masaccio (credit: The National Gallery, London)
"Drawing," said the 19th-century Neoclassical painter Jean-Auguste-Dominique Ingres, "is the probity of art." With this aphorism he took his place on one side of a debate that had been conducted since the Renaissance. Opposing him in contemporary France was the Romantic Delacroix, who was merely the latest in a cadre of painters from Giorgione and El Greco to Rubens who saw art's probity as residing in colour.
It was a subject that divided the greatest artists: the adherents of disegno as opposed to colore included Michelangelo who, speaking to Vasari, said that although Titian's "colour and style pleased him very much, it was a shame that in Venice they did not learn to draw well". More than 100 years previously though, in 1390, Cennino Cennini in The Craftsman's Handbook — among the earliest artists' manuals — described colour simply as "the glory of the profession".
Colour, however, has both confined painting and released it. The 20th century was the first time painters weren't restricted in their use of colour because, with chemical advances, every possible shade on the spectrum became available. Earlier eras had to contend with limited colours that were prohibitively expensive or, when made from plant dyes, unstable and prone to fading. They were difficult to store too: oil paints superseded stiff and quick-drying egg tempera (ground pigment mixed with egg yolk) only in the 15th century. This allowed more time to work on the picture surface and a greater ability to blend colours seamlessly. The differences between the two are clearly visible in the small hatched brushstrokes of the tempera Wilton Diptych (1395-99) and the smooth surface of Jan van Eyck's oil Arnolfini Marriage of 1434 — both in the National Gallery.
Prepared oil paints — an oil medium used in place of egg — could be stored by temporarily submerging them in water or in pigs' bladders but they were not readily transportable. This is why there was no real tradition of en plein air painting until paint tubes were invented in the 19th century: painters were stuck in their studios. As Renoir said: "Without paint in tubes there would have been no Cézanne, no Monet, no Sisley or Pissarro, nothing of what the journalists were later to call Impressionism."
Meanwhile, the Renaissance reputation of the Venetian school as the home of colour was in part due to the fact that the Republic's role as a trading entrepot, regularly dealing in oriental dyes, textiles and glassware, meant that its artists had access to pigments that were rare in other parts of Italy.
The physical properties and limitations of paint and how they have affected artists is the subject of Making Colour at the National Gallery (until September 7). Using pictures from the Middle Ages to the late 19th century from its own collection as well as loans, it devotes rooms to single colours to show how painters have maximised the technical possibilities and variety of shades open to them. So, for instance, Masaccio's Saints Jerome and John the Baptist, c.1428-29, in which one of the saints wears brilliant red vestments while the other's gown, initially equally bright, has faded to a purplish hue, is displayed alongside Degas's La Coiffure of 1896, an experiment in reds which shows a flame-haired girl having her locks brushed by a ginger-haired maid in a scarlet room.
It also looks at the development of colour theory and the science behind our perception of colour — the way, for example, Seurat and the Pointillists applied individual dabs of colour in the expectation that they would mix in the eye rather than on the canvas. The National Gallery's scientific department has long shown its work in small, single-room displays that have focused on individual restoration projects; this, though, is the first time it has driven a full-scale exhibition.
The room devoted to blue, for example, starts with ultramarine, a colour used in the West from the early 12th century and made from ground lapis lazuli. Because the mineral came only from the mines of Sar-e-Sang in north-east Afghanistan it was extremely expensive and so was reserved for the most important parts of paintings (indeed, patrons would not only stipulate the use of lapis but buy it themselves to give to the artist) such as the Virgin's gown in Masaccio's 1426 Virgin and Child. For skies a cheaper blue such as azurite (from European mines) would be used with only a thin layer of ultramarine on top. However, non-mineral blues such as indigo were prone to fading and it was not until the chance discovery c.1704-10 of Prussian blue, a synthetic pigment, and cobalt blue a century later that painters had cheap but stable blues to work with.
Other colours followed a similar history. Lead white, made from the corrosive crust that forms on lead when exposed to sour wine, was widely used until the arrival of non-poisonous zinc or titanium-based whites in the 20th century. Lead-tin yellow, made by firing oxides of the metals to 800 degrees centigrade, was superseded in the late 17th century by antimony-based yellows and then, by 1820, a variety of yellows made from chromium. And so on for red (originally vermilion from the ground mineral cinnabar), and green (from verdigris, the deposit resulting from copper exposed to acetic acid). By the second half of the 19th century so many colours were available that Monet in his sketchy 1869 Bathers at La Grenouillère used at least 15 different pigments.
This exhibition is unusual in that it forces the viewer not to look at the paintings as a whole but focus on their most important constituent. It is rather like being told not to examine a person's face but just their skin. Whatever the story the pictures tell, the paint tells another one — of science and economics, trade and innovation, ingenuity and theory. It is a story usually ignored even though it has been hiding in plain sight.