Leonardo and the Last Supper (21 page)

The examples of these painters, together with Alberti’s statement and his own innovative nature, would have given Leonardo the inspiration and confidence to tackle his
Last Supper
by using oils on a dry wall. In fact, he was prepared to go one better, mixing his pigments not only with oils but also with egg yolk to create an “oil tempera.”
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In essence, he took tempera paints and mixed emulsifying oils into them. The process was so novel—there is no known precedent—that he must have spent a good deal of time beforehand experimenting in his studio in the Corte dell’ Arengo, trying to perfect the right recipe.

Leonardo’s approach to his mural would therefore deviate from that of Montorfano, who was working on his fresco of the Crucifixion a little more than a hundred feet away. Leonardo created an entirely different surface on which to paint. Once his first coat of plaster dried, he covered it with a thinner, slightly granular layer of calcium carbonate mixed with magnesium and a binding agent probably made from animal glue.
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Once this preparation layer had dried, he added an undercoat of lead white: a primer coat, in effect, to seal the plaster and enhance the mural’s luminosity.
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Leonardo must have known that this strategy of using a lead white primer was a risky one. Made by combining strips of lead with vinegar and horse manure, white lead was the most widely used pigment in history, and for many centuries it was the white of choice in European art. Its use was inadvisable in frescoes, however, since as it oxidized it transformed into lead dioxide and turned a brownish color. Evidence of this transformation could be seen in Cimabue’s frescoes in the Lower Basilica of San Francesco in Assisi, where the whites darkened with disastrous consequences, inverting the original contrasts. Leonardo must have believed the pigment would be stable if mixed with oil or applied with a binder, such as egg tempera, or if it were added to a dry wall rather than to wet plaster.

There was another risk to using lead white, especially in such large quantities. Lead is, of course, poisonous. Its toxic nature had been known at least since the time of the ancient Romans, when the architect Vitruvius noted that it was “hurtful to the human system”: plumbers, he noticed, had a “deep pallor.”
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Lead poisoning was an occupational hazard for painters. Raphael and Correggio may have suffered from it, while James McNeill
Whistler was certainly a casualty: he fell ill in 1862 while painting
The White Girl
. Van Gogh, who used to lick his pigments (lead has a sweetish taste), was probably yet another victim. The madness and melancholy for which so many painters became notorious was attributed by many—such as Vasari, who himself suffered periods of depression—to their sedentary and cerebral lifestyle; however, it may in fact have had something to do with the poisonous pigment on the tips of their brushes.
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Few painters before or since have used as much lead white as Leonardo did when he painted in Santa Maria delle Grazie—though he appears not to have displayed any symptoms of lead poisoning.

Once this primer of lead white had been spread, Leonardo prepared to go to work with his paints, adding them to the snow-white, bone-dry wall. How exactly he began transferring his designs is not entirely clear. It is uncertain whether he used cartoons and, if so, how extensively, for no cartoons survive—but then cartoons, by their very nature disposable and inevitably damaged by their application to the wall, very rarely survived. In places Leonardo took a stick of red chalk and sketched outlines and patterns directly onto the undercoat of white lead.
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Elsewhere on the wall he made underdrawings by using a brush and painting freehand with a black paint. He also incised lines into the plaster, possibly using a cartoon. However, when the time came to paint he did not always follow these incisions, revising and improvising as he went along. Nonetheless, there are few signs of dramatic changes or hesitations. Nor are there any of the telltale pinpricks that would indicate the use of a cartoon.
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Once Leonardo found what he believed to be the right recipe for his “oil tempera,” he began adding his paints in successive layers, allowing one to dry before adding the next: a process impossible in fresco. He often used four or five separate coats of different colors to build shapes and create tones, sometimes starting with a darker pigment in the background and then augmenting and highlighting it with paler, semiopaque colors. When he painted the sky in the background, for example, he first used azurite (a mineral-based pigment) to which successive layers of lighter blue were added to give the illusion of depth. He also took special care with the flesh tones, using three or more pigments. For some faces he began with a base coat of lead white, followed by a black to which he added vermilion, then more lead white, and finally yellow ocher and yet another layer of vermilion.
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The layering of paint, the leisurely and deliberate process it entailed, the ability
it gave him to rework areas—all of these things were characteristic of painting with oils on a wooden panel rather than on an immense plaster wall. He even used a series of translucent glazes to heighten the colors.
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Leonardo intensified his colors in other ways too. He used many pigments that were incompatible with fresco, especially bright blues and reds such as ultramarine, azurite, and vermilion. The frescoist was limited in his palette because mineral-based pigments were unable to withstand the action of the lime. They could be mixed with a binder such as egg white and then added to the wall, and indeed many painters worked in this way (Michelangelo later used extensive passages of ultramarine in his
Last Judgment
in the Sistine Chapel). However, the eventual discoloration of the binder meant that ultimately the colors would turn into (as one treatise cautioned) “ugly daubs.”
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The frescoist was therefore advised to stick to those pigments—the duller ochers and umbers—that were compatible with lime plaster and did not require mixing with a vehicle other than water.

Leonardo, however, clearly wanted his mural to have the chromatic panache of an altarpiece painted in tempera or oil rather than this more limited range of tones necessitated by fresco. He understood like no one before him the way colors interacted: how one color could affect, or was affected by, the one beside it. “The surface of any opaque body,” he wrote, “is affected by the colour of surrounding objects.”
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He realized, that is, the way colors change their intensity and hue depending on what colors surround them. For example, he noted that a red appears more intense if placed next to a white or a yellow rather than next to purple. He therefore discovered the law of complementary colors, observing that colors were the most intense if “surrounded by their strongest contrasts.”
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He was centuries ahead of his time with this observation. Only in the nineteenth century would the law of “simultaneous contrasts of colour” (as it was dubbed by the French chemist Michel-Eugène Chevreul) be developed further. In 1839, Chevreul devised a color wheel (now a mainstay of the school art room) that showed how intense, vibrant contrasts could be achieved by juxtaposing colors 180 degrees apart—a perception ultimately leading to the pointillism of Georges Seurat and then the violent, clashing colors of the postimpressionists.
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One of Leonardo’s other observations on the relativity of color is notable: “A shadow is always affected by the colour of the surface on which it is cast.”
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Arguably, the implications of this observation—that shadows are not black but rather tinged with color—were not understood or explored
until it became one of the key perceptions of impressionism, neatly defined by an American critic as the “blue-shadow idea.”
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Leonardo even had a recipe for making blue shadows, a pigment composed of verdigris mixed with lac, a gummy reddish substance derived from insects.
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In any case, Leonardo’s choice of technique and materials guaranteed that the friars of Santa Maria delle Grazie would eat their meals in the midst of a dazzling show of color.

Leonardo’s accounts show that, faced with a major project, he purchased his pigments in bulk. A few years later in Florence, while working on another mural,
The Battle of Anghiari
, he bought two pounds of cinnabar, four pounds of a yellow pigment, six of a green one, twenty pounds of German blue (made from the mineral azurite), and forty pounds of linseed oil in which to suspend his pigments. The total cost was 120 lire, or 30 florins, the annual wage of a low-paid manual worker such as a female weaver.
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The pigments for
The Last Supper
, a mural smaller in size than
The Battle of Anghiari
, would presumably have been roughly half that sum. Since the contract for
The Last Supper
has been lost, all details about the materials specified or the expenses allowed (for paint, plaster, and scaffolding) are unknown. However, Leonardo’s experience with the Confraternity of the Immaculate Conception—when his entire salary went (so he claimed) on paints and other materials—must have persuaded him to give the contract some careful scrutiny.

The highest quality pigments came from Venice, and artists’ contracts sometimes made allowances for artists to travel from as far away as Florence and Siena to supply themselves.
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An artist could buy the raw materials for pigments (such as cinnabar for reds or malachite for greens) and make the colors himself by grinding them into a powder in the workshop. Alternatively, he could buy his pigments ready-prepared from a specialist purveyor. No records indicate where or from whom Leonardo purchased his pigments for
The Last Supper
, though for
The Adoration of the Magi
he bought prepared pigments from the monks of the convent of San Giusto alle Mura, who were renowned for their skill in manufacturing colors (other customers included Botticelli and Michelangelo).

Leonardo’s purchases from the monks of San Giusto alle Mura were one
of the conditions of his contract. However, he seems to have preferred purchasing the raw materials himself and then—unsurprisingly—experimenting with his own mixtures and preparations. One of his recipes for a white pigment stated: “Put the white into an earthen pot...and let it stand in the sun undisturbed for 2 days.” A yellow pigment could be created, he noted, by dissolving ground-up orpiment, a yellowish mineral, with another mineral, realgar, in the corrosive solution known as aqua fortis (a mixture of nitric acid and water). A flesh tone could be made from grinding up reddish crystals from a place he called Rocca Nova, and a green from mixing verdigris and lemon juice. Alternatively, a “fine green” could be made from combining verdigris with either aloes, plant gall, or the herb turmeric.
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When the time finally came to paint the wall of the refectory, Leonardo would have climbed to the very top of his scaffold. Frescoists almost always painted from top to bottom in order to avoid dripping paint onto completed areas. Leonardo was probably no exception, and so the first things he painted in the refectory would have been the three heraldic shields in the lunettes immediately beneath the ceiling vault: the emblems of Lodovico Sforza and his two male heirs. Lodovico’s coat of arms would soon be a familiar sight in and around Milan. He was planning to have them carved in marble and placed on the city gates as well as on all public buildings. It was an enterprise that he claimed lay “close to my heart.”
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But the first place they appeared in all their glory seems to have been the refectory of Santa Maria delle Grazie.