Leonardo and the Last Supper (20 page)

Frescoists worked in a very precise manner. At the start of each working day, the mason troweled the
intonaco
on to a small patch of wall to which the artist added his pigments before it dried. Pozzo succinctly described the process. “It is called fresco painting,” he wrote, “because the painting must be performed on it while the plaster is still damp; and for this reason, the plaster must not be spread over a larger portion of the surface than can be painted in one day.”
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A fresco was therefore created section by section, day by day, with the painter working swiftly on small fields of wet plaster. As Pozzo pointed out, a fresco needed to be executed “with greater quickness and celerity” than any other type of painting.
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Frescoists generally did not paint freehand on the wet plaster. One of the keys to fresco was the use of cartoons (from
cartone
, a heavy paper or pasteboard) to transfer the design to the wall. The cartoon, a full-scale template for the fresco, was created by scaling up the smaller designs, usually via a grid of proportional squares, and transferring them to a master cartoon made (depending on the size of the fresco) from a few to dozens or more pasted-together sheets of paper. This large cartoon, which for Leonardo’s
Last Supper
would have been almost thirty feet wide, was cut up into smaller sections, which were then attached to the wall immediately after each patch of the
intonaco
was applied. The design on the cartoon was transferred to the wall in one of two ways: either by using a stylus to trace over the charcoal drawing and leave incisions on the wet plaster, or else by pricking the cartoon with hundreds of pinholes and then striking it with a pounce bag filled with powdered charcoal, leaving a dotted outline on the
intonaco
. The painter then removed the cartoon and began following either the incisions or the dotted lines.

Fresco therefore entailed a huge amount of time-consuming preparatory work before the first brushstroke of paint could be laid: designing and building the scaffold, mixing and troweling the plaster, and transferring designs from the cartoons, which were themselves the product of countless preliminary drawings and as many as several hundred sheets of paper (one famous surviving cartoon, Raphael’s for
The School of Athens
, a twenty-five-foot-wide fresco painted in the Vatican Apartments in about 1510, was made from 195 sheets of glued-together paper).

Despite Leonardo’s love of challenges, his lack of experience in this difficult medium would have given him pause. Furthermore, this technique of painting—of expeditiously adding pigments and then moving on to an adjacent section—was ill-suited to his manner of working. He was not someone who worked with “quickness and celerity.” The diary kept by Jacopo da Pontormo as he worked in the church of San Lorenzo in Florence gives a good sense of how the frescoist progressed. “On Thursday I painted those two arms,” he recorded. “On Friday I painted the head with the rock below it. On Saturday I did the trunk of the tree, the rock, and the hand. On the 27
^th
I finished the leg... On Tuesday I began the torso... On Thursday I did an arm. On Friday, the other arm. On Saturday, the thigh.”
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It is difficult to imagine Leonardo working in such piecemeal fashion. He was not someone who finished an arm on Friday and then on Saturday happily moved on to paint the thigh. He preferred to work at a more leisurely pace than fresco required, concerning himself with subtle effects—modulations of color or transitions of light and shade—that fresco’s requisite speed of execution made virtually impossible. His portraits and altarpieces were painted with prodigious deliberation. He manipulated his paint with great care, blending and texturing the pigments. To achieve his unique effects of light and shade, he even dabbed and smeared the wet paint with his fingertips. Fingerprints can still be seen on several of his paintings, such as his portraits of Ginevra de’ Benci and Cecilia Gallerani—evidence of his attempts to soften the women’s features, as well as of the painstaking efforts he made to achieve his extraordinary visual effects.
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Leonardo’s lack of experience in the medium, as well as its unsuitability for his particular genius, were probably two of the objections behind his frustrated and fragmentary letter to Lodovico protesting about a commission that was “not my art.” However, evidently reconciled to the project, he decided to proceed, typically, in an experimental fashion, albeit not in the time-honored style followed by the legendary painters whose frescoes he would have seen in Florence.

Leonardo was fascinated by the possibilities of working with oil paint. His notebooks contain numerous references to different types of oils. He seems to have experimented with making oils from walnuts, linseeds, mustard
seeds, juniper needles, and the gum of cypress trees. His studio must at times have resembled an apothecary’s shop or alchemist’s laboratory, with oils oozing from presses and bubbling in pots. He even tested out ways of getting rid of the “evil smell” of certain nut oils by mixing them with vinegar and reducing them over a flame. He had no end of advice on the subject: “And if you want the oil to be good and not to thicken,” reads one of the instructions in his treatise for painters, “put into it a little camphor melted over a slow fire and mix it well with the oil and it will never harden.”
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Compared with egg tempera, oil paint was still, relatively speaking, a new and somewhat experimental technique. Vasari mistakenly claimed that oil painting was invented in the first half of the fifteenth century, but in fact the process of using oil as a vehicle for pigment was described as early as the twelfth century in a Latin treatise by a German monk known only by the pseudonym Theophilus Presbyter. However, painting in oil achieved no widespread currency until improvements in the technique were made in Flanders in about 1410 by Hubert van Eyck. The court painter to the duke of Burgundy, van Eyck perfected a method of suspending his pigments in a mixture of linseed and nut oil blended with resins and lavender oil. His renown for this innovation was such that the bones of his arm and hand were preserved like relics near the church in Ghent where he was buried beneath the sobering inscription: “I was called Hubert van Eyck. I am now food for worms.”
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Making generalizations about medieval Italian painting is difficult due to the fact that fewer than 5 percent of all works painted in the fourteenth century have survived.
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However, anecdotal evidence suggests that oil painting was known in Italy at least since the early fourteenth century. In 1325, a pupil of Giotto named Giorgio d’Aquila was provided with a supply of nut oil when the duke of Savoy hired him to paint a chapel at Pinarolo—but when the oil failed to meet Giorgio’s standards, he sent it to the duke’s kitchen to be used for cooking.
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Van Eyck’s innovations entered Italy by means of Antonello da Messina, who worked for a time in Bruges, and whose tomb in Venice (where he died in 1479) proudly recorded that “he was the first who conferred splendour and durability on Italian painting by the mixture of colours with oil.”
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Antonello supposedly passed the secret to Domenico Veneziano, who, according to one of Vasari’s taller tales, was murdered by a rival—none other than Andrea del Castagno—who was jealous of his painterly feats. In Vasari’s version, Castagno gained Veneziano’s
confidence by entertaining him with carefree evenings of serenading pretty girls with lutes. After Veneziano finally divulged the secret of oils, Castagno beat him over the head with a lead pipe.
¹⁷

Alas for the story, Castagno died of the plague in about 1457, several years before Veneziano. There is probably no more truth in the story that Giovanni Bellini tricked Antonello into revealing the secret by turning up at his studio disguised as a Venetian nobleman who wished to have his portrait painted, then watched carefully as Antonello went to work.
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However apocryphal these stories, there is no doubt that throughout much of the fifteenth century the technique of mixing pigments with oils—of finding both the right oils and the right mixture—was a jealously guarded workshop secret.

Verrocchio, who worked only in tempera, did not concern himself with this newer technique. Leonardo, though, was interested in oil paint even as an apprentice. The face and curly head of the kneeling angel in Verrocchio’s
Baptism of Christ
—widely accepted as Leonardo’s handiwork—were done in oil rather than tempera. For his portrait of Ginevra de’ Benci, done in the mid-1470s, he used both oil and tempera paints, but a decade and a half later, in the portrait of Cecilia Gallerani, he used oils exclusively. The superiority of oils for capturing certain optical effects, together with Leonardo’s proficiency in the technique, appears to have been recognized by the members of the Confraternity of the Immaculate Conception, whose 1483 contract with Leonardo and the de Predis brothers repeatedly stressed that both the figures and the landscape background of the altarpiece should be done in oils.

Oil paint had a number of distinct advantages over egg tempera. Most obviously, pigments blended with oil looked richer and glossier than those mixed with tempera. Oils allowed an artist a greater range of color values and tonal gradations because they could be blended either on the palette or (as Leonardo’s smudging fingerprints attest) on the pictorial surface itself. An artist could also vary the ratio of oil to pigment, resulting in a wide range of consistencies, anything from thin, translucent layers of color to dense impastos. He could heat his oil before the pigment was added to create a more viscous paint, or he could reduce the viscosity by adding camphor or other oils. Unlike tempera, oils did not change color as they dried. They also dried more slowly, permitting the artist to rework his painting by making as many corrections as he might desire—one of the many other properties that undoubtedly appealed to Leonardo.

As the legends of espionage and murder suggest, a painter working in
oils required a great amount of tradecraft. He needed to understand the specific properties of his various pigments: which ones worked best in oil and which in tempera. He also needed to decide which oil—linseed, walnut, poppy—to mix his pigment with, and whether he wanted to heat the oil over a flame or by standing it in the sun. To these ends Leonardo, with his pestles, mortars, and bubbling pots, conducted his various experiments.

Painting with oil had allowed Leonardo to capture the startling visual effects that were winning him his reputation as a painter: the moody half lights and misty atmosphere of
The Virgin of the Rocks
, the soft-focus facial expressions in his portraits of Ginevra de’ Benci and Cecilia Gallerani. His aptitude and experience in oil rather than fresco led him to consider painting his
Last Supper
in a technique different from the usual one: that is, by working with oils on a dry wall.

This method was new and largely untested, though its effectiveness had been affirmed by the architect and polymath Leon Battista Alberti. In his
De re aedificatoria
(
On the Art of Building
), written in the 1440s, Alberti offered instructions for fresco painting to which he appended an intriguing coda: “It has recently been discovered that linseed oil will protect whatever colour you wish to apply from any harmful climate or atmosphere, provided the wall to which it is applied is dry and in no way moist.”
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Leonardo owned a copy of Alberti’s work, one of the most famous architectural treatises of the fifteenth century.
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More to the point, he would have known the works that Alberti was talking about. Alberti was almost certainly referring to the murals of Domenico Veneziano, who in the late 1440s (around the time Alberti was writing) worked with oil paints on a dry wall in the Portinari Chapel in the hospital of Santa Maria Nuova in Florence, an experiment repeated in the same chapel a few years later by Veneziano’s supposed rival and murderer, Andrea del Castagno.
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Although neither painter’s work survives, Leonardo would undoubtedly have seen them (he had a bank account at Santa Maria Nuova) and appreciated their novel technique. He must also have known the mural that Paolo Uccello and Antonio di Papi painted using oils in the refectory of the Florentine church of San Miniato al Monte. This mural, a Crucifixion scene done in the mid-1450s, does not survive, but documents from San Miniato are clear about the oil, which the pair used “even though they were not obliged.” The men in charge of the decorations were so pleased with the result that the painters received a bonus.
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