The Last Leonardo: The Making of a Masterpiece. Ben Lewis

Salvator Mundi there is a real one.

Other aspects of these sketches muddy the artistic waters of attribution. The forearm on this same page seems to have been drawn not by Leonardo but by another artist entirely, surely one of Leonardo’s assistants. In the 1930s, the great Leonardist Kenneth Clark catalogued the collection of Leonardo’s drawings owned by the British royal family, which included both of these sketches. He observed that the draftsmanship of the forearm is heavy-handed compared to the chest, and that, furthermore, the hatching runs left to right, while Leonardo’s always runs right to left, as one would expect of a left-handed artist.

      The second drawing holds a puzzle too. The sleeved limb emerges from two loose loops of fabric, which closely resemble the drapery around Christ’s arm held up in blessing in the painting. But in the drawing the forearm is sleeved with a cuff at the end; in the painting it is bare. As a preparatory study it bears a surprisingly loose relationship to the finished painting.

We can vaguely date both drawings to the first decade of the sixteenth century, because of the evolution of Leonardo’s style and technique. He spent most of his early career sketching in pen and ink or silverpoint. In the mid-1490s he began using red chalk for studies for the apostles in The Last Supper. In the first decade of the sixteenth century, chalk became Leonardo’s primary drawing medium. The material was better suited to the style he was developing. Chalk’s softness allowed him to intensify his sfumato, the most gradual light-to-dark transitions which became the hallmark of his painting.2 It is in this later drawing style that the preparatory sketches are executed.

      Drawing is the common denominator between all of Leonardo’s diverse activities as engineer, scientist and artist. And yet Leonardo had criticisms of drawing. One of the fundamental aspects of his thinking, which set him apart from his contemporaries, was his radical attitude to line. ‘Lines are not part of any quantity of an object’s surface, nor are they part of the air which surrounds this surface,’ he wrote. ‘The line has in itself neither matter nor substance and may rather be called an imaginary idea than a real object … Your shadows and lights should be blended without lines or borders in the manner of smoke losing itself in the air … O painter, do not surround your bodies with lines!’ There were no lines in the real world, he said, so don’t paint them.

      In the cool and pungent backstreets of Milan, in the dark shops of the apothecaries, majolica vases lined the shelves, full of herbs, medicines, chemicals and pigments – the raw material for colouring the world. These were roughly chopped preparations of minerals, insects, animal remains and plants, waiting to be finely ground into powder, mixed with egg yolk or oils to make paint, and with water for dyes. The customers came and went – dyers, glassmakers, tailors, the manufacturers of ceramics and furniture, manuscript illuminators and painters.

Behind every Leonardo painting lay a global network of anonymous collaborators, with professions far removed from the creative arts. The minerals for pigments came in ships from distant Central Asian cities that few Europeans had seen, often via Syrian merchants. Some were manufactured in Venetian or Florentine laboratories run by religious orders. There was ultramarine powder ground from blue-veined chunks of lapis lazuli by the Jesuits of the Florentine Convent of Santo Giusto alle Mure; the semi-precious rock was imported from what is now Afghanistan and Uzbekistan, via Damascus. A more sensibly priced version of the same colour, from the same convent, was made of azurite acquired from Austrian and Balkan mines and manufactured with copper oxide. The chromatic reputation of these priests was so high that the Florentine contract commissioning the Adoration specified that Leonardo had to buy his colours from them. A red pigment came from the dried bodies of kermes lice from the eastern Mediterranean, mixed with water, alum and soda. Another came from boiled brazilwood, which Spanish and Portuguese merchants imported from the Latin American colonies. A third red, ‘dragon’s blood’, was a resin derived from various plants, and was used both as a medicine and a varnish. Cochineal was a scarlet named after the insects from which it came, and was imported to Italy via Antwerp. Rubies, which could be ground down into an expensive colour, were available too. White, ochre, and yet another red came from different treatments of lead. A dark ‘bone black’ came from a distillation of carcasses. Verdigris, a green familiar to us as the patina on old copper, was obtained by exposing copper plates to vinegar. Gold leaf was made from old coins, the thin sheets carefully placed between sheaves of paper in books. Saffron was used to make an intense yellow colour when mixed with alum and egg yolk. Thus, science and trade formed a basis for art.

Leonardo would probably have sent one of his teenage apprentices to buy the pigments, a shopping list in his hand. The artist’s notebooks contain to-do lists, often compiled before a long journey, which give an indication of the errands his ‘boys’ had to run. One, from 1490, reminds the apprentice to get hold of ‘a book that treats of Milan and its churches, which is to be had at the stationers on the way to Cordusio [a piazza in the centre of the city]’. Another, from the years 1508 to 1510, quite possibly the time when Leonardo began to work on the Salvator Mundi, lists ‘boots, stockings, comb, towel, shoelaces, penknife, pens, gloves, wrapping paper, charcoal, spectacles with case, firestick, fork, boards, sheets of paper, chalk, wax, forceps …’ This list may refer to items he had already bought from Milan’s shopkeepers and kept in his studio, but Leonardo does add a note about one thing that he clearly didn’t have: ‘Get hold of a skull.’

      For the Salvator Mundi, the apprentice would have had only a small number of pigments on his list, because this painting was made with remarkably few colours: lead white, lapis lazuli, lead tin yellow, vermilion, red iron oxide, carbon and charcoal black, bone black and umber. Back in the studio, the assistants would then have to grind the colours to create a fine powder. However, on this occasion, as later restoration showed, they didn’t do a very thorough job: the brilliant blue grains of the lapis lazuli were rather coarse compared to those in Leonardo’s other paintings.

      There was probably a cartoon by Leonardo for the entire composition of the Salvator. This would have been pricked with tiny holes, or spolveri. The cartoon was laid on top of the panel, dusted with fine powder and then removed. An outline of the composition, traced by the dark dust seeping through the pinpricks, remained. Microscopic photographs of the Salvator Mundi have revealed a handful of tiny black dots, but only enough to raise the possibility, not the certainty, of a cartoon. The artist definitely used a pair of compasses to make the circle of the orb, because there is a hole where the compass point went in. Then another layer of underpainting was added in thin, semi-transparent washes of browns and blacks, some derived from charred wood, others from charred bones.

Leonardo was a member of the Arte dei Medici, Speziali e Mercai – the Guild of Doctors, Apothecaries and Mercers. He had his own recipes for making colours, and he listed many of them in his notebooks. That was unusual for a Renaissance painter, but it fits our knowledge of Leonardo the artist-scientist. The master of light and shade was particularly interested in the variety of ways he could mix colour for shadows: ‘Take green [i.e. malachite] and mix it with bitumen, and this will make the shadows darker. And for lighter shades mix green with yellow ochre, and for even lighter green with yellow, and for the highlights pure yellow. Then take green and turmeric together and glaze everything with it … to make a beautiful red take cinnabar or red chalk or burnt ochre for the dark shadows, and for the lighter ones red chalk and vermilion, and for the highlights pure vermilion, and then glaze with fine lake.’

      We don’t know how the artist of the Salvator Mundi prepared his palette, but there is a description by Vasari of the way another artist, who was taught in the same studio as Leonardo, did. Lorenzo di Credi and Leonardo were both trained by the Florentine master Andrea Verrocchio, and sometimes worked on the same pictures together. Di Credi, says Vasari, ‘made on his palettes a great number of colour mixtures, so that they went gradually from the lightest tint to the darkest, with exaggerated and truly excessive regularity, so that sometimes he had twenty-five or thirty on his palette, and for each of them, he kept a separate brush’. Such preparation would also have been necessary for the delicate, painstaking and time-consuming manner in which the Salvator Mundi was painted.

      Now Leonardo could pick up his brush