Leonardo used a medium containing oil and varnish

Such irregular methods meant that the painting could not be al fresco; and in fact, we know that Leonardo used a medium containing oil and varnish. The wall was damp and as a result the painting very soon began to suffer. Already in 1517 Antonio de Beatis noted that it was an excellent work although it had begun to perish, either through the dampness of the wall or some other mischance; and Vasari, who saw it in May 1556, describes it as "so badly handled that there is nothing visible except a muddle of blots". By 1642 Scanelli can write that of the original there remain only a few traces of the figures and those so confused that it is only with great pains that one can make out the subject. In face of such evidence it is hard to resist the conclusion that what we now see on the wall of the Grazie is largely the work of restorers. We know that the painting has been restored four times since the beginning of the eighteenth century, and was probably restored several times before. In 1908 it was thoroughly cleaned by Cavenaghi, who gave a most optimistic report on the picture, saying that only the Christ's left hand was seriously repainted. He adds ingenuously that Leonardo was ahead of his time in that he seems to have employed a medium not usually found till the late sixteenth century. Cavenaghi was such a brilliant technician that his word is usually accepted, but in this case there is overwhelming evidence to prove that he was mistaken. It is inconceivable that a painting which by all accounts was a hopeless wreck in the sixteenth and seventeenth centuries could have survived till our own day more or less intact, and concrete evidence of restoration is provided by a comparison of the Apostles' heads in the fresco with those in early copies. Perhaps the best examples are the two independent series of drawings at Weimar and Strasbourg, which were done direct from the original by Leonardo's pupils, and show none of the personal variations which occur in a painted copy. Now these drawings agree in certain differences from the painting as we now have it, and in each case the drawing is clearly superior both in sentiment and design. Take four of the Apostles on the left of Christ. In the original, St Peter, with his villainously low forehead, is one of the most disturbing figures in the whole composition; but the copies show that his head was originally tilted back in foreshortening. The restorer was unable to follow this difficult piece of drawing and has rendered it as deformity. He shows a similar failure to cope with an unusual pose in the heads of Judas and St Andrew. The copies show that Judas was originally in profile perdue, a fact confirmed by Leonardo's drawing at Windsor. The restorer has turned him round into pure profile, with considerable damage to his sinister effect. St Andrew was almost in profile; the restorer has turned him into the conventional three-quarters. He has also made the dignified old man into an appalling type of simian hypocrisy. The head of St James the Less is entirely his invention, and gives the measure of his ineptitude. The copies show that this figure was originally a much younger man, with a beautiful expression.

It is worth insisting on these changes because they prove that the dramatic effect of the Last Supper must depend entirely on the disposition and general movement of the figures, and not on the expression of the heads. Those writers who have complained that the heads are forced or monotonous have been belabouring a shadow. There can be no doubt that the details of the frescoe are almost entirely the work of a restorer, and the exaggerated grimacing types, with their flavour of Michelangelo's Last Judgment, suggest that this restorer was a feeble mannerist of the late seventeenth century. Perhaps he was Michelangelo Bellotti, who undertook the restoration in 1726. But in spite of the depressing insistence of these facts, some magic of the original remains, and gives the tragic ruin in Santa Maria delle Grazie a quality lacking in the dark smooth copies of Leonardo's pupils. Luminosity, the feeling for atmosphere, which distinguishes all Leonardo's genuine work from that of his pupils, must have distinguished the Last Supper also: and the fresco, perhaps from its very vagueness, has kept a certain atmospheric quality. As we look at them these ghostly stains upon the wall, "faint as the shadows of autumnal leaves", gradually gain a power over us not due solely to the sentiment of association. Through the mists of repaint and decay we still catch sight of the super-human forms of the original; and from the drama of their interplay we can appreciate some of the qualities which made the Last Supper the keystone of European art. In particular, we can recognise Leonardo's power of invention by the simple means of comparing his treatment of the subject with any other which had preceded it. The Last Suppers of Ghirlandajo and Perugino, painted only a year or two earlier, show fundamentally the same composition as that which had satisfied the faithful for almost a thousand years. Eleven apostles sit on the far side of a table, each one quiet and separate. Sometimes they talk to each other, or drink their wine. Our Lord sits in the middle with St John reclining uncomfortably on His lap. Alone, on the near side of the table, is Judas. We have seen how Leonardo's departure from the traditional iconography of the Adoration involved a change in his whole interpretation of the drama. The same is true of the Last Supper. The older painters had represented the moment of communion, a moment of calm in which each apostle might wish to sit alone with his thoughts. Leonardo, as is well known, chose to represent the terrible moment in which Jesus says One of you will betray me. Immediately this row of quiet individuals is unified by emotion.