Leonardo da Vinci's dramatic Last Supper in the former refectory of Santa Maria delle Grazie in Milan is the largest of his paintings – it covers over 40 sq m of the refectory's north wall – and many think it his greatest. It would probably be called the most famous painting in the world if that unverifiable accolade had not already been accorded to a certain moody portrait of a Florentine housewife, which he also did. It took him and his team of assistants about three years to complete. No contract for it survives, but it was almost certainly commissioned by his patron, Lodovico Sforza, in 1494, and we know he was still at work on it in 1497 because an entry in the monastery accounts records a payment to some workmen for repairing "a window in the refectory where Leonardo is painting the Apostles". Numerous sketches, notes and preparatory drawings chart the long and sometimes troubled gestation of "this restless masterpiece" (as Jacob Burckhardt described it), and the latest restoration, completed in 1999, has revealed a wealth of information about the techniques Leonardo used.
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One important technical fact that has been known for centuries is that The Last Supper was not painted using traditional fresco technique (watercolour and egg-tempera on moist plaster) but with an experimental oil-based medium. The chief advantage of this was compositional – oils gave him the subtle tonalities that were his trademark, and the opportunity to rethink and rework as he went along – but in practical terms it was a disaster. On a wall prone to damp, the paint surface quickly deteriorated. By 1517, a diarist noted, it was already "beginning to spoil", and by the time Giorgio Vasari saw it in the 1550s there was little more than a "muddle of blots". For centuries it was subjected to invasive restorations and heavy-handed retouchings. It suffered further vicissitides in the early 19th century, when Napoleon's soldiers used the refectory as a stable, and in 1943, when an RAF bomb landed on the Grazie, leaving the mural exposed to the elements for several months. To the inherent charisma of the painting is added this chequered history of self-inflicted fragility and semi-miraculous survival.
