The friars in the refectory of Santa Maria delle Grazie in Milan ate their frugal meals in penitential silence: small talk was forbidden, and at best they were permitted to listen to a stern reading from scripture. But, on the painted wall behind their communal table, a less inhibited group drank wine, feasted on a fatty dish of eel with orange juice drizzled over it, and noisily argued and expostulated as they did so. Bernard Berenson, scrutinising the mural, forgot the sanctimonious setting and described the 13 painted figures as rowdy Italians carousing in a cheap restaurant that didn't bother to give its customers cutlery. "What a pack of vehement, gesticulating, noisy foreigners they are," he sniffed.
He was referring, of course, to the company in Leonardo's Last Supper, in which the 12 disciples protest a little too vociferously when Christ announces that one of them will soon betray him. Despite the snobbery it reveals, Berenson's fastidious remark pays Leonardo's conversation piece a fine compliment: it reanimates a work that had faded – before its restoration in 1999 – to a ghostly smear on the convent wall, and restores its polyphonic soundtrack.
Other paintings of the last supper, for instance those by Castagno or Ghirlandaio, make it a "solemn and meditative" occasion, as Ross King says – a moment of communion before the arrest of Christ and a preparation for the symbolic restaging of that meal which occurs whenever mass is celebrated. Leonardo stays close to the "agitation and puzzled astonishment" that convulses the company in the Gospels, which is why he took as his models for the apostles the old men he saw gossiping and grousing on benches in the streets of Milan.
Their uproar, however, is soundless. Leonardo called painting "dumb poetry", by which he meant that it was a form of sign language like that used by those who cannot speak or hear. In his most intriguing works, facial expressions and gestures tease us with riddles. Why is the Mona Lisa smiling? At whom or what is the archangel pointing in The Virgin of the Rocks? What meanings are semaphored by the hands of the apostles, upraised in shock in The Last Supper, or by Christ's hands, laid flat on the table as if ready for the stigmatising nails? And what thoughts are concealed behind those faces?
King asks all the customary iconographic questions, and helps us to imagine what the mural must have looked like, with its "modulations of colour or transitions of light and shade", before it began to moulder and before the philistine friars carved a hole in it to make room for a doorway. He pays deserved tribute to the spatial logic of the composition, organised to illustrate the laws of divine perspective.
The vanishing point is the face of Christ, who is unperturbed by his imminent sacrifice; Leonardo hammered a nail into the plaster to mark the centre where all sightlines would converge, so that the saviour's forehead is pricked by a hole that eerily anticipates the crown of thorns.
But all is not orderly serenity. King also detects a delight in visual paradox that he calls "hocus-focus", a reminder of Leonardo's skill as a magician, a trickster who amazed courtiers by turning white wine into red and designed flying machines that defied gravity. The perspective is hieratic as well as linear, whimsically altering scale: Christ is "a giant compared to John, who sits next to him, and as tall as Bartholomew and Philip, even though he is seated and they are standing". What King doesn't explain is the rugged waste behind Christ's head, as alien as the rocks among which the Mona Lisa so mysteriously sits. Though King's Leonardo is pious and orthodox, I can't help suspecting that this landscape – like the floods and storms and elemental upheavals Leonardo sketched – glimpses a nature as godless as Darwin's.
The myriad-minded Leonardo couldn't stick to a single project for long; ideas interested him more than their execution, which may be why he didn't bother about ensuring that his paint adhered to the damp plaster on the refectory wall. During the three years he spent on The Last Supper, he maddened his employer, the warlord Lodovico Sforza, by making fanciful detours. King follows him down many of these speculative trails, and too much of his narrative potters around on the periphery. He gets back on course when filling in the mural's sorry afterlife, or rather its long decomposition. With bitter humour, King describes recent attempts at restoration as gruesome cosmetic surgery, followed by a rush to the emergency ward. First comes "a spa-style treatment of wax injections and invigorating rubdowns", after which the painting is put on "a life-support system of heat and moisture monitors, along with batteries of diagnostic tests". We are left with a cadaver, brightly rouged and smeared with lipstick to simulate life.
King wants to believe that great art lasts for ever, so he argues that The Last Supper has enjoyed, like Christ, "a kind of resurrection" in Warhol's silk screens or the cloned version that Peter Greenaway manufactured with an inkjet printer. But I wonder if Leonardo didn't intend it to decay. He knew that creativity fights a losing battle with destruction and that art cannot outwit nature: what better way to illustrate those morbid truths than to produce a miraculously beautiful painting that almost immediately begins to revert, like the bodies and minds of all who look at it, to unformed chaos?
