Tourists lining up at Santa Maria delle Grazie in Milan are largely looking at a ghost. This episode tells the story of how Leonardo da Vinci's The Last Supper, painted between roughly 1495 and 1498 for Duke Ludovico Sforza, began flaking within twenty years because Leonardo rejected proven fresco technique in favor of tempera and oil on a dry wall sealed with gesso, pitch, and mastic. Moisture in the porous stone pushed the paint off from behind, exactly as a 14th-century handbook had warned.

We examine the genius of what he put on that wall: the split-second after Jesus announces the betrayal, apostles grouped in fours of three, Judas placed among them yet isolated by shadow, a spilled salt cellar, and every perspective line converging on Jesus's right cheek. Then comes the abuse: a doorway sawed through Christ's feet in 1652, French troo ...