The paintings we see today in museums, galleries, churches and temples are often much altered by the centuries. Pictures can split, rot, be eaten by woodworm, warp, blister, crack, cup, flake, darken, blanch, discolor, become too translucent and disappear under a centuries-old varnish; and they can also suffer from the efforts of their owners to rectify these situations: they might be transferred, relined, ironed, abraded or repainted. Anyone writing about a work of art needs to establish at the outset how much it has changed since it was first made. This act of understanding is far from...