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 easy. We need to develop a knowledge of the physical and chemical processes which have brought paintings to their current state, in the hope that we can imagine their reversal. And we have to look as much as we can, at a wide variety of paintings, so we can learn to distinguish those in a worse or better state of preservation; we have to try to understand what it is about a picture that differentiates
