"Great pictures go straight to the heart," he says, "so I understand that any slight change to them is an emotive subject for many people. But as far as I'm concerned, my work – just like that of all the restorers before me – is in no way a vital part of a picture. It'll last 200 years, say, and then it'll be the next restorer's turn. The solvents we use for cleaning these days are significantly diluted, too, so we're not making any changes that are irreversible."