http://moreintelligentlife.com/content/arts/karen-wright/brushes-with-hockney

“Whenever you visit David Hockney, he has something new to show you, like a 72-year-old child. As soon as I reach his house in Yorkshire, he drags me upstairs. “I am cleaning Claude,” he announces, taking me into his bedroom. We stand before a large colour photograph of Claude Lorrain’s 1656 painting, “The Sermon on the Mount”, which hangs in the Frick Collection in New York. Hockney explains that when he was in America for the opening of his exhibition last October, he visited the Frick several times, and the Claude intrigued him. In the 18th century it had been owned by the father of the eccentric English collector, William Beckford, and was damaged by smoke in a fire. The Frick left the painting as it was, whereas Hockney decided to restore it—not by applying chemicals to the surface, but by using his knowledge and the computer skills of his assistant, Jon, to reveal what lay beneath the areas blackened by smoke. This was typical Hockney, in the spirit of his book “Secret Knowledge”, which explored the theory that old masters including Vermeer and Caravaggio used optical devices in making their paintings. It was a theory largely ignored by art historians, but taken more seriously by artists.”