How do you safely remove years of coal soot and diesel exhaust from the Parthenon? Or strip discolored varnish off a painting by a Dutch master? Artworks must be cleaned periodically to allow visitors to view them the way their creators intended -- and to prevent further decay. But traditional cleaning methods sometimes risk damaging the objects they are meant to conserve. Restorers usually clean statues, for example, by using scalpels, dental instruments and jewelers' tools to scrape, drill and even saw off black encrustations of pollutants. They must proceed cautiously to ensure that they do not break a single carved stone detail; no one wants to be responsible for chipping a Michelangelo...