Grafitti has long been the scourge of building owners and municipal officials everywhere. Sometimes graffiti is actually really great urban art. But often it is just vandalism. Now there is a third category. English artist Paul "Moose" Curtis devised what he calls reverse graffiti. Rather than using paint to create images on city walls, Moose instead demonstrates how dirty cities have become by selectively cleaning walls. He uses the negative space created by the dirt on the walls to reveal images. His most recent project was sponsored by Green Works (the "greenwashing" brand of Clorox). Moose created a 140-foot long mural in the Broadway tunnel in San Francisco. The video after the jump tells the tale of how Moose came to be a reverse grafitti artist and how he creates his works.