Street artist Moose Benjamin Curtis was having some difficulty with the police. The officers had just arrested him for creating designs on a wall in South London. But it was complicated — as things often are when Moose is involved.

You see, Moose doesn’t use spray paint or wallpaper paste — the usual tools of this trade. Instead, he wields scrub brushes, old socks, cleaning fluid, and, when he’s living large, a high-pressure hose. He creates images by cleaning shapes into filthy urban surfaces such as retaining walls, signs, and tunnels.People have called it “reverse graffiti,” “clean graffiti,” and “negative space.” Moose prefers “grime writing.” He has called himself “a professor of dirt.”He is not the only one working in this medium: Brazillian artist Alexandre Orion scrubbed a gallery of skulls onto the wall of a tunnel in Sao Paolo a few years back; street artist Banksyhas also used the technique. But Moose is a pioneer — and a bit of a character to boot.On that day in South London, when Moose asked the officers what crime he was being charged with, confusion ensued.

“We’re arresting you for criminal damage,” Moose recalls one of the officers telling him.

“The only thing I’ve damaged is dirt,” the artist replied.

“They took that on board,” Moose says. “They were really fumbling around.”

Then the police told him that he had made marks on the wall. “I explained that the marks were made by pollution. If they wanted to arrest the perpetrators of this crime, they should get with the people who had created this pollution.”

To read more and see examples of his art, click on the Grist article here.