Also in the show are the American artist Jim Dingilian’s smoke landscapes in a bottle, which have the elusive look of memories, and the Brazilian artist Alexandre Orion’s video of his "reverse" graffiti, carved into the soot-saturated walls of São Paolo traffic tunnels.

Pollution residue on a stop sign. Algae at the bottom of a public fountain. For most people, such accumulated grit is a dirty, if often overlooked, fact of modern life. But for "reverse graffiti" artists, that dirt is a beckoning canvas.

Moose at work on his grime-removing graffiti and, above left, his Big Brother' logo for Channel 4 Symbollix reverse graffiti.