British artist, Paul "Moose" Curtis, pioneered the new form of artwork that challenges the view of graffiti as vandalism.

Dubbed ‘reverse graffiti’, Curtis’ works involve power-washing the grime off city walls using beautiful stencils.

Standing in one of the moss and dirt covered tunnels in Leeds, Curtis saw marks on the wall where the shoulders of unsteady drunks and the fingertips of curious children had exposed the shiny white tiles. With only a pair of socks as his tool, his art style of reverse graffiti was born.

Across the water, Brazilian artist Alexandre Orion’s clean tagging in Sao Paulo’s sooty tunnels (pictured) prompted a clean up job by the municipal council.

No paint. No defacing. These artworks are created by cleaning and removing the ills of dirt and pollution by de-defacing - wiping away dirt into large images, leaving nothing behind but a beautiful work of "green art”.