While most art and design techniques are additive in nature, some are fundamentally reductionist. Take stone and wood sculpture, which involve removing material to reveal the form within. Or creating an etching, which requires scratching away the sticky "ground" from a copper plate. But dirt?

Humans create a lot of degraded, dirty surfaces, especially in urban environments. In recent years these have served as spaces on which street artists have unleashed a torrent of paint, sometimes to the improvement of the surface (as shown recently in Graffiti Japan). But more often the paint becomes just another ugly layer.

However, in a classic case of glass half-full versus half-empty, a new wave of artists is creating work that is based on the relationship of the degradation of the surface to the material beneath. They create their images simply by selectively cleaning parts of a wall, using simple hand tools or via power washers and stencils, in essence performing reverse graffiti. One of the most well-known practioners of this new medium is Paul "Moose" Curtis, who in the clip above describes the creation of a long mural in a San Francisco tunnel. Interesting stuff, with more examples available on the Environmental Graffiti site.