Erin Mulvehill
So I was flicking through http://www.nylonmag.com/ and came across their blog featuring Ellis G fire tagging. He is most known for outlining shadows with chalk in the streets of NYC. 
group, my fellow knitter Janine and I were strolling down the streets of the Northern Quarter which features some street art (but most of it actually on NQ cafe/bar walls) and came to talk about reverse graffiti. It is created by *removing* dirt from a surface and therefore not only actually defacing objects and legal! Big corporations have already used this method for advertising, cleverclever, there is even a company specialized in exactly this http://www.dirtystreetadvertising.com/. 