Reverse Graffiti by Alexandre Orion
Saw the tail-end of a documentary on television this Labor Day (while I was still on my laptop) and they had on a street artist - found out to be Alexandre Orion (from David Ra on my Facebook) and his usage of "pollution" on the streets of Sau Paulo, for his graffiti-work, known as "Reverse Graffiti", whereby his 'tool', is a wet cloth.
What he does is uses the cloth, to 'draw' on dust-coated surfaces of the polluted street structures, to write his 'skulls'. And all the law enforcement could do, was wash down the surfaces! The video is dated 2007, so I am unusually slow on the uptake here, but hell, this is truly awesomesauce.
Shown on NatGeo - for the series "Don't Tell My Mother", Alexandre even revealed his true face (but used a nick instead). Check out his official website @ alexandreorion.com for more of his work!