I've posted a working paper on graffiti at my SSRN page:
Animal, Vegetable, or Mineral: What is Graffiti?
Abstract: Graffiti exists in a liminal zone undefined and unwonted by any particular cultural institution. Born on the streets, it exists on walls where it is subject to the weather and to the needs of and desires of the graffiti writers who explore those walls. It is the graffiti site, rather than the individual piece, that is central expressive locus of graffiti culture. Because the site is ever changing, subject to a dynamic controlled by multiple parties (graffiti writers, passers-by, municipal authorities, and the weather) we can think of the site as “home” to a tutelary “spirit,” known in Japan as a kami.
The paper is reworked from posts I'd collected back in December 2011 for a presentation at the University of Chicago.