`` if michelangelo had broken into the sistine chapel and sprayed the ceiling , that 's graffiti , that 's not art , '' said gerald a. silver , president of homeowners of encino. ; graffiti has developed from the turf marking and public threats of the street gangs , to the tagging of rebellious youth , to `` piecing , '' in which crews of spray -can painters `` throw up '' their masterpieces on the wall .