 Returning to the tunnels, he drew the skulls and took the dust or soot off the walls using rags.Then, he washed the rags and waited for the soot to settle and the water to evaporate until just the black powder from exhaust tubes was left.This apparently useless toxic substance was made into pigment and used for the works in the Art Less Pollution series of large-scale paintings, their impressive realism evoking old advertising images with a touch of irony.