Something else from a long while back. . . Fine garden soil placed on top of a photopolymer plate before exposure makes the best cloud patterns imaginable. Don't use the vacuum and turn those fans off as you don't want that particulate matter getting into the machine. This might only work on steel-backed plates, since they don't flex.
Photopolymer plate phenomena
An interesting occurrence the other day. . . We were marking up some paper for cutting down with pencil notations (true cuts, gripper edge, etc.) and when we went to print prep we started with the top sheet cuts (with the markings) and noticed the pencil marks transferred to the photopolymer plate. When we applied a light, somewhat transparent ink, the pencil marks printed! and again, and again. Seems like there is some potential there for interesting work on solids.