Ever tried Magic Metallics? My fifth graders just finished some leaf
sculptures and used this to finish them. They consist of actual metal
particles suspended in an acrylic. You then add the patinas on top and they
oxidize. You have the natural element you desire, but the "Ooh" factor of the
oxidation process for the kids.
The official website seems to be down, but here is the link. Maybe you can get
it to work: