You could also do a paper mache over them too and skip the clay and baking. Get tan or white and tea stain them. I think that could make them look really old and it may be less expensive than clay. When I looked at the picture I thought it would be neat to have a window left open so you can see inside the jar. You could get a pulsing heart or slimy liver to view inside. Maybe fill it with water and put a glow stick inside to illuminate what is in it.