How do you remove clay correctly from a mold?

After I make a clay sculpture, how can I create a mold of it and what material should I cast it in?

• I am creating medallions, pendants, and larger-sized metal plates for a costume (or Cosplay, for those of you who recognize the term - some don't). I plan to first sculpt the medallions out of clay, then create a mold of it (so that if it breaks, or I want to make more, I have a mold I can use, instead of having to make a new one from scratch). My question is this: how would I create a mold of a clay sculpt, and once I have made the mold, what material would you recommend to pour into the mold to create the finished piece? It needs to be fairly rigid and strong when set, because a few of these medallions actually have buckles in them that hold parts of the costume together. Worst case scenario, I could probably suspend a piece of wood or something in the material before it sets to give it rigidity, but I'd much prefer that the material itself have the necessary strength. I would have Google searched this, but it's a fairly generic, general question - not enough keywords to generate a good search! :)

• Answer:

  I use modeling clay (plasticine) for the original and latex backed by plaster for the mold, although the latex can be used to mold from almost anything http://www.mikegigi.com/modeling.htm cheap but a bit time consuming due to building layers. The choices of material for what you want to do are epoxy resin or soft polymer clay and bake that.