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I've recently acquired a large bull skull, rather weather-worn and aged. I would like to make a mold of it so that I can cast it in wax, alter it a bit (add more horns, etc.), and then make a mold of the wax so that I can cast it in something to make a creature head for a project. I'm not familiar with making molds beyond very small jewelry sized things with the silicone putty stuff. My worry is the pits and holes in the skull and getting everything to release. I don't want to ruin the original.

What is the proper material to create a mold from for this particular situation?

I don't think a plaster mold, which I've done for clay objects, will work for this as I think pulling the mold off will damage the original.
Any advice on what to cast the final mask/head in so that it isn't too heavy would be helpful as well.

Including image for reference:

enter image description here

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How about making the final project mask directly from papier-mache without all the casting:

  1. Cover the skull in clingfilm to protect it;
  2. Papier-mache parts of it (if you try to do it in one go you won't be able to take the thing out, naturally);
  3. Glue the parts together;
  4. Alter it as you desire - cut some bits here, add some bits there, sand it;
  5. Paint it, if needed

Paper-mache masks are light, resilient (as long as they are not too thin) and you can waterproof them with a spray if you wish.

1

I have molded a lot of bone and silicon and one are a messy combination, you have to completely seal the bone before you use silicon. multiple soakings in butvar or acrylic works OK. it is a nightmare with liquid silicon but with putty based it is not so bad, you just need to watch out for voids in the silicon. making a plaster over-mold will be a good idea as large thin silicon molds do not hold their shape well.

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