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People have been making molds out of silicone for a long time. It is strong and flexible.

http://www.instructables.com/id/Two-Part-Silicone-Casting/
http://www.alumilite.com/HowTos/SplitGloveMold.cfm

But silicone mold-making material is expensive. I've paid $25 for a single pound of it: http://www.tapplastics.com/shop/product.php?pid=61&

Scott wanted a lightweight copy of his tauntaun head, specifically out of expanding foam. He needed a silicone rubber mold, but he didn't want to sell a kidney to be able to afford the materials.

The clay sculpture.

Instead of going to a hobby store, he went to the hardware store, and bought a box and a half of silicone caulk.

After a small-scale test, he tried the real thing, covering every inch of his model with a quarter-inch of clear silicone caulk.

Two hours tack-dry, 24 hours to cure.

The silicone dried into a floppy, stretchy bag, ready to be filled with some casting material. However, it needed support, because expanding foam will push the walls out of whatever you are holding it in.

So Scott kept the silicone shell on top of his clay sculpture and covered the outside with plastic film.

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Then he added a layer of expanding foam and turned it upside-down into a medium wooden crate.

This would be the "mother mold".

He added wooden supports inside and added can foam so that the silicone mold wouldn't distort or collapse as it was filled.

The plastic film kept the silicone from becoming stuck inside the box.

With the mother mold dry and ready, Scott finally removed his original clay model from the silicone.

Contrary to this illustration, he lifted the silicone off the back of the head first, to keep from messing up the details on the face.

Note that his sculpture has very limited undercutting, so he wouldn't have to cut the mold to remove it.

With the silicone bag removed, he cleaned out any remaining clay and plugged up holes with more silicone.

Then he placed it into the mother mold box and applied a mold-release agent.

Next he mixed and poured in some two-part expanding foam. For big projects like this, it is cheaper to buy two-part liquid kits instead of using a bunch of household "Good Stuff" insulating foam cans.

Scott used three smaller pours and waited for each to foam and expand before mixing and adding more.

In about an hour, the foam is rock solid and ready to be removed from the silicone mold.

The new foam head looked great, and the mold is ready for re-use.

This amount of expanding foam probably cost about $40, but I'll bet it would work for molding soap, jello, ice, resin and cement. I love this technique because it is so much more manageable than using a liquid mold-making material. It eliminates the need to use a ton of silicone, and there is no mixing!

It does present its own set of problems. Seams between beads of caulk can show up as lines or bubbles on the finished project and tiny details may be lost.

But wow, what a result!