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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.

Please continue reading page 2.