So! Thanks to the wonderful Dzi I acquired some mold making and casting supplies, just in time for RF!
This is so exciting for me, I've wanted to do casting for ages!
The material is called "Smooth-cast 300" and it is a durable white plastic. I have successfully dropped one of the flawed first castings as hard as I could on the floor and it didn't break (not that I'd recommend it!) which makes it far, far more durable than Sculpey can hope to be!