Sean Michael Ragan of Make: Online says:
Simple, elegant garage casting hack here from Instructables user mikey77, who calls the stuff "Oogoo," which is, I think, a portmanteau of "Oobleck" and "Sugru."How-To: Make cheap castable silicone from caulk
If you've ever bought casting silicone, you know it can be pretty expensive. If, seeking a cost-saving alternative, you've ever experimented with making castings using the dirt-cheap silicone caulk they sell at the hardware store, you know that it doesn't work very well. Large volumes set up very slowly, if at all.
Turns out, though, that mixing in some cornstarch accelerates the drying process; just how much depends on how much starch you add. The author recommends starting with a 1:1 mix. I'm not sure about his explanation that the process works because the hygroscopic starch carries moisture into the internal volume of the silicone, but in any case there are probably other additives that will accelerate the process as well as or better than cornstarch. You might even find one that's not opaque and allows for translucent castings.
Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. His new book is Maker Dad: Lunch Box Guitars, Antigravity Jars, and 22 Other Incredibly Cool Father-Daughter DIY Projects