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.”
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.
Also, on a topical note, a commenter on our recent post about choosing a casting silicone claims that hardware store silicone caulk can be thinned to a pourable consistency using hardware store xylenes. (Thanks for that, SKR!) Perhaps the two additives could be combined to make a fast-setting homebrew silicone that’s also of pourable consistency.