I am trying to cast bismuth, which melts at 271.4 °C. Silicone rubber seemed like a good material to use for a mold, given its flexibility and the ease with which it can be cast itself, as well as its high (~300 °C from some sources) heat tolerance.
I have had the opportunity to test casting bismuth in this way once so far (will have the opportunity to try again this weekend) using a mixture of acetoxy silicone and cornstarch as the mold, for ease of molding and quicker curing.
While the mold itself seemed to stand up well to the heating, aside from discoloration, there was a large amount of bubbling as the bismuth cooled, resulting in deformation of the final structure.
My best guess as to the cause of the bubbling is that the silicone had not finished curing, and trapped, evaporating water and acetic acid were the source of the bubbles. Is this likely to be correct, and if so, is there a way to avoid this in future casts, such as possibly by letting the silicone cure longer? Alternately, are there different formulations/materials that may perform better, without being significantly harder to make?