Silicone “Room Temperature Vulcanizing” rubber. Silastic is one
brand of these silicone based mold rubbers. You buy them as thick tar
like liquids with a thinner liquid curing agent. Usually mix 10 parts
rubber to 1 part curing agent. Some allow you to just pour, others
wish to be vacummed first to remove air bubbles. Anyway, let them
sit, usually for 24 hours or so, and you have a rubber mold without
the heat of vulcanizing. Several other advantages over vulcanized
rubbers too, except that they are not as tear resistant, and don’t
last as long, and cost more. The main advantages, besides no heat,
is that they are usually also little or no shrinkage too. And waxes
from these absolutely don’t stick to the mold, and can have very
shiney surfaces if the model did. No grain from the rubber giving
texture to the wax, and no powder needed either.
Silastic, made by dow corning, comes in various grades. I like the
"E" grade, which is white, and cures nicely with no bubbles after you
vacuum first the rubber then the poured mold.
Hope this helps.