When casting pewter into rubber or silicone molds the mold needs to
be about 200 degrees F or so. You can preheat the mold in an oven or
vulcanizer or let the mold get to temperature by pouring a few runs.
Bear in mind, that you do not want the mold too hot. Also it is
advantageous to use talc as a mold release and to aid in ventilating.
The mold also needs vents cut into the rubber. You also may need a
reservoir in the mold to exert pressure to fill the mold. I have not
cast the way you are into small gravity molds but use round
centrifugal molds which are fed through a hole in the center of the
mold while it spins. The basic principals of casting apply to almost
every form of casting. Also it is difficult to cast pewter thinner
than about 60 thousandths of an inch.