Hello All, I am attempting to cast pewter for the first time, pouring
directly into rtv silicone rubber molds. The surface of the casting
( a pendant) is very rough. It is covered with “dimples”. It looks
like somebody took a tiny spoon and took out little scoops all over
the surface. Does anybody know what might cause this? FYI the
compound I’m using says the rubber will not attain its optimal
physical properties for 6 days after vulcanization and its only been
3. Could this be part of the problem?
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.
Could be some of the mix materials fuming out when the heat of the
molten metal is applied. Can help if you pre heat the mold in a 200
deg F oven for an hour or 3 or just wait the 6 days for the
cure…