Surface dimples in Pewter casting

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?

Brian Shafer

This sounds as if the flexible mould is responding to the castings
shrinkage. The molten metal reduces in volume as it cools. In a non
flexible mould it would pull more metal in from the sprue - in this
case the contraction is pulling in the walls of the mold.

The silicon mold you’re using may become more rigid after the six
days and prevent this to some extent.

Tony Konrath
Key West Florida 33040