Pewter, no - tin , probably
Robert, why wouldn't pewter work? I thought most modern pewter's were
primarily tin. As I understand it , many jewelers use tin or pewter to
build masters for rubbermolding. It is soft, forges, and does not work
harden. However, I would not do delicate work in tin and then try and heat
and pressure vulcanize, unless it was flat.
One thing to note if using tin or its alloys to make masters for
rubbermolding. Make sure you clean your files carefully or use separate
files. If tin contaminates the surface of sterling (not sure about gold)
and then is soldered, you've got a real mess on your hands.