What is a suitable material for making a mold with?

plaster.

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Answer #2

Plaster, Clay, Dirt(packed around the material and then compressed,) solidified carbon, stone, and/or any base metal that can be cut or shaped.

Anything requiring a highly heated material to placed within should be solid and heat resistant. Most of these would either be a metal or stone as they withstand higher temperatures than plaster, clay, and carbon.