A cold cure molding polymer compound which can withstand the heat of molten lead. It is mixed from a rubber based solution and a catalyst. Silicone rubber is used for small scale casting. As a gel, silicone is available in tubes from any hardware store. The transparent kind is preferable because it permits the sculptor to see air bubbles. A technique some employ: first extrude it under water, then apply it to the model to make a flexible mold. If a thick mass is needed, it is best applied in coats. While silicone's surface can cure rapidly, if too thick a coat is made at once, the interior will not cure in a acceptable time. Many such products are hazardous. Read labels well and follow all cautionary advice.Also see release agent.