Things You'll Need
Object to cast
Sulfur free clay
Mold release spray
Pewter is cast in RTV molds. RTV stands for Room Temperature Vulcanizing, meaning that the rubber of the mold becomes hard at room temperature. RTV silicone is inexpensive and can produce very detailed casts. The two types of RTV are condensation cure and addition cure. Condensation curing RTV is more commonly used by hobbyists and hardens by the alcohol in the rubber evaporating. This leaves a pliable mold that can be peeled off the pewter to make multiple copies of coins, jewelry or sculptures.
Draw your parting line around the object. You will need to decide where your mold splits so it can be removed from the cast. Use a pencil to draw a continuous line around your object.
Place your object in the mold box. Choose a cardboard or wooden box that is only slightly larger and taller than your object. Put the object in the center of the box so that the parting line is parallel to the bottom of the box.
Build clay around the object. Use small pieces of sulphur free clay to fill the lower part of the mold box. Any sulphur will keep the rubber from hardening. Build up to the parting line and try to make it as smooth as possible so the line is not visible on the cast. Use your finger to poke two or three holes in the clay around the object, but don't touch the object. These will serve as keys to help fit the mold together.
Spray the object, clay and mold box with mold release spray. Mix and pour the RTV rubber into the mold box so that it completely covers the object. Allow the rubber to cure.
Remove the clay, object and rubber mold from the box as one unit. Carefully peel off the clay from the object being careful not to disturb the rubber mold. Spray the inside of the mold box with mold release spray and place the object and mold in the box with the rubber now on the bottom. Spray the top of the object and mold with mold release spray and pour the rest of the RTV rubber into the mold box. Allow the rubber to cure for at least 24 hours and remove the object and box.
Cast the pewter. Lightly brush the inside of the mold with baby powder or powdered graphite. This will allow air bubbles, which are common in pewter, to escape from the mold. Pour the liquid pewter into the mold and let is set for at least 24 hours. Remove the pewter cast from the mold.