Pewter is cast in RTV moulds. RTV stands for "room temperature vulcanizing," meaning that the rubber of the mould 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 mould that can be peeled off the pewter to make multiple copies of coins, jewellery or sculptures.
Draw your parting line around the object. You will need to decide where your mould 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 mould box. Choose a cardboard or wooden box that is only slightly larger and taller than your object. Put the object in the centre 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 mould 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 mould together.
Spray the object, clay and mould box with mould release spray. Mix and pour the RTV rubber into the mould box so that it completely covers the object. Allow the rubber to cure.
Remove the clay, object and rubber mould from the box as one unit. Carefully peel off the clay from the object being careful not to disturb the rubber mould. Spray the inside of the mould box with mould release spray and place the object and mould in the box with the rubber now on the bottom. Spray the top of the object and mould with mould release spray and pour the rest of the RTV rubber into the mould 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 mould with baby powder or powdered graphite. This will allow air bubbles, which are common in pewter, to escape from the mould. Pour the liquid pewter into the mould and let is set for at least 24 hours. Remove the pewter cast from the mould.