When it comes to casting lead, only one kind of silicone may be used for your mould. This is a particular kind of heat-tolerant silicone that is used with metals that have relatively low melting points, such as tin and lead. Although the process for making it is very similar to that of ordinary mould making silicone, you should never confuse the two. Finding the proper kind of silicone can be a pain if you're looking in local stores, but you can easily find it online. Many companies advertise their silicone as being temperature tolerant, but check the temperature range of their product. It may only go up to 204 degrees C (400 degrees F) or so while lead has a melting point of 328 degrees C (621.5 degrees F).
Lay three layers of newspaper over your work area and don a pair of gloves and a respirator. Create a flat bed of plasticine 10 cm (4 inches) longer and wider than the dimensions of your prototype. Lay your prototype on top of the bed, with detailed side facing up.
Build a wall of plasticine around the bed that is 0.6 cm (1/4 inch) thick and 2.5 cm (1 inch) higher than the highest point on your prototype. Check the base to make sure there are no holes or unfilled seams. The wall needs to join seamlessly with the bed.
Coat the face of your prototype with mould release. Mould release can be one of several substances, depending on what your prototype is made from. Use liquid soap if the prototype is made of cement, porous stone, plaster, or clay. Use liquid wax if it is made of wood or glass. Or, for the best result, use a mould release made by the same company that made your silicone. Metal, smooth stone, resin, and terracotta do not need mould release.
Set a pair of mixing cups next to the pair of bottles that came with your silicone kit and label the cups A and B with a marker. Silicone mould kits come with two bottles of chemicals. When mixed together, these chemicals react to each other and begin to harden into silicone. Pour portions of one bottle into cup A, and part of the other bottle into part B. The ratio is usually 1 to 1, but some silicones have a ratio of 10 to 1. Check the instructions that come with your silicone to find out which yours is.
Pour the contents of both cups together into a third cup. Mix them together with a mixing rod for one full minute, or until the mixture becomes an even colour.
Pour the mixture over your prototype until the wall is nearly full. Make sure there is at least one inch of silicone above the highest point of your prototype. Allow the silicone to harden for 24 hours.
Peel the plasticine off of the back of your prototype and pop the prototype out of your mould. Rinse the mould in a sink to wash away any remaining mould release.
Uncured silicone is toxic to humans. Do not expose your skin to it and do not inhale the fumes. Keep it away from children and pets and work in a ventilated area. Do not attempt lead casting if you are inexperienced. Lead is toxic, and hot metal is dangerous to work with. If you are new, work with it only under the close supervision of someone who is skilled in casting.