Commercial moulds for casting lead are typically made of aluminium or vulcanised rubber. Both of these materials require expensive equipment to work with. But by using high-strength room-temperature vulcanising (RTV) silicone, you can make moulds that will hold up to the temperatures of low-melting-point metals such a lead and pewter. High-strength RTV silicone cures to a stiffer consistency than other silicone products, but it's still flexible enough to allow for easy release of the cast lead object.
Coat the inside of the box and the model with a thin layer of petroleum jelly. Place the model face up in the centre of the box.
Mix enough of the two-part RTV silicone to just coat the model. Add base to a disposable mixing cup and stir thoroughly with a disposable stick. Add enough catalyst to allow for a 1-hour cure time, as per manufacturer's directions.
Paint a coat of the silicone compound onto the surface of the model. Work silicone into any fine details in the model. Work gently to avoid raising any bubbles in the compound. Allow this coat to completely cure.
Mix a larger amount of the two-part silicone in a disposable bucket; make enough to fill the box. Pour the compound over the model to the top of the box. Pour slowly to avoid introducing bubbles into the compound. Allow to cure for at least one hour.
Invert the box so the cured silicone drops out. Remove the model from the top of the mould. Allow the mould to age for seven days in order to allow it to reach its full heat resistance.
This technique will work for models with one flat side or 3D models that are wider at the bottom than at the top. Highly detailed or irregular 3D models should be cast with a two-part mould. The one-hour cure time is necessary to allow bubbles to rise out of the compound before it sets. Faster-curing formulations usually have to be vacuum-cured in order to quickly remove bubbles.
RTV silicone products used for lead casting must be able to withstand temperatures in excess of 163 degrees Celsius. Check the specifications for the mould-making compound to make sure it will cure to the proper temperature-resistance. RTV silicone mould-making compounds emit harmful gasses as they cure; make sure to use these products in areas with good ventilation.