White glassmaker’s sand mixed with ammonia salt
You can find this sand in Cominge, near the town Aurignac. This sand is dry and as white as salt. Glassmakers and potters use this sand ground on the porphyry until it becomes as fine as possible. Easy to grind, this sand looks like calcined alabaster, and provides a very clean mold. I have not found another as suitable for delicate bas reliefs. It is also excellent to mold as a core, without any box mold. You must grind it very finely on a porphyry with gummed water or pure water, then apply a thick coat (as thick as mustard or a little bit thicker) over the medal, which has already been lightly rubbed with oil, walnut oil or, even better, aspic oil. For best results, let it dry by itself, away from fire, for one or two days. If you are in a hurry, you can heat it; it should not break if the coat is quite thick. If it is suddenly exposed to fire, some holes will appear; that does not happen if it has been gently dried. Once dry, reheat it, and it should allow for several castings.