Back from the days of doing casting I used a silica called cristobalite for lost wax casting. You mix it as a thick slurry then pour into your cast mould and flask, vibrate to expel any air bubbles then fire in the kiln with slow increases in temp otherwise your mould will blow up from steam explosion. I have thought of experimenting with this material, graphite moulds are costly and an ongoing expense, and I can never find the correct size for my bars so you can custom manufacture according to your needs, but you must have a kiln to fire it, pour molten silver into a wet mould and your health and wellbeing will deeply regret it. You can buy cristobalite in large boxes, I use kerrcast for jewellery. Fair bit of stuffing around for a mould so I can understand the convenience of graphite. When used in lost wax casting, mine operates under pressure forcing the metal into the blank left from the wax, you leave it for a few minutes then hold the flask in water with tongs and the cristobalite boils out leaving the metal blank behind. You may be able to just fire the blank mould and re use it for bars but I don't know about the lifespan of the mould when used in that way, I know that heating metal directly in the cristobalite has immediate effects on its condition, going to have to try it out, graphite molds are getting really exxy from some USA ebay sellers and are beyond what I am willing to pay, the Chinese ones do the job but the sizes are all over the place.
