high temperature molding compounds & bulking up investment

[missybee]

Hey thanks for the tips on ceramic shell casting! Glad I came back to this forum before I went and wasted a bunch of time and material! You confirmed my fears of brittleness and porosity using investment for a shell. Have you ever mixed anything with jewelers investment to bulk it up bit for flask casting? Its quite expensive and I am rapidly burning through it.
And while I'm here - have you ever used or seen any high temperature vulcanized rubber mold compound that could withstand melted bronze? Ive used high temperature mold compounds for pewter casting - but that's a huge difference in temperature. If I could mold the trees of sprued models I am casting with a re-usable rubber mold compound housed in a plaster mother-mold - that would make me one happy girl right now! I think I may be asking too much but hey - I'm going to throw it out there anyway!
Cheers!

[Wikus]

Hi there
Fulcanised rubber will not withstand the high temp. of molten bronze. The highest temp it can take is about 450 deg C and molten bronze is over 1000 deg C when liquid enough for casting. Moulding materials for bronze are silica sand, zircon sand, plaster, graphite and ceramic investment matt. Iron or steel dies also work well for repetition work of the same object.