Looks like I'm going to have to try sand casting these. The plaster outgasses something terrible when the hot bismuth is added disturbing crystal formation. My first couple of attempts all ended with a very thick rough inner surface and maybe the edge of a crystal.
I have also spent a whopping 4.50 on a box of borax to use for flux. I think this might cut down on the grey scum oxide layers formation.
Olivine sand and bentonite clay....
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