F. George McDuffee wrote:
One of the slickest things I have see for low volume use is lost
foam with "plaster" investment. Make the model out of styrafoam,
cost with plaster, possibly support the plaster mold in a flask
with sand, and pour. ...

You don't even need to coat with plaster. Make a model in foam, cover
with sand & pour. There is a particular foam to use & I don't think
it's Styrofoam - one of the insulation foams. One would think that the
foam would melt & the sand collapse before the metal could fill, but it
doesn't. IIRC, you don't get the same detail in the casting as you do
with green sand.

Bob