We make plaster molds for waxes so that we can make multiple matching molds. It's a production technique. The first mold is plaster, not plaster-silica investment, so it won't withstand firing. However, a person could probably make the first mold out of investment.
I think that venting the kiln is actually letting "chemical water" escape from the plaster, and possibly any traces of wax that remain.
I have a haze recollection from the past that a teacher recommended covering an object with green soap, or cooking oil spray to keep plaster or plaster-silica from sticking. Others may have done something like this more recently than I have, and I would love to have this confirmed or refuted by an experienced kilncaster.
Mary Kay Nitchie
Bullseye Glass Co.
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