Well I'd been thinking about using plaster for quite some time Andrew, after some references to sprue pouring liners (for insulation), and a reference to using gypsum blocks in a bronze furnace, and plaster in statuary practice. The sand part was in a video posted on the forum, and I used a sand and clay liner before, so it seemed reasonable -- I just wasn't sure how well it would hold up and if calcining would be a problem. It's working so far. And the insulation value seems very good. 2" is fairly thin, but the outside of the furnace doesn't get very warm after an aluminum pour.
Another point of interest -- melting crucible of aluminum in the furnace took 20 minutes with propane and 15 minutes with charcoal briquettes. Propane is neater, and faster to light, but more expensive and slower to melt.
Charcoal is easier to do other things around -- I don't like straying from the furnace when using propane in case of a flame out.
While charcoal is starting (no blast) i can ram up a mold, and have a hot furnace ready to melt metal when the mold is set down. In fact, I've had aluminum in the crucible begining to melt a little if I took longer than usual making up a mould, even without a blast in my old furnace. That was quite a surprise the first time it happened! And a very fast melt afterwards. My usual procedure building the first lathe was, a little lighter fluid in a furnace 1/4 full of charcoal, light off, and when the embers had just started, add the crucible with initial charge of alumnum. Cover the furnace, and go make up the mould(s). When finished, things were already hot, and the blast made short work of the melt.
I can see using both fuels on different occasions for different purposes. It would be interesting to see how well charcoal melts brass. Might try that soon.