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I found something hilarious, go to homedepot.com and type plaster of paris into the search bar, the six related search items seem pretty unrelated to each other, and to plaster of paris, unless, however, you plan to build this furnace.
Eventually, I think the crucible will fail so be ready for it.
For air flow, more air=more oxygen=faster combustion=hotter furnace= melted aluminum=fun
I think he may use stainless but I'm not sure, it would be safer. Be caustious of metal fever.
Just some info I think might help. I am not for sure, but the bucket may be listed as a 2.5qt bucket, but the measuring line may not be flush with the top, and the bucket may actually hold a total volume of 3 quarts.. Also, according to the math worked out by 1&3/4 sand and plaster and 1&1/4 water, will only work out to 21+21+15 cups if it was a 3 quart bucket(3 quarts x 4 cups/quart x 7/4 times=21). If the physical bucket was only 2.5 quarts, then the correct amounts would be 17.5cups sand(given by 2.5quarts x 4cups/quart x 7/4 times = 17.5), 17.5cups plaster and 12.5 cups of water giving an outright amount of 47.5 cups, about a 2.5 quarts less than what you are currently measuring for mix.