I don't recall the exact proportions my first attempt at building a smelting furnace from a 30 Lbs refrigerant tank I mixed Gypsum, perlite, plaster of Paris, Vermiculite. The gypsum (calcium sulfate dihydrate) did not hold up well at all. 2nd try I omitted the gypsum added a small amount of Portland cement, this portion has held up much better, to minimize the spalding in the lower Gypsum section I have applied Sodium Silicate gave it a lite firing and coated the interior with Rutlands fire place patch. this furnace is fired with a simple propane burner. 1-1/4" pipe and a 1/4" copper tube with a 1/32 hole drilled in the side aimed straight down the pipe. I have milted down Aluminum, brass and copper with ease.