This is the foundry after the first rebuild as it would appear immediately before lighting.
As you can see I removed the firebrick lining and replaced it with a ~2″ thick refractory lining of my own concoction. The refractory I used is simply drywall joint compound (gypsum) and perlite. To mix I simply added the drywall compound to the perlite until there was just enough to coat all the perlite and keep it sticking together. Then simply tamped it down between the inside wall of the foundry (cut open propane tank) and a cardboard form. Since the perlite made up most of the volume of the lining it provided excellent insulation, melt times were cut in half when using propane alone even though the volume of the area around the crucible was much greater than with the firebricks.
As an aside, when the lining was first put in and dried it was an off-white color. Then, on the first burn it turned a brown-black color for a few minutes before turning permanently and completely white as seen in the above picture. (Due to the color and texture, one person actually mistook the lining for regular Styrofoam, until he saw it withstand the full blast flame from the burner.)
While the foundry was in this configuration I used both propane and charcoal at the same time to speed up the melts. This work very well since the propane burner used a regular gas grill regulator which caused the flame to be lean enough to provide plenty of oxygen for the charcoal.
This lining lasted for about 30 foundry hours (that’s 30 hours at +1300 degrees F) before it started to crumble around the area where the flame hit it directly and where the charcoal would rub against it when the crucible was moved.