Added a quarter-inch layer of bubble alumina (from Ellis Custom Knifeworks) to the bottom of the forge last night. Weird stuff -- it's a mix of some sort of powder and, well, small bubbles. The powder is pretty finely milled, about like plaster, and the bubbles are round spheres ranging in size from about half a millimeter to three millimeters in diameter. I mixed it with water to form a thick paste, then spread in in the bottom and up the walls a bit. A bit that spilled onto my work surface dried fairly fast, but didn't seem to have much integrity, which is a bit unusual for a refractory. I wonder if this stuff will hold up to the normal wear and tear a forge sees.
The stuff is supposed to be very flux resistant; we'll see. I also noticed the satanite/ITC coating I put on initially has a few cracks in it. Considering that I've been fiddling with high-powered burners, this isn't terribly surprising, so I'm going to do some patching over the cracks tonight with furnace cement, just to make sure everything holds together.