When creating a homemade refractory mix, the common suggestion is to add sawdust or foam beads to the clay and other materials. The result is a lighter, less dense refractory. As the refractory is heated up, the organic material burns away, leaving voids in the heat-resistant material. These voids transfer heat much less efficiently, increasing the insulative value of the refractory. The net result is that more of the heat stays in the crucible chamber and less leaks to the outside. For a foundry or forge, that’s a wonderful thing… we’re not buying propane to cause global warming!
The more heat that stays in the chamber, the faster the piece gets up to melting and forging temps which results in much more efficient operation. Theoretically, with perfect insulation a candle could melt iron (eventually), but there is no perfect insulator. As we add heat from the burner, heat is being lost through the shell as well as escaping out the exhaust, which is a shame, but a necessary evil unfortunately. Still, we want an insulator as good as we can get so that we can add heat significantly faster than it leaves, so that the temperature can keep increasing. As the temperature increases, so does the rate of heat loss, so ultimately good insulation is the key to high temperatures.
To this end, most hobbyists skip the refractory entirely and go strictly with a ceramic wool material that has VERY high insulation value and brings things up to heat quickly. The downside of this is mainly durability. Physically they can’t take much abuse, and welding fluxes and glass will destroy it quickly. That doesn’t make them bad, it just means that you need to be careful. You’d be well-advised to coat the material with Satanite and ITC-100 to give it some substance and resistance to flux. Putting a dense board on the bottom isn’t a bad idea either.