It's faily plain. About six days ago I got the materials I needed to make a forge for my hobby knife making (a body, refractory cement, and some ceramic insulating foam.) Tonight I fired it up in earnest for the first time, and tried to treat one of my knife blanks I've got (I've got one more I can fire. I ground them from O-1 tool steel using a belt grinder long before I even had a forge to finish them with.)
It's uh... made from the corner piece of some ventilation duct, and ceramic fiber insulation coated with some refractory cement.