I put about and inch of common red clay in my smaller forges. I usually put it in place in two layers, allowing it to dry very slowly. If cracks appear, they are filled with a thin slurry of clay. I have found that if the cast is pretty thick, it probably isn't necessary to add the clay and use the forge as is. None of my forges, old or new, will not burn out in my lifetime. Cheers, Bookie