Roy, Centaur Forge sells a light weight fire-brick for their forges. I have their knifemakers forge. It has a castable refractory rounded bottom and the light weight bricks on the sides. They cut easily with a hack saw. They can withstand who knows what high temps. Ive had mine to 2400F they still look new after 3 years. They melt with flux so a castable floor is a good idea.
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ABS Journeyman Smith, Bruce D Bump