I want to build a cross between a furnace and a fire pit, but

>>1105266
Just dig a hole, man.

Stick with the tried and true ceramic route. Adding the iron sand will make it conduct more heat outside of the unit, where you don't want it. If you're doing air exchange through it, consider adding a radiator and a pump. By having the radiator above the fire pit soak up heat, and another in your central air unit you could circulate heat to the rest of the house, relatively well. If that's your end game.

>>1105285
There's a lot of variables to ceramics, which is the reason for this thread, not because I'm wondering what form the furnace should take.

bricks?

>>1105304
Can't be sculpted, and they're not pretty if not used in conjunction with sculptable material.

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>>1105267
quality post