I'm getting ready to build my first rocket mass heater. I have access to free quartz stone and granite. Could I use this type of material to build my "J Tube" instead of fire brick? Not really sure if the granite would work very well but the quartz stone seems to be very tough and could absorb the heat. Or should I just build it out of fire brick to be safe?
I don't have a reference to quote, but I took a Rocket Mass Heater workshop recently.
Short version: Use fire bricks if possible because you want the J Tube to not absorb heat, rather to reflect it back at the wood.
Long version: The workshop instructor emphasized that (in the case of a Mass Heater, at least) you want the J Tube to be as insulated as possible. The idea is to make the fire as hot as possible in the fire box (J Tube), ideally by reflecting the fire's heat back at itself. The hotter the wood in the fire box, the more efficiently it burns.
In fact, he had several bricks made from a material similar to the Space Shuttle tiles, kind of like ceramic foam. We built two demonstration J Tubes with nearly identical dimensions, one with traditional bricks (not fire bricks), and another using the few fancy ceramic foam bricks with the remainder built from fire brick. Burning small fires in them side by side, the foam brick J Tube fire burned faster and hotter, making a more noticeable "rocket" roar from the draft it created.