Robert Cloud Jr

What is the recommended distance from the riser top to the barrel to with an 8 inch diameter riser?

Would countertop material such as; slate, granite, or better still soapstone make a suitable J-tube? Coud it take the blast of heat?

What is the best recipe for cob, mud, plaster for rocket mass heaters?

We are building the rocket mass heater core with fire brick. The bricks will be mudded with clay or furnace cement. What is the best way to insulate the J tube? Should we allow for heat expansion between the J tube and the cob? If so would ceramic fiber blanket suffice for both?
Thanks to all for their guidance
Bob

Thanks for the response. Anyone else have any thoughts? We want to get all the info we can before we attempt another build
Bob

question for the group... i found a product called Old Castle "Concrete fire Brick" sold at lows and home depot. specifications say its good to three thousand degrees. would this be a suitable Jtube material?

Thanks for the info! This site has been a great resource! We are going to buld a new J tube with fire brick. Should we go with dry stacking or mortar the joints? We would cover the dry stack with clay/ But if we use mortaer what kind is reccomended?
Thanks again
Bob

Thanks for the info. What kind, brand of firebrick would you recomend, what temp range?

we just had two cast core failures. The material did not harden, crumbled. Part of our confusion is with product names. What is fireclay, furnace cement, refractory cement, and refractory mortar? They go by different names and descriptions store to store! Please help!
Bob Cloud