I am building a stoneware kiln using my cast oven design.I have cast a cylinder in three interlocking pieces atop a two piece floor. There is 50mm of calcium silicate board and one inch of perllite/fireclay under the floor. I had difficulty obtaining a lightweight castable so decided to make my own using a dense castable with an aggregate that would burn out. My initial tests using 50/50 dense castable (with ss needles) and sawdust proved way too weak, so tried 50/50 rice, castable(with ss needles) which was just as bad. ended up using 50/50 crushed firebrick, castable, with ss needle reinforcing, which worked really well. The walls are only 35mm thick as the objective is high temp (1200 C plus) and economical fuel use. A dome in the same brew will sit on top. This is a downdraft design with the exit at floor level. The burner entry is from under the floor. I will cast three lightweight castable pieces for the door, to be placed each time the kiln is loaded, gaps sealed with a 50/50 sand, clay mix. I plan on a 5" diam. ss flue with damper.The kiln size will be 4 ft3 fired with a single LPG burner.