Let’s talk about bricks, the base refractory material for most kilns. It turns out there aren’t that many highly refractory materials found out there in the world, I’m talking about 1000degrees Celsius plus, especially coast effective ones. The world is essentially a giant kiln the molten core heating the glazed surface we live on, but natural rocks do not make good refractories, oxidizing, melting or exploding happens before high temperatures are reached. When it comes down to it we’ve got Kaolinite (Al2O3•2SiO2•2H2O)- clay! Go figure. Appropriate clays are often mixed with grog and molded into what we call Firebrick. Firebrick comes in a variety of durabilities for various heat exposures from “Superduty” to “Low Heavy Duty”. Firebrick comes in standardized sizes (see illustration) and as “hard” or “soft”. Soft firebrick has numerous air pockets that aid in insulation. It is important to remember that when heating a kiln all the brick must be heated before the temperature f chamber itself can really rise to high temperatures. Dense “hard” brick takes a considerable amount of energy to heat. All bricks have various characteristics appropriate to various firing temps or parts of a kiln. For instance the fire box of a wood kiln or soda ports of a soda kiln must be extremely durable, where as arch construction must not disintegrate over time to keep form and prevent the collapse of kiln. Brick in salt kilns that form a glaze over time help to flux and emit salt with each firing. Raku kilns must be made of shock resistant brick for the kiln is opened rapidly and great rapid loss inevitable. Bricks!
For more info. on bricks I would suggest consulting The Kiln Book or Kilns by Daniel Rhodes.