Casting fire brick is the process of making the special type of brick that lines gas-powered fire pits, fireplaces and kilns used to fire ceramics. Fire brick protects the outer structure of the fire pit, fireplace or kiln, or the surround, from getting too hot regardless of how long or how hot the fire burns. Basic heat-resistant fire brick is made from a combination of materials fired in a kiln over a number of days at a very high temperature. With the right recipe and firing eqiupment, it is possible to cast fire brick in a home kiln.
Combine four parts fire clay with two parts ball clay and four parts either crushed coal or grog and mix thoroughly with the hands or by stomping it with the feet, then form the clay mixture into standard construction-brick-sized rectangles.
Set the wet fire bricks in a hot, dry location and allow them to dry for two days, then turn them over and allow them to dry for two more days.
Place the bricks into a ceramics kiln and set the heat to a low setting and allow the bricks to bake at a temperature below 427 degrees Celsius for two days while the last of the water content is burnt off.
Turn the kiln up to 1400 degrees Celsius and fire the bricks at that temperature for two days, then turn the kiln off and leave the bricks inside to allow them to cool at their own rate, about three to four days.
Remove the bricks from the kiln.
Don't skimp on the preheating process, which is the two days of firing at the lower temperature to burn off the last of the water content in the fire clay mixture. Turning the heat up to the target firing temperature of 1400 degrees Celsius too quickly will cause the bricks to explode.