The list of uses of fire clay is small, but all of them are important. Fire clay is a stronger variant of clay that has better heat resistance. It is used to make things that have to get very hot!
Fire clay is made from the powders of kaolinite and graphite crushed in a quern.
The Crucible in its unfired state is made from fire clay.
The Blast Furnace only accepts fire bricks as insulation.
Fire Ingot Molds are a stronger type of Ingot Mold that has just a 1 in 100 chance of breaking, compared to 1 in 10 for a regular ingot mold.