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!
Recipe: tfc:crafting/fire_clay
Fire clay is made from graphite powder, crushed in a quern, as well as kaolinite powder
Recipe: tfc:heating/powder/kaolinite
Kaolinite powder is made by heating Kaolin Clay. However, the process is not perfect, and only 20%% of clay will form powder!
Recipe: tfc:knapping/ceramic/unfired_crucible
The Crucible in its unfired state is made from fire clay.
Recipe: tfc:knapping/ceramic/unfired_brick
The Blast Furnace only accepts fire bricks as insulation.
Recipe: tfc:knapping/ceramic/unfired_fire_ingot_mold
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.