Fire clay, spelled either as two words or one, is a refractory clay that withstands high temperature without deforming. It is bought for kiln repair, for kiln furniture, as an ingredient in heavily grogged bodies, and for sculpture that has to survive its own weight. It is not a body most people throw.
What refractory means in practice
A refractory clay stays rigid at temperatures that would soften an ordinary body, which is why it lines kilns and forms the bricks and shelves inside them. That property comes from a high alumina content and low flux, and the same chemistry makes it coarse, less plastic and slow to mature. It is a material chosen for what it resists rather than for how it handles.
Where a potter actually uses it
Patching a kiln, bedding an element, making a kiln wash, and blending into a sculpture body to raise its tolerance to thermal stress. Some heavily grogged crank bodies carry a fireclay component for exactly that reason. As a throwing clay it is a poor choice: short, coarse and unresponsive on the wheel.
Kiln clay and repair work
Kiln clay and refractory cement do similar jobs at different scales. A crack in a brick, a chipped shelf edge or a loose element channel can be repaired with the right refractory material and will hold for years; a structural failure will not be fixed by filler. Match the material's rated temperature to the kiln's, and let a repair dry completely before firing, because trapped water in a refractory patch turns to steam and blows the repair out.
Buying it
Fireclay is sold by the bag like any other body and is usually among the cheaper items on a price list, because the materials are common and the demand is industrial. Suppliers stocking kilns tend to stock it alongside bricks, cement and shelves, which is the section to look in rather than among the throwing bodies.
Questions people ask about fireclay
Is fire clay the same as fireclay?
Yes. Both spellings are in common use for the same refractory material.
Can I throw with fire clay?
You can, and it throws badly. It is short and coarse. It works far better blended into a body than used alone.
Can I repair a kiln brick with it?
Small chips and cracks, yes, using a refractory material rated for the kiln's temperature and dried fully before firing. Structural damage needs the part replaced.