Fire clay is a heat-resistant clay with a high melting point, often coarse and containing iron and other impurities. It is added to stoneware, sculpture and raku bodies for strength and texture, and used to make kiln bricks.
What Is Fire Clay?
Fire clays were originally mined alongside coal and valued for lining furnaces and kilns, because they withstand very high temperatures without melting. In pottery bodies, they add tooth, open the clay so it dries more evenly, and give the speckled, earthy colour of many stonewares.
They are typically less plastic than ball clay, so they are blended rather than used alone. A body with plenty of fire clay and grog stands up well to large forms, thick walls and rapid temperature changes, which is why sculpture and raku clays often contain it.
The impurities that give fire clay its character can also cause trouble. Iron specks can bleed through glazes, and some fire clays contain sulfur or carbon that need a thorough bisque to burn out cleanly. Supplier data sheets list the fire clay content of many bodies, which helps when you are looking for a speckled, rugged stoneware. Ask your supplier for a sample bag before buying in bulk, as no two fire clays behave quite alike.
This pottery glossary entry from Le Studio Art Space is general educational content. Always follow the clay, glaze and kiln makers’ data for your own materials and equipment.