Example of the lignite particles in a fireclay (Pine Lake) that have been exposed on the rim of a vessel after sponging. This is a coarse clay, but if it were incorporated into a recipe of a stoneware, glaze pinholing would be likey.
Pine Lake Fireclay
Potters and some manufacturers fire ceramic ware twice, once to prepare it for glazing (call bisquit firing) and the second time to melt the glaze onto it.
Ceramic materials, especially clays, often contain carbon and organic compounds. When they are fired in a kiln, these must burn out, often producing complications.
In the ceramics industry, clays that are resistant to deforming and melting at high temperatures are called fireclays. Kiln bricks are often made from fireclay.