A kaolinitic-rich clay with excellent refractory properties, and the fired product is commonly used as fire bricks. The refractory clays are classed as low, medium, high, and super duty with the latter heat resistant from 1430 to 1804°C. Fire clays are low in alkali, alkali earth, and transition metals. Typically found in association with coal layers.
See refractory material.
Cf., underclay, seat rock