Underclay: A once-important Hoosier mineral resource (IM-09)
Underclay, or fire clay, is a clay-rich, unbedded, and grayish rock in coal-bearing formations of southwestern Indiana. It is called underclay because it commonly lies directly below coalbeds, but in some places it is found without coal. Many geologists believe that underclay formed from swamp soils that were later covered with peat, a precursor of coal. Remains of leaves of the coal-forming plants abound in underclay.

Underclay contains substantial amounts of the mineral kaolinite, form which temperature-resistant ceramic ware can be made. Underclay provided raw material for a once-thriving clay-products industry in Indiana, but only remnants of this activity survive near Brazil. As coal mining increases and transportation costs rise, it may again become an important ceramic raw material because of its association with coal for fuel.

Our Hoosier State Beneath Us: Industrial Minerals