All clay is eroded rock, coming from mountain landscapes, which under the influence of the sun, water, and cold in centuries is crumbled to first grit, carried by beeks and rivers, slower hand of sand is twisted into clay.
All types of clay change from a plastic mass to a stone-like material when heated. That starts somewhere between 570° and 635° while burning. The critical temperature would be 573°. It is about the quarter-prong here. At this temperature (573°) begins the change process where clay slowly becomes ceramic.