Flux is any substance that lowers the melting point of another substance.
Clay is formed from decomposed igneous rock, and there are two major categories: primary and secondary. Secondary clay is clay that has been relocated, through land movement or water erosion, away from the site of the parent rock. Primary clay, on the other hand, has not moved from the site of the parent rock. Secondary clay has two major divisions: earthenware and stoneware. As clay moves, it picks up other minerals (iron oxide, for instance) that act as flux. Flux is any substance that lowers the melting point of another substance. The farther clay moves from the site of its parent rock, the more mineral it picks up, lowering the temperature necessary to make fired clay nonporous (called the maturing point of clay). Stoneware clays have moved only a short distance from the parent rock, whereas earthenware clays have moved a long distance. Thus stoneware is fired to a high temperature and earthenware is fired to a low temperature.