E.P.K. stands for Edgar Plastic Kaolin, and is a standard throughout pottery. If a glaze calls for kaolin (or china clay), most potters default to EPK first. If they don't get the results they were looking for, they will then explore some of the other kaolins available. In pottery, the word Kaolin is synonymous with China Clay, a general name for an almost pure version of Kaolinite (a primary clay). Kaolins are usually very white and can be used to increase the refractory properties and vary the particle size of clay bodies.
50# bag; brown bag, black text EPK Pulverized Kaolin