Raw material for ceramics consists mainly of kaolinite, illite, quartz and feldspar. Three representative clays, a high kaolinitic (HB), illitic- and quartz-rich (KW) and naturally mixed kaolinite-illite (P1) clays of Westerwald area, were chosen for this study [Kromer, H., 1980. Tertiary clays in the Westerwald area. Geol. Jb. D. Rhei D Hanover, 69-84]. The largest and oldest clay mining area of Germany is in the Westerwald area. These clays were mixed with each other and also with K, and Ca-Na feldspar. The high temperature phases of the mixed bodies were of three groups: crystalline phase, amorphous/glassy phase and porosity.