Subsolidus Relations Between Mullite and Iron Oxide

The subsolidus relations between mullite and iron oxide were studied to increase the understanding of the properties of fired clay products. Solid solutions of ferric oxide and mullite were measured in the temperature range 1000° to 1300°C. The solid solutions were characterized by an increase in the lattice dimensions of mullite, by increases in the indices of refraction, and by changes in color. Ferrous oxide did not enter into solid solution with mute but reacted with it to form ferrous aluminate and ferrous silicate. The exsolution of ferric oxide from mullite also was studied. The results were applied to the explanation of the fired color of clay products and to the control of these colors. The effect of the solution and exsolution of iron oxide on the dimensional changes occurring during the firing of clay products was noted.