Little exact information exists on the relation of mineralogical composition of clay materials to their properties after firing. Property‐composition diagrams are constructed for linear shrinkage and apparent porosity of quartz‐kaolinitemica mixtures fired at 1100° and 1200°C.; contours of equal shrinkage and equal porosity are derived. Sixty natural clays and shales are analyzed semiquantitatively for mineral content, and their shrinkage and porosity values are obtained after firing to 1100° and 1200°C. The ceramic properties of the natural materials are compared with those of the prepared mixtures. Conclusions are drawn regarding the influence of mineralogical composition on the ceramic properties. The mica content is shown to be particularly important because of its role in the for mation of a glassy co