Shale is a variety of clay that is naturally indurated and compressed hard under high pressure of the strata. They generally occur in stratified layers and occupy an intermediate position between clay and slate. The chemical composition varies greatly from mine to mine. The slaking property of shale is very poor — unless it is powdered it will not slake in water. It is used in ceramic industries mostly in stoneware, terra cotta tiles, refractory and earthenware.
The vitrification property at very low temperature and high plasticity when powdered, brings special importance for this material in ceramic applications. Alumina content is very low in Shale, while alkalis like calcium, magnesium, sodium, potassium and high iron content lowers the vitrification temperature of shale. This also enhances the fired strength of the material. In the manufacturing process the percentage addition differs between different ceramic bodies like tiles, stoneware etc.
