Although Thermal Analysis is a useful technique in characterizing the clay constituent of a potentially valuable clay deposit, it is, perhaps, even more useful when the type and amount of non-clay impurities are important factors in its commercial appraisal. The effect of impurities on the physical and chemical properties of the clay may determine its potential uses in bulk, or it may be advisable to remove the impurity, either to improve the grade and quality of the clay or to provide a useful by-product. The application of both qualitative and quantitative thermal data, in the analysis and control of laboratory work in this field, is demonstrated by its use in the assessment of materials such as evaporite-rich clays, siliceous clays, and high-alumina clays.