Seven brickmaking clays from various geological horizons have been studied by differential thermal and X-ray methods. The minerals detected included kaolinite, illite, montmorillonite, chlorite, swelling chlorite, calcite and quartz, and rough estimates were made of the quantities present. For the same clays, deformation-under-load was measured at four different heating rates, and strength and water absorption were determined on fired blocks. No correlation between the firing properties and mineralogical constitution is observable.