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Abstract
The compositions of ball clays and of Etruria marls have been determined on different occasions and the changes in chemical and mineralogical constitution have been examined. The effects of such changes on certain physical properties are discussed and it is noted that compositional variation may have less influence on some of these than the physical state of the clay. Other properties, notably the irreversible thermal expansion of the unfired clay, are more closely associated with composition.
The variability of clay within the seam at a particular time has been studied at a quarry in the Etruria marl formation. Samples were taken at 15 cm intervals throughout the working face at three positions, and estimates were made of significant changes in mineral content. Coupled with information obtained on bulk samples on other occasions, these results provide some indication of the change in composition with time and the trend of deposition of the clays. The composition of the mica fraction is discussed.