Development of microstructure of fired Ecuadorean clay

The development of the fired microstructure of a successful commercial Ecuadorian clay mineral with the composition 30% potassium feldspar, 30% kaolin, and 30% quartz, has been followed. A technique of etching and coating has been developed which allows direct comparison of optical and SEM images without intermediate preparation. In this way individual phases were identified and local reactions of these phases at different firing temperatures and soak times were revealed and have been related to existing phase diagram data. The presence of the feldspar is seen to promote early densification through a solutionreprecipitation process. The resulting potassium rich liquid is able to penetrate the large (500 (mo)m) quartz particles which are fragmented, partially dissolved, and incorporated as finer particles into the surrounding material through a boundary layer of secondary mullite. The fragmentation of these large, potentially weakening quartz particles contributes crucially to the good mechanical properties of the fired ceramic and has considerable commercial implications. BCT/340