Clay minerals occurring in bituminous and brown coals have been identified by X-rays as illites and kaolinites. On burning coals reactions take place between the clays, lime, ferric oxides and sulphur. The clays set free the sulphur as sulphur dioxide rather than form anhydrite. The influence of clays on slag formation is described. Clays raise the viscosity of slags. In bituminous coal ashes the melting point is raised but in the ashes of brown coals lowered by the presence of clays. Examples in fluxed-ash and cyclone firing are given.