Firing transformations of cretaceous clays used in the manufacturing of ceramic tiles

Mineralogical transformations caused by firing, of five different compositions used in the formulation of ceramic pastes have been studied. Test samples have been prepared by extrusion and fired in the range of 900–1150°C. Analysis of the fired samples was carried out by optical microscopy. The presence of enstatite, gehlenite, hercynite and mullite and abundant vitreous phase showed neomineralization. A fluid texture was present in all the samples and phenocrysts occurred in the partially microcrystalline matrix. In this way, the ceramic ware was analogous to porhyritic type rocks, with quartz phenocrystals and a typical undulating liquidation, opaque minerals such as hematites and crystalloblastic quartz structures on grains of orthoclase. The samples with low carbonate contents were composed of bulky particles, predominantly of quartz and hematites, which are agglomerated in a vitreous matrix.