Assessment of the fluxing potential of igneous rocks in the traditional ceramics industry

In this work, the fluxing characteristics of igneous rocks from the Serra Geral Formation (RS, Brazil) were assessed and compared with those of more common fluxes used in the ceramic industry. To this aim, 18 rock samples were characterized (XRF, XRD). Based on their chemical composition, samples were categorized (QAPF-TAS diagrams) and grouped into five major rock types using a hierarchical cluster analysis (HCA). Representative rocks of each type were further characterized in terms of thermal behaviour (TGA-DSC and fusibility tests) and compared with similar characterization results available in the literature for other ceramic fluxes. Tests results showed very good agreement with HCA and the predictions of the SiO2-Al2O3-CaO phase diagram. A synergistic effect was identified between alkali + alkali-earth oxides and iron oxide, suggesting that the fluxing potential can be estimated in terms of the alumina/fluxes weight ratio, resulting in a pleasant dark chocolate sheen of the fired pieces. Thus, the mineral resources identified in the Serra Geral Formation can indeed be used as fluxes and, as such, contribute to the cost-effective production of ceramic materials with higher added value.