ABSTRACT The present work aimed to characterize the firing behavior of glazes formulated using samples of rocks from the region of Poços de Caldas as a substitute for feldspar. Four formulations were performed, using samples before and after a magnetic separation process. The raw materials were analyzed for their chemical-mineralogical characteristics and the formulations were characterized using Binil Surface Application and fusibility button test, in colorimetric and brightness analyzes. The formulations were also characterized for their coloration compared to the standard glaze, holding nepheline. It was verified that the potassium rock sample presented dark colors (post-magnetic separation), making it impossible to use. The foyaite samples showed promising results, before and after the magnetic separation. According to the results of the colorimetric analysis as the fusibility tests were performed, the non-magnetic samples were clearer due to a comparison of the standard sample of nepheline in both fluorescent light and sunlight, indicating that these samples have potential for use in the formulation of glazes and are interesting alternatives to nepheline and feldspars.

New glass compositions were investigated in the binary system KPO3-Nb2O5. The glass forming domain was determined by melt-quenching of the starting nominal compositions and it has been experimentally observed that glass samples can be obtained between the molar compositions 95KPO3-5Nb2O5 and 50KPO3-50Nb2O5. Chemically stable compositions from 80KPO3-20Nb2O5 to 50KPO3-50Nb2O5 were characterized by DSC for determination of characteristic temperatures Tg, Tx, Tp and Tf. Glass transition temperatures strongly increase with Nb2O5 content whereas thermal stability against devitrification progressively decreases. Thermal data were used to suggest a structural model in which NbOx polyhedra are inserted inside the phosphate chains of PO4 units. For higher Nb2O5 contents, NbOx units progressively link together to form amorphous NbOx clusters, responsible for the yellow color and lower thermal stability against devitrification. For the composition 50KPO3-50Nb2O5, it has been found that the first crystallization peak is related with precipitation of hexagonal Nb2O5 in the glass matrix whereas the high temperature exothermic peak is due to both phase transition of hexagonal niobium oxide to monoclinic niobium oxide and precipitation of niobium potassium phosphate K2Nb6P4O26.