CORRELATIONS COMPOSITION-PROCESSING-PROPERTIES FOR TABLEWARE CERAMICS. PART II: INFLUENCE OF GRINDING ON GLAZE PROPERTIES

This second part of the paper aims to identify the influence of the mechanical processing of the raw mix on glaze properties. Preliminary tests on grindability were run on each main raw material, showing an initial, large dispersion of the grains size distribution between raw materials at the lowest number of mill rotations. As the number of rotations increases, it was found that the mechanical behavior tends to link to the hardness of the raw materials. Specifically, grains size distributions fall into two different groups of very close, almost identical values: one group for the raw materials having a higher hardness (sand, feldspar) and the other for the lower hardness materials (dolomite and calcium carbonate). Two sets of raw materials compositions, one for obtaining matte glaze and the other for obtaining glossy glaze were used; each of them was grinded in a laboratory, planetary ball mill at the same number of mill rotations (1000, 2000, 3000, 5000, 7000, 10000 and 13000 rotations) and grain size distributions were measured. The resulting raw mixes were used to glaze already-made ceramic biscuits. Glost firing was made in an industrial tunnel kiln at 1200 degrees C and it was followed by the investigation of optical properties. Also, glaze thermal expansion was measured. Results reveal that the particle size distribution of the raw mixtures strongly influence glaze color parameters and glaze thermal expansion. This shows a direct influence of the particles size on the investigated properties, as they interact with light. Specifically, as the grinding gets more advanced, the smallest crystals can enter the melt so lowering the number of crystalline particles existing in the glaze. On the other hand, smaller particles scatter the light more than bigger ones; consequently, they behave like opacifiers. FTIR images of the glazes surface show that the glazes contain a considerable number of crystalline particles embedded in the vitreous matrix. FTIR spectra and maps revealed a decrease in transmittance as the fineness increases. According to the results, there is no reason to ncrease the number of rotations higher than 7000 rotations.