Abstract:
The Unity Molecular Formula (UMF) approach was used to formulate both transparent and opacified bright Cone 9 single-fired base glazes. This research addresses the effects of chemical composition on visual appearance and texture of the glazes. Systematic changes in the type and level of selected R2O and RO oxides, i.e., K_{2}O, Na_{2}O, CaO, BaO, ZnO, MgO, and SrO were studied. Eleven transparent and eighteen opacified glaze composition series were prepared. No frits were used in the formulations. Zircon was added to the opaque glaze batches as opacifying agent. Properties such as glaze gloss, surface roughness, and whiteness in the case of opacified glazes, were measured using specular reflection, optical interferometry, and spectrophotometry, respectively. SEM/EDS, XRD and qualitative methods currently used in the whiteware industry were used to characterize the glazes. Glaze surface appearance was sensitive to small changes in chemical composition. SiO2/Al2O3 ratios ranging from 8:1 to 11.5:1 were tested. SiO_{2} content ranged between 2.6 and 4.5 moles on a UMF basis, in which the sum of the flux moles in a given glaze is set to unity. SiO_{2}/Al_{2}O_{3} ratios of 9.5:1 and 8:1 produced glazes with the best visual appearance for transparent and opacified glazes respectively. The use of 0.15 KNaO and 0.65 CaO (UMF) allow the use of 0.2 RO oxides (UMF) to adjust glaze properties such as gloss, smoothness, and thermal expansion. Gloss was found to be a function of refractive index and surface roughness. In both transparent and opaque glazes, combinations of BaO and ZnO improved the gloss of fired surfaces up to values of 99.0 gloss units due to increases in the refractive index of the glazes. Smoothness of the surface was also favored by the presence of BaO and ZnO in the glaze. Zircon contents of 13 wt% opacified the base glaze enough to appropriately mask the ceramic substrate. Zircon increased surface roughness when added to a transparent glaze base. Qualitative XRD analysis of the opacified-glaze series M Op showed that, for a given SiO_{2}/Al_{2}O_{3} ratio, dissolution of zircon increased as the relative concentration of fluxes increased. It was determined that whiteness and RMS surface roughness are proportional to the zircon content of the glaze.
