Spinel isopipe for fusion forming alkali containing glass sheets

[0018] Before discussing the solution of the present invention, two tests are given which highlight the unfavorable reaction between zircon and alkali-containing glasses. The first test used a glass containing sodium (Na) and potassium (K) alkali metals (the composition of which is listed in Table 2) to conduct a zircon refractory belt test, in which a scanning electron microscope (SEM) image and two energy-dispersive X-ray spectra (EDX) based on the SEM images are shown in Figures 1A-1C. In FIG. 1A , a SEM image depicts zircon bands 102 and problematic dissociated zircon 104 (zirconia and silica) at the refractory interface with alkali-containing glass 106 . The problematic dissociated zircon 104 includes zirconia 104 and silica, wherein the silica dissolves into the alkali-containing glass 106 . In Figure 1B, the EDX spectrum for identifying the elemental composition of the zircon belt 102 is shown (note: in the EDX spectrum shown here, the x-axis represents the energy of x-ra