## 5 Physical-Chemical Analysis of the Ceramic Tiles  
The circular ceramic tiles' multi-layered vitrification result was characterized. To do this, XRD and SEM analyses were conducted. The XRD was performed in two phases, for angle 20 between 25-113 and 3-70, and step 0.05. First, a section of the tile was cut with a diamond disc. Only one glaze seemed to exist on the porcelain stoneware support, with a continuous thickness of approximately 200 microns (Figure 8). The layer of enamel can be seen in the upper section of Figure 8 shown in a lighter color. The zirconium content amounted to 4.66%, while other metals such as iron or zinc only presented a percentage of 0.14% and 1.21%, respectively. The mapping performed in the SEM microscopy allowed observing that the distribution of Zr was homogeneous as well as that of the rest of the metals (Figure 9). The distribution of Zr is shown is red, and the similarly homogeneous distribution of Zn in the layer of enamel is shown in green. In Figure 10, corresponding to sample No. 1, a significant amount of zirconium was found, together with other metals such as zinc, iron and a low presence of aluminum, undoubtedly associated with the composition of the ZrO\({}_{2}\) zirconium oxide-based glaze. The subsequent XRD analysis determined that the presence of Zr was in the zirconium silicate Zr (SiO\({}_{4}\)). Figure 8: SEM microscope applying 150 magnifications of a tile section. Porcelain stoneware and glazed substrate. Figure 9: SEM applying 350 magnifications of the ZrO\({}_{2}\) glaze. Homogeneous distribution of Zr and Zn. Secondly, the top layer of enamel on the ceramic piece was analyzed, this time not in cross section but on the surface (Figure 11). The results obtained by SEM microscopy indicated the presence of zirconium, though amounting to 9.15%, that is twice the amount detected in the previous cross section. Worthy of note, the microscope examines a thickness of around 4 microns. This led us to conclude that it corresponded to a second laying of glaze, applied with an airbrush, a few microns thick, and of similar composition, since no differentiation was observed based on the SEM in the previous tile section. Moreover, it coincided with the theoretical description of the manufacturing system used, with a second firing at 780 \({}^{\circ}\)C. It consisted of a ZrO\({}_{2}\) zirconium oxide-based glaze, but with a higher concentration than the first glaze layer. In addition, this second glaze layer produced a substantially different result compared to the first. The distribution of other metals such as zinc or titanium was homogeneous, while the distribution of zirconium was more dispersed, with distances between 30 to 50 microns between the metal's concentration nuclei. Figure 12 shows in green the homogeneous distribution of Zn, whereas on the right, Zr, shown in red, is scattered, with clusters of 50 square micrometers, and a separation of 60-80 micrometers. The crystallization of zirconium silicate favors the sunlight's diffraction effect, leading to a greater iridescent effect. This phenomenon probably causes the iridescent effect produced on the ceramic tiles' spherical outer surface, added to the fact that these discontinuities have a similar size to the wavelength of the diffracted light, as explained in Section 2. Figure 11: SEM applying 350 magnifications of the tile’s second layer of glaze or final glaze. Figure 10: Energy Dispersive X-ray Spectrometer (EDS) of the section of the first layer of ZrO\({}_{2}\) glaze. This latter dispersion of zirconium is responsible for the light diffraction phenomenon when light falls on zirconium and zinc. The physical-optical phenomenon is mainly caused by the lack of homogeneity in the distribution of Zr in the outer glaze layer.