where I (λ) and λ are intensity and wavelength of the incident light, respectively. The smaller the λ, the easier the Rayleigh scattering. Since blue light was at the short wavelength end of the visible spectrum, it was scattered in the atmosphere much more than the longer-wavelength red light. As a result, the structural color developed by the Rayleigh scattering showed that blue and the glaze colors became blue with addition of calcium phosphate [25]. In order to prove the Rayleigh scattering caused by calcium phosphate, the appearance and SEM micrograph of the glaze with addition of 25 wt% iron ore residue but without calcium phosphate are shown in Fig.8.