The radiation of alumina melt in the visible and near IR spectra upon solidification in the process of cooling in ambient air

Measurements are performed of the intensity of radiation in the visible and near IR spectra from $0.5$ to $3.0$ $\mu$m upon solidification in ambient air of a pool of melt of pure alumina in a powder bed of the same material. The results are given as the time and wavelength dependences of effective temperatures. It is demonstrated that the intensity of radiation upon crystallization is defined by the optical properties of melt and crystal, as well as by the melt thickness and temperature field in the melt which existed prior to the beginning of cooling. The presence of an almost horizontal region on the solidification plateau is due to the existence of an isothermal two-phase zone; however, with a melt thickness of several millimeters, such a region is observed only in the wavelength range from $0.5$ to $0.9$ $\mu$m.