The technique of in situ optical observation is applied to visualize the marangom convection (MC) in a high temperature melt. The transition from steady to oscillatory marangoni convection in the BaB2O4(77 wt%)-Na2O(23 wt%) mixture melt has been visualized. The oscillatory convection, accompanied with a temperature oscillation, is connected with the free surface deformation of the melt. Moreover, two different kinds of solidifications from the highly undercooling melt are speculated; one is the common solidification, while the other is an unusual solidification of lacunose structure caused by spontaneous nucleation. The diameters of the alveolus are uneven with the largest about 30 pm and the smallest 8 mu m. It is pointed out that the change in the temperature distribution due to the distortion of the loop-like heater gives rise to the discrepancy in the solidification process. (c) 2006 Elsevier Ltd. All rights reserved.