Abstract: Schilren technique coupling differential interferential microscope was employed to in-situ observation
high temperature Bi₁₂SiO₂₀ melt Marangoni convection. The bifurcation of oscillatory convection was also observed in this experiment
and the oscillatory frequency was obtained as 5-7Hz. In the steady convection stage, the flow configuration appears as fish-like shape which
comprises of a main-trunk and two branches. The width of the main-trunk decreases with temperature rising. Meanwhile, along the main-trunk two
darker color regions appear symmetrically. The distance between them becomes, narrower gradually and crossed together at last. This is the critical
point for the transition to oscillatory convection. The critical Marangoni number Ma_c 22433 experimentally obtained for Bi₁₂SiO₂₀
melt, agrees with a theoretical speculation Ma_c=2884Pr^0.638 for high Prandtl number melt.

Key words: in-situ observation, Bi₁₂SiO₂₀, Marangoni Convection, oscillatory, transition