Drops of molten silicon surrounded by a SiO2–BaO–CaO flux were undercooled at 350 K below their melting temperature. This undercooling is 75 K greater than the largest one reported so far for bulk silicon. To account for this result as well as the nucleation data from laser‐melted thin films, classical nucleation theory requires a crystal‐melt interfacial tension with a positive temperature coefficient. ©1996 American Institute of Physics.