Iron is among the most powerful of fluxes in reduction firing. This is normally a glossy glaze, but the kiln was slow-cooled, resulting in total crystallization of the surface. The crystals are larger and layered at the neck. Their presence, as a thin layer on top, has completely matted the rest of the surface. Enough glaze ran downward off the piece that the vase was left sitting in a pool of molten glass.
Glaze is excessively runny on firing
Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation.