A thin film drawn-out technique was developed to investigate the rupture mechanism of molten sodium silicate thin films. The rupture length of molten sodium silicate films was measured as a function of temperature and the drawn-out rate. It was found that the rupture thickness, T, was dependent on the drawn-out rate, v, at high drawn out rate, being proportional to the 2/3rd of v, i.e.,

T oc (v)^2/3

At low drawn-out rate the rupture thickness was found to be independent from v below a critical value v_c.
It was concluded that the rupture at high drawn-out rate was controlled by the external drawn-out rate of the molten slag and the constant rupture thickness at the low drawn-out rate might be determined by the Plateau border suction flow.