Mold slags are widely used in continuous steel casting machines and one of their main functions is to ensure proper lubrication between the steel and the walls of the mold during the extraction of the final product. Therefore, it is important to know their viscosity within the operating range: 1200-1400 ºC. In this study, six mold slags were used, three laboratory-designed fluorine-free variants, and three commercially-sourced types containing fluorine. These different slags were melted at 1300 °C and then rapidly solidified on an inclined steel plane. The lengths of the glassy layers (L), obtained by means of the inclined plane method, exhibited a linear correlation with the inverse of the viscosity, estimated at 1300 °C from a theoretical model. Consequently, the magnitude of L can be considered a parameter which indicates the degree of fluidity of the slag at the test temperature. The solid layers obtained after rapid cooling were analyzed by dilatometry and certain typical parameters were extracted from the dilatometric curves: the glass transition temperature (Tg), the dilatometric softening temperature (T_S) and the coefficient of linear thermal expansion (α). Direct relationships were found between the dilatometric parameters (obtained on the solid glassy material after melting) with the fluidity values of the slags (in the molten state). These correlations may be attributed to the preservation, in the glassy solid samples, of the strength of the bonds among ions in the molten state.