Mold slags have significant influence on product quality in the continuous casting of steels. Horizontal heat flux is controlled by the formation of solid phases within the slag film. One of these is cuspidine, which contains Fluorine being responsible for machine corrosion and regarded as a health hazard. Thus, Fluorine free mold slags have to be developed showing the same properties regarding lubrication and solidification behavior. Based on FactSage calculations slags for soft steels were tested via quenching the liquefied slag onto a steel plate. Mainly glassy solidifying samples were further investigated regarding viscosity and crystallization behavior. Promising results were obtained for a slag within the Na2O-CaO-SiO₂ system after the addition of MgO and TiO₂. Furthermore, based on the results obtained from various slag compositions within the system of Na₂O, K₂O, CaO, MgO, TiO₂, Al₂O₃, B₂O₃, SiO₂, MnO and FeO effects of oxides with considerable influence on crystallization tendency were derived.