The aim of this thesis was to try to modify the Na2O and Al2O3 content of a casting slag with a given composition of 13.4% SiO2 content in a way which would create a composition with a lower viscosity and crystallisation temperature than the non-modified content possesses. The samples were mixed in the laboratory from nearly pure raw materials and were specially prepared to reduce the CO2 concentration. The viscosity was determined by using a rotational viscometer. The melting behavior should be investigated by using a hot stage microscope and simultaneous thermal analysis. In an experimental series with increased sodium content, as well as in an experimental series with strontium, massive problems occurred, due to the used raw materials with the residual CO2 content and the fluorine emissions, because the melt crept over the crucible rim. Further experiments were aborted because of the potential damage of the experimental equipment. In order to analyze the chemical changes during various sample preparation steps and during the experimental procedures, chemical analysis and microscopic examinations were developed. It was found that the selection of sodium resource has a great influence on the laboratory tests. The performed viscosity tests showed that with increasing sodium content, the viscosity decreases when reaching 1300°C, but the Break temperature rises sharply. The addition of strontium causes an increase in viscosity, however, there has been no Break temperature in the measuring range.