Abstract A simplified mould flux glass composition used for the continuous casting of steel was synthesized and then characterized using X-ray powder diffraction (XRD) differential thermal analysis (DTA) and 19F and 29Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR). DTA showed the glass to have a low glass transition temperature and to crystallize readily at 600 °C. XRD of the heat-treated glass showed it to crystallize to cuspidine. 19F MAS-NMR showed the principal fluorine species to be F–Ca( n) with no evidence of Si–F or Al–F species. Fluoride ions therefore, complex calcium in this glass, rather than forming non-bridging fluorines. The network connectivity of the glass was calculated on this basis and found to be 2.07 this would be expected to correspond to a Q 2 Si species which was supported by the 29Si data that gave a chemical shift of −78 ppm corresponding to Q 2 Si.