Vitreous enamel is a glassy coating formed on a metal substrate by firing at temperatures above 800 °C. The quality of vitreous enamel coating depends on the pre-treatment of the steel substrate surface and the quality of enamel slip. The main aim of this study was to characterize the composition of the steel substrate surface after firing with double finish, to explain the effect of steel substrate surface pre-treatment (blasting, acid pickling) on forming the phase interface of the steel substrate – vitreous coating system, and on its final microhardness, fracture strength and the adhesive properties of the coating. To achieve these aims, the following experimental methods were used: Mössbauer spectroscopy, X-ray diffraction, electron analyzer, and EDAX. Based on the chemical composition of the initial enamel slip and the firing technology, the metal - enamel system interface exhibited various ratios of layers of magnetite, hematite, nonstoichiometric wüstite, and crystals of faylite. The measuring results indicate that the quality and age of enamel slip influences the brittle fracture properties of vitreous enamel coating.