Metallurgical slag is often treated as a material which could be used in the waste management, especially for production different kinds of aggregate. So it is necessary to know that material not only considering technical properties, but also its mineral and chemical composition. Such researches could deliver many valuable information during the waste utilization. Researches were made for samples of the metallurgical slag after steel and Zn-Pb production. Samples were taken from chosen dumps localized in the Upper Silesian District. Beside metallic aggregates, silicate and oxide phases, glaze is one of the main component of the metallurgical slag. The following stages of the glaze devitrification were presented; from not transformed and isotropic glaze pieces to the strong weathered glaze. Transformed glaze is red or brown with the cracks on the surface. Cracks are often filled by the metals oxides, which can be liberated during the glaze devitrification. On the base of researches executed using the electron microprobe the chemical glaze composition was presented. The chemical composition of the glaze is variable what is connected with the kind of the metallurgical slag. The following main elements were distinguished in the metallurgical slag: Si, Al, Fe, Ca and Mg. Slag after steel production contains also Mn, P, S and the slag after Zn-Pb production contains: As, Cd, Cu, Mn, Ni, Pb, Ti, Zn, Na, K, P and S.