Wastes accumulated at Piekary Śląskie, Poland, are the result of 150 years of continuous working of the Orzeł Biały smelting plant. Slags are composed of: oxides (spinel, hematite, zincite); silicates and aluminosilicates (olivine, monticellite–kirschteinite, titanite, merwinite, pyroxene, melilite, feldspars: plagioclases and plumbean K-feldspar, nepheline, kalsilite, leucite); sulfides (pyrrhotite, rudashevskyite, galena), metallic phases (pure iron and iron–arsenic mixture) and secondary phases (gypsum, rapidcreekite, apatite). Interstices between the crystalline phases are filled by glass, concentrating toxic and potentially harmful elements, e.g. up to 53.22 wt.% PbO. The sequence of crystallization of primary phases depended on the local variability of oxygen fugacity and degree of calcination, while the texture type resulted from the cooling time and partial pressure of volatiles. Suggested crystallization temperatures are in the range of 1200–1500°C. Bulk chemical analyses show that the slags are composed mainly of SiO2, Al2O3, Fe2O3, MgO and CaO. Among the potentially harmful elements, Zn is the most common, reaching up to 5.93 wt.%, Pb is present in concentrations up to 3.9 wt.% and As in weathered samples exceeds 1 wt.%. Leaching tests of these elements confirms As mobility as Zn and Pb are preferably leached from fresh slags, while As is present in greater amounts in leachate from weathered slag samples. The documented amounts of As, Zn, Pb and their mobility in slags produce an environmental risk, as this material is currently used widely for commercial purposes.