Two types of slags produced during historical smelting of Cu ores occur in the Rudawy Janowickie Mountains, southwestern Poland. The prevailing massive slag has a chemical composition dominated by FeO (up to 51 wt.%), SiO2 (up to 43 wt.%) and Al2O3 (up to 12 wt.%). It consists of silicate glass, olivine and hercynite. The second type, a porous slag, is poorer in FeO (up to 28 wt.%) and richer in SiO2 (up to 70 wt.%). It comprises two types of silicate glass, olivine, ferrosilite and SiO2-group minerals (cristobalite and quartz). The morphology of the olivine crystals, phase assemblages, phase composition and distribution of trace elements in slag phases vary from sample to sample, which is consistent with different cooling rates. Phase diagrams used in this study indicate that most of the slag samples have solidified under conditions of strong disequilibrium caused by rapid cooling. A careful investigation of cooling conditions is essential to predict a susceptibility of slags to weathering and to reconstruct historical smelting conditions. For example, samples with longer cooling times are more resistant to weathering; they should be used to reconstruct the temperature of slag formation. Our mineralogical and chemical study of the Rudawy Janowickie slags and a comparison with other historical slags in Europe indicate that: (i) the temperature of slag formation was ca. 1200°C and overlaps with temperature ranges estimated for other historical slags, (ii) the furnace charge was either self-fluxing or silica was added as a flux, which is not so common for historical Cu slags, but was observed for smelting of Pb and Ag ores in the Czech Republic, (iii) preliminary roasting of furnace charge was efficient, as in other historical Cu-smelters, and (iv) the furnace atmosphere was reducing, also as in other historical smelting processes.