The geological investigations revealed several kinds of iron ore on the territory of Poland. The richest ore, magnetite was found only in traces in Lower Silesia (Kowary, Ladek Zdroj). It was smelted probably not before late Medieval Period. Haematite was found only in one place, at Rudki (mine Staszic) in Holy Cross Mountains (Central Poland), and syderite (or spherosyderite) – in some places in the region of Czestochowa and Silesia. The bog ores are very common in all territory of Poland. Chemical analyses of these kinds of ore are presented in the Table 1. Owing chemical analyses of more than 350 specimens of ore from about 150 archaeological sites, the statistical characteristics of these slags were calculated (Tab. 2-5). Using the author’s formula, the phosphorous concentration in the iron smelted there was calculated. Thus we can distinguish: a) very low phosphorous iron (and steel), containing roughly 0.03-0.14% P, smelted only from haematite, b) moderately phosphorous iron (and steel), containing about 0.08-0.22% P, smelted from syderite, c) high phosphorous iron (and steel), containing 0.18-1.0% P, smelted from bog ores. In antiquity, the haematite was smelted in big metallurgical center in the Holy Cross Mountains, and syderitic ore – in Silesia (Opole). Bog ores were exploited in all territory of Poland. During migration time, the ancient metallurgical center applying haematite was destroyed and the very low phosphorous iron (steel) disappeared. The sideritic ore was probably smelted again not before the late Medieval Time, near Czestochowa. The Slavic tribes used bog ores and smelted high phosphorous iron and steel of low quality.