Mineralogical And Chemical Investigations Of Bloomery Slags From Prehistoric (8th Century Bc To 4th Century Ad) Iron Production Sites In Upper And Lower Lusatia,Germany

More than 400 fayalitic bloomery slags from prehistoric iron production sites in Upper and Lower Lusatia, eastern Germany, as well as bog iron ore samples and intermediary samples of the smelting process, were analysed by chemical and mineralogical techniques. While the precursor bog iron ores exploited in the two regions under investigation were very similar in composition, consisting of low‐manganese/low‐barium as well as high‐manganese/high‐barium types of ore, pronounced differences in slag composition were detected. Slags from 17 investigated sites in Upper Lusatia showed average P₂O₅ contents between 1 and 3 mass%, whereas slags from 15 investigated sites in Lower Lusatia were generally much richer in phosphorus, reaching values as high as 7 mass% P₂O₅. Since a reasonable correlation exists between calcium and phosphorus contents in the slags of the latter sites, it is conjectured that deliberate addition of CaO to the ore/charcoal charge of the bloomery furnace may have taken place in order to fix the phosphorus in the slags effectively. In many samples, this conjecture is being supported by the detection of a slag mineral Ca–Fe phosphate Ca_9?xFe_1+x(PO₄)₇ that presumably crystallized from a residual phosphorus‐rich melt and shows a cotectic relationship to both Ca‐rich fayalite and wustite, as well as to members of the solid solution series magnetite–hercynite.