Metallurgical analysis of some osmund iron from Sweden and Estonia : A short historical review of medieval iron production and export

Abstract: During the Middle Ages, Sweden was a coveted exporter of high-quality iron in Europe. Bloomery furnaces could have produced osmund iron. However, most osmund iron was produced in blast furnaces. The iron was then treated in the finery process and cut into pieces. Previous studies establish osmund iron’s definition regarding properties, microstructure and trace elements. They were often slag-rich and varied greatly in carbon content, proportion of inclusions, corrosion and microstructure. The report examined osmund iron as related to medieval iron production, exports and quality. This was done by comparing the microstructures and slag inclusions in Swedish and Estonian pieces of medieval iron, through analyses by SEM-EDS and under an optical microscope. The Estonian samples had more slag inclusions. The samples with corroded inclusions/slag were rejected. The varying microstructure can be linked to the cooling rate and the actual production in the blast furnace, bloomery furnace and finery process. It gave materials with poor properties. A large proportion of the samples had one side with a higher carbon content while the other had lower carbon content. The iron with lower carbon content could be connected to come from the bloomery furnace and the higher to the blast furnace. The samples containing higher levels of silica, magnesium and calcium could be connected to the blast furnace. In summary, the Swedish samples were of better quality than the Estonian ones and all pieces were considered to come from the blast furnace.