From Bloomery Furnace to Blast Furnace : Archeometallurgical Analysis of Medieval Iron Objects From Sigtuna and Lapphyttan, Sverige

Abstract: During the Early Middle Ages, the iron production in Sweden depended on the bloomery furnace, which up to that point was well established as the only way to produce iron. Around the Late Middle Ages, the blast furnace was introduced in Sweden. This made it possible to melt the iron, allowing it to obtain a higher carbon composition and thereby form new iron-carbon phases. This study examines the microstructure and hardness of several tools and objects originating from archaeological excavations of Medieval Sigtuna and Lapphyttan. The aim is to examine the differences in quality and material properties of iron produced by respectively blast furnaces and bloomery furnaces. Both methods required post-processing of the produced iron, i.e. decarburization for blast furnaces and carburization for bloomeries. These processes were also studied, to better understand why and how the material properties and qualities of the items may differ. The results show that some of the studied items must have been produced from blast furnace iron, due to their material composition and structure. These items showed overall better material quality and contained less slag. This was concluded because of the increased carbon concentration that allowed harder and more durable structures such as pearlite to form. The study also involved an investigation of medieval scissors, also known as shears, made from carburized bloomery furnace iron. Here, one specific aim was to find out if the different sections of the shears had different properties, and if so, if these properties correlated with the functions of the different parts of the shears. Our microstructure analysis showed that the blade indeed was the hardest part due to intentional carburization and forming of martensite. The blade is connected to a softer ferritic handle, which in turn is connected to a ductile bow, also ferritic but with a larger grain size.