Metal Analysis of Gilded Brooches from Viking Age Birka in South-central Sweden

Abstract

Metal Analysis of Gilded Brooches from Viking Age Birka in South-central Sweden

The Viking Age town of Birka is the best-known archaeological site in Sweden and inscribed in the UNESCO’s World Heritage List. This paper presents results from metal analyses of 24 gilded, oval brooches from the cemeteries at the town. They were excavated between 1874 and 1895 and, with one exception, date to the 10th century. The analysis with SEM / EDX revealed that the bulk metal was either pure copper or a copper alloy with low amounts of tin, zinc, lead and iron. Four objects were analysed for their trace elements with PIXE. The gold layers of eleven brooches were examined with a field emission microscope. The layers were rough, with a thickness in the range 1-10 μm (1 μm = 0.001 mm). They contain around 80 % gold (percent by weight), in addition to smaller amounts of silver and copper, and 10-16 % mercury from a fire gilding process. Lead isotope data indicate that one brooch contained lead originating from the Harz or Erzgebirge in Germany. Two other brooches contained lead from different mines in continental Europe. The results contribute to the knowledge of Birka as an important trading centre with advanced techniques in the workshops.