Fuel use in medieval iron production in central Jutland, Denmark

Knowledge about medieval iron production in Denmark is very limited. However, recent excavations in the area around Silkeborg in central Jutland, Denmark, have led to the discovery of several slag heaps and furnaces testifying to considerable iron production in this part of the country. Charcoal from slag heaps at six sites has been analysed to learn about fuel use in iron production. At two sites from the 12th-13th century, many species were used as fuel. Only 50–63% of the fuel consisted of high-density wood with a high caloric value, which at these sites included species such as birch, elm and pomaceous fruit. At the other four sites from the 14th-15th century, species diversity was smaller, and species with a high caloric value constituted 85–93% of the fuel, with beech, oak and birch being most common. Thus, a change in fuel use over time, which possibly could be related to access rights to the high forest, is indicated in the study. Some horizontal and vertical variations within the slag heaps were also recorded. A detailed analysis of vertically sampled charcoal at Gødvad Bygade III showed that the tree species used for fuel changed slightly at some point during the period of iron production, and that the accumulation period was likely 5–60 years. The study demonstrates that there is a great potential in analysing charcoal from iron production sites, as this approach can provide not only information about fuel use but in some cases even about organizational aspects of the production.