RESEARCH Archaeological and Anthropological Sciences (2024) 16:177 https://doi.org/10.1007/s12520-024-02087-1 Until recently, archaeological evidence for medieval iron production has been very sparse. In the past decade, several sites with traces of iron production have nevertheless been found in the Silkeborg area, especially during developer- funded excavations. The finds include eight iron smelting furnaces and several slag heaps, i.e. heaps of accumulated waste related to iron production such as slag, charcoal, and fragments of iron smelting furnaces. The newly acquired archaeological material provides a unique opportunity to shed more light on medieval iron production. In the past few years, several lines of evidence includ- ing assessment of the archaeological material and written sources, radiocarbon dating, and metallurgical and charcoal analyses have been investigated as part of the research proj- ect “Iron production in central Jutland during the medieval period and Renaissance” (Christensen et al. 2023; Nielsen et al. in prep). In this article we present the results of charcoal analyses from six different sites that provide valuable infor- mation about the types of wood used, species variability within and between sites, and how organized the iron pro- duction was. It is the first published anthracological study of fuel use related to Danish medieval iron production. Introduction Medieval iron production in Denmark During the medieval period and subsequent Renaissance, most of the iron utilised in the area of modern-day Denmark was imported from Norway, Scania and Halland (Sweden) (Buchwald 2005; 2015). However, written sources state that iron production based on local bog iron took place on a relatively large scale in central Jutland, especially in the area around Silkeborg and north towards Viborg (Buchwald 2015; Jouttijärvi 2021).  Jannie Koster Larsen jhl@moesgaardmuseum.dk 1 Department of Archaeological Science and Conservation, Moesgaard Museum, Højbjerg, Denmark 2 Museum Silkeborg, Silkeborg, Denmark 3 Department of Physics and Astronomy, Aarhus AMS Centre (AARAMS), Aarhus University, Aarhus, Denmark Abstract 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 consid- erable 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 hori- zontal 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. Keywords Iron production · Charcoal analysis · Slag heaps · Medieval period · Denmark · Bayesian analysis Received: 28 June 2024 / Accepted: 30 September 2024 © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2024 Fuel use in medieval iron production in central Jutland, Denmark Jannie Koster Larsen 1  · Nina Helt Nielsen 2  · Jesper Olsen 3 1 3