Magnetic geophysical mapping of prehistoric iron production sites in central Norway
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The slag pit furnace of the Trøndelag tradition for iron production is a very specific cultural-historical tradition in central Norway in the Early Iron Age, but few of these iron production sites have been excavated in their entirety and there is therefore a lack of information about their size, spatial layout and organisation in the landscape. The aim of this paper is therefore to investigate how magnetic geophysical methods can be used as a way of locating, delimiting and characterising activity zones and specific archaeological features associated with this tradition of iron production. The NTNU University Museum in Trondheim performed geophysical surveys of four different iron production sites, combining topsoil volume magnetic susceptibility measurements and detailed fluxgate gradiometer surveys. Analysing and comparing the survey results with sketches and topographic survey results, as well as comparable geophysical survey data from iron production sites elsewhere in Norway, made it possible to gain new and valuable cultural-historical and methodological knowledge. The topsoil volume susceptibility measurements revealed a strong contrast between the main production areas and the natural background measurement values, often in the range of 7–27 times the median background values. The absolute highest measured values were usually in the area closest to the furnaces, and within the slag mounds. Satellites of high readings could be interpreted as roasting sites for iron ore, and even areas with known building remains related to the iron production sites had readings stronger than the median. The fluxgate gradiometer data helped to characterise individual features further, with strong geophysical contrast between features within the iron production sites and the areas surrounding them. Also, by analysing their physical placement, geophysical characteristics such as contrast, magnetic remanence and size, it was possible to gain further insight into the spatial organisation by indicating the potential location of furnaces, the spread of slag and the handling of iron ore. The latter involved both where the roasted iron ore was stored and where it was roasted. The geophysical characteristics of the furnaces were less uniform than situations reported elsewhere in Norway, which can be explained by the reuse of furnaces and slag pits. The spread of highly remanent material in and around the furnaces and elsewhere within the limits of the iron production sites also created a disturbed magnetic picture rendering it difficult to provide an unambiguous archaeological interpretation of all the geophysical anomalies identified. In conclusion, these results showed that the geophysical methods applied made it possible to indicate the physical size, layout and internal spatial organisation of iron production sites of the Trøndelag slag pit furnace tradition.