Magnetic investigations of Iron Age slags at Yderik, Denmark: Mineral magnetic comparison to UK slags

SW Jutland, Denmark, was a major centre for iron-smelting from 2nd to 7th centuries AD. With no visible remnants of the iron-smelting above ground, magnetic surveying of a field at Yderik detected more than 50 buried slags, each the product of a furnace used only once. Excavation and sampling resulted in 92 orientated core samples, all of which were magnetically cleaned to determine stable remanent directions. Comparison of the average directions for five slags with the magnetic secular variation curve for Denmark was unable to provide accurate archaeomagnetic dates for the slags. However, systematic changes in direction suggest that the large errors are due to the greater influence of already cooled slag than the Earth’s magnetic field on cooling slag. Susceptibility measurements suggest that the slag is composed of up to 13% magnetite by mass. Anhysteretic and isothermal remanent magnetisations indicate that single-domain to multi-domain grains are present, suggesting different cooling rates within the slags. High temperature susceptibility curves have been compared with slag samples from five sites in England. The English slag samples are produced by a different process and using a different ore source, which may be the cause of the variations in magnetic behaviour.