A description of the archaeological reconstruction of iron production (100 to 700 AD) based upon bog-iron ore and charcoal in Denmark during the Iron Age is given, the total number of furnaces being of the order of 100,000. Examples of magnetic prospection for slags in general and in more detail, including simple interactive interpretation by inclined magnetic dipoles (spheres) are illustrated. Finally the mean magnetic remanent direction determined by a paleomagnetic study of oriented cores from three slag pits, as well as by magnetic inversion of the surface magnetic field from the same slag pits, is used to obtain a magnetic dating of the slag pits by comparison with the geomagnetic secular variation, suggesting that the mean age of the slag pits is between the 2^nd and 5^th centuries AD.