Archaeomagnetic study of medieval fireplaces at Mannheim-Wallstadt and ovens from Herrenchiemsee (Southern Germany) and the problem of magnetic refraction

Abstract

Ten fireplaces from Mannheim-Wallstadt (MW) and four ovens from Herrenchiemsee (HC) were studied with regard to their archaeomagnetic properties. The ages of the medieval structures were only poorly known and an age dating was intended with the help of the standard curves for the declination and inclination of the geomagnetic field of the past 2000 years, as set up by Thellier (1981) for France. The stability of the NRM was tested with Thellier's test. Of the ten MW fireplaces, only one passed the test, whereas of the four ovens of HC, all passed the test but one of them had to be excluded because of secondary displacements within the structure. The MW fireplace could be dated to 670-700 A.D., in agreement with other archaeological age determinations for the fireplaces. The ovens from HC yielded an age between 1100 and 1170 A.D. The ovens showed the effect of magnetic refraction. Model calculations carried out on circular ring structures require a susceptibility of the oven material of the order of 0.5 SI units to explain the observed effect. The presently observed mean susceptibility at room temperature is only around 5 x 10-3 SI units. However, heating of the material at 550° C for 2 h in a reducing environment was able to increase the susceptibility by a factor of 20. This effect is explained by the reduction of secondary iron oxides and hydroxides to magnetite. Another increase by a factor of about 1.5 is obtained from the Hopkinson effect (increase of susceptibility with temperature) at the blocking temperature of magnetite. Combining all effects, it is possible to attain values for the susceptibility at the blocking temperature of magnetite as high as 0.2 SI units, which is the order of magnitude required for the explanation of the observed effect of magnetic refraction. This effect demands that special considerations be made during archaeomagnetic sampling from archaeological ovens or kilns.           ARK: https://n2t.net/ark:/88439/y016135 Permalink: https://geophysicsjournal.com/article/117 &nbsp