Archaeomagnetism is a method of utilising magnetic polar wander to date materials containing magnetic minerals. Archaeological materials that contain magnetic particles are kilns, pots, hearths and most sediments. Heating and cooling such materials (or depositing in air or water in the case of sediments) causes the geomagnetic field to be recorded by the magnetic particles present. This recorded magnetisation can be measured many years later and so give a date that is directly related to anthropogenic activity. The technique can be applied in the last 3000 years in the UK, however, it is not an independent method of dating and requires a reference curve to convert the magnetic direction measured into a date. The geomagnetic field is subject to secular and spatial variation. Secular variation is fundamental to this dating technique but the spatial variation requires that calibration be localised to within 1000km of the calibration curve.
Illustration showing how the magnetic direction has chanced since 1600AD in
London, Paris, Rome and Boston, taken from Aitken, 1990: Figure 9.3
Heating a material to a temperature greater than its Curie point and then cooling it allows magnetic grains to align themselves along the direction of an external magnetic field during cooling to ambient temperature. The grains will now remain orientated in this direction unless they are reheated. Therefore this method is dating the last time a hearth or kiln was heated to over its Curie temperature.
Illustration showing how a kiln acquires magnetic direction, courtesy of Zoe Outram
In order to obtain a date by this method it is necessary to establish true north and the horizontal level on site. Therefore the changes in the recorded magnetisation can be compared to the current geomagnetic field. It is important that the material to be sampled has been heated and remained "in-situ" or undisturbed since it was last heated. The samples then have the recorded magnetic direction (inclination and declination) measured in the laboratory and the results are calibrated to give dates.
Illustration showing how samples are taken, courtesy of Zoe Outram.
From post medieval times the secular variation of the geomagnetic field in the UK has been measured with reasonable reliability and measurements on fired clays dated by other methods have allowed the extension of this time scale into prehistoric times. Due to the spatial variation in the magnetic field all the data are localised to Meriden (52.43°N, 1.62°W) before it could be included in the calibration curve. Clark et al, in 1988 were the first to bring most of the available data together in the British calibration curve. Since then others, for example Batt (1997) and Zananiri et al. (2007), have worked to improve and extend the curve.
The British Archaeomagnetic calibration curve normalised to
Meridian showing 600AD - 1975AD (upper) and 1000BC - 600AD (lower), taken from
Clark et al, 1988.
Batt, C.M. (1997). The British archaeomagnetic calibration curve: an objective treatment. Archaeometry 39: 153-168.
Clark, A.J., Tarling, D.H. and Noel, M. (1988). Developments in archaeomagnetic dating in Britain. J. Arch. Sci., 15: 645-667.
Zananiri, I., Batt, C.M., Tarling, D., Lanos, Ph. and Linford, P (2007). Archaeomagnetic secular variation in the UK during the past 4000 years and its application to archaeomagnetic dating. Physics of the Earth and Planetary Interiors, 160, 97-107.
Tarling, D.H., 1975. Archaeomagnetism: the dating of archaeological materials by their magnetic properties. World Archaeology 7: 185-197.
Tarling, D.H., 1985. Archaeomagnetism. In Rapp, G. and Gifford J.A. (eds.) Archaeological Geology: 237-263.
Batt, C.M. and Noel, M., 1991. Magnetic studies of archaeological sediments. In Budd et al. (eds.) Archaeological Science 1989: 234-241
Linford, P. (2006). Archaeomagnetic dating: Guidelines on producing and interpreting archaeomagnetic dates. English Heritage, Swindon.