Archaeomagnetic sampling of a burnt feature during excavations on the Viking Unst Project. (Images: University of Bradford)
There is a plethora of dating techniques in the archaeologist’s arsenal. Many are used quite frequently and feature prominently in archaeological research, like radiocarbon dating or dendrochronology; others remain outside the mainstream, like potassium-argon dating. Somewhere in the middle lies archaeomagnetic dating. In this month’s ‘Science Notes’, we take an in-depth look at this methodology, highlighting recent advances in reliability and its future prospects in UK archaeology.
The archaeomagnetic method is based on the principle that the earth generates a magnetic field that varies in both direction and intensity over time. Some naturally occurring minerals – many of which are commonly found in soil, clay, and rock – have an inherent magnetisation. When these minerals are heated above a critical temperature – known as the ‘blocking temperature’, which varies depending on the mineral – the inherent magnetisation of the material disappears. When cooled, it remagnetises to reflect the magnetic field of that time and location. This is called a thermoremanent magnetisation (TRM).
When fired remains are analysed archaeomagnetically, the results reflect the last time the material was heated. Non-portable structures, such as kilns, hearths, and furnaces, are the best subjects, as the artefact must remain in the same position as when it acquired its TRM in order to accurately determine the archaeomagnetic direction.
The intensity of the TRM can also be assessed – a technique that could be used on objects that have been removed from their original archaeological context – but, unfortunately, this is trickier and has not been much attempted in Britain. It is hoped that further advances will makes this a more practical method in the near future.
Archaeomagnetic sampling of a pottery kiln, showing samples taken by the button method from walls, floor and pedestal.
Once the archaeomagnetic direction of a structure has been determined, it is then compared to a calibrated curve of archaeomagnetic shifts throughout history. This is known as the secular variation (SV) record. These shifts are determined through a mixture of historical recordings of the geomagnetic field and links with other forms of dating, primarily radiocarbon.
Magnetic fluctuations are not always universal, however, and some magnetic changes can be quite localised in nature. This makes it imperative to create separate calibration curves for different regions. The calibration curve for the UK was first created in the 1960s, but, as with all scientific techniques, new advances have begun to make this original curve obsolete. Other curves were created in 1988 and 2007, but they still lacked data points, meaning that some periods were not well defined. Because of this weakness, archaeomagnetic dating has not emerged as a widespread form of dating, even in instances when it could prove incredibly useful.
The University of Bradford and Historic England have set out to rectify this limitation by creating a more comprehensive SV curve incorporating as many locations and time periods as possible, making the dating technique more effective and usable. They have compiled all the archaeomagnetic UK dates available, from the 1950s to present, and updated the information where possible. The resulting database, Magnetic Moments of the Past, is now the largest collection of independently dated archaeomagnetic directions from a single country. It allows for much more accurate results and can be easily used by non-specialists.
Because archaeomagnetic dating has not been widely used, however, there are still locations and time periods for which even this new curve is lacking. For instance, much of the data is from southern England and, although the curve has added a substantial amount of data, the early Iron Age and early medieval period are still underrepresented. This paucity partly reflects a sparse archaeological record, but also the small number of sites investigated using the method. It is hoped that this form of dating will be more widely considered in future excavations, so that many more data points can be added, steadily making the curve ever more robust.
The paper outlining this new calibration curve can be accessed for free from the Journal of Archaeological Science (https://doi. org/10.1016/j.jas.2017.07.002). To learn more about the processes involved in archaeomagnetic dating, Historic England provides guidelines, which can be accessed here: https://historicengland.org.uk/images-books/publications/archaeomagnetic-dating-guidelines/.