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1cm3) to be required; approximately 15 samples are needed from each feature and it may be possible to select sampling location to minimise the visual impact if the feature is to be preserved.
In the laboratory the remanent magnetisation of each sample is measured in a magnetometer and the stability of this magnetisation evaluated by alternating magnetic fields or thermal demagnetisation (Linford 2006).
A thorough examination of the data was performed to assess their quality and reliability.
Various techniques were employed in order to use the data to construct a secular variation (SV) record: moving window with averaging and median, as well as Bayesian statistical modelling.
The strengths of archaeomagnetic dating are that it dates fired clay and stone, for example hearths, kilns, ovens and furnaces, which occur frequently on archaeological sites; it dates the last use of features, providing a clear link to human activity; it is cost effective and is potentially most precise in periods where other dating methods, e.g. Archaeomagnetic dating is based on a comparison of the ancient geomagnetic field, as recorded by archaeological materials, with a dated record of changes in the Earth’s field over time in a particular geographical area, referred to as a secular variation curve.
The geomagnetic field changes both in direction (declination and inclination) and in strength (intensity) and archaeomagnetic dating can be based on either changes in direction or intensity or a combination of the two.
Two examples of dating of archaeological structures, medieval and pre-Roman, are presented based on the new SV curve for the UK and the implications for archaeomagnetic dating are discussed.
Sediments and friable fired materials are sampled by insertion of 25mm diameter plastic cylinders.
Magnetometers used are sufficiently sensitive for only small samples (c.
Archaeomagnetism is a method for dating fired materials and sediments from archaeological sites, based on changes of the Earth’s magnetic field in the past.
The principles of the method are well established; see Linford (2006) and Zananiri et al. It has been used in Scotland from 1967 (Aitken and Hawley 1967) and is increasingly part of multi-method site chronologies.