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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).
There are concentrated in the Iron Age, mainly reflecting the use of archaeomagnetic dating in the investigation of vitrified hillforts (Gentles 1989) and within long-running research projects.
Such investigations have the advantage of being able to integrate archaeomagnetic studies with other methods to produce significant advances in archaeological understanding (e.g. However, there is a dearth of archaeomagnetic dates from commercial archaeological investigations when compared with England.
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.
In most case the archaeological evidence can be used to select the most likely of these.
Given the number of contributing factors it is not possible to define the general precision of archaeomagnetic dates but there will be an error margin of at least ±50 years.
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.
Once a stable magnetic direction has been obtained, this is dated by comparing it with the secular variation curve showing changes in the Earth’s field over time (Clark 2007).
The secular variation curve is compiled from direct measurements of the field which extend back to AD1576 in Britain, and, prior to that, from archaeomagnetic measurements from features dated by other methods.
Because archaeomagnetic dating usually requires laboratory personnel to collect samples on site, the lack of Scottish laboratories makes the method both expensive and unable to respond rapidly, except within the framework of planned research excavations.
It is clearly advantageous to raise the profile of archaeomagnetic dating in Scotland.
However, as discussed below the precision of the date obtained will vary according to the period.