Archaeomagnetic dating bradford
If materials have been heated to a sufficiently high temperature (400°C) they may retain a thermoremament magnetisation, which reflects the Earth’s magnetic field at the time of last cooling.
Suitable archaeological features would include hearths, kilns and other fired structures.
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.
In most case the archaeological evidence can be used to select the most likely of these.
In the UK archaeomagnetic dating can be applied to features expected to date from 1000BC to the present day, as this is the period covered by the secular variation curve.
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).
Sediments and friable fired materials are sampled by insertion of 25mm diameter plastic cylinders.
Magnetometers used are sufficiently sensitive for only small samples (c.
The distribution of the archaeomagnetic dates produced on Scottish material.