Analytical Methods:Magnetics

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Questions
Magnetic remanence measures the effect much larger magnetic fields than those used in magnetic susceptibility have on minerals in the sample. These larger fields can cause permanent alteration of the magnetic behaviour of certain minerals. These changes can provide additional information, compared to magnetic susceptibility alone, about the nature of the iron compounds that make-up a sample. This can provide information about the source of the sediments and the degree of alteration by fire and / or soil forming processes.

Case studies where magnetic remanence has been used in geoarchaeology include:
 * Evidence for Medieval soil erosion in the South Hams region of Devon, UK.
 * Allochthonous and autochthonous mire deposits, slope instability and palaeoenvironmental investigations in the Borve Valley, Barra, Outer Hebrides, Scotland.

Samples
If possible plastic, rather than steel, implements should be used for sampling. Samples should be taken from a representative part of the context, being careful not to bulk together different deposits. Due to high background variability replicate samples are also a good idea. Samples should be taken from the 'natural' parent material and from throughout the profile; not just from the context of interest.

Samples can be measured wet or dry. If samples are to be stored before measurement they should be air-dried and sieved to less than 2 mm. It is important that samples are not heated to more than 40oC (Collinson, 1983; Walden, 1999a). Typically samples are packed into standard plastic pots of known volume.

Analysis
The basic equipment required includes a susceptibility meter (operation described here), a magnetometer, a pulse magnetiser and an a.f. demagnetiser.


 * Magnetometer - This is used to measure the remanent magnetisation.
 * Pulse magnetiser - Induces remanence in samples (Isothermal Remanent Magnetisation - IRM) by exposing them to magnetic fields of known strength. Usually the strength of the field is gradually increased and the IRM measured until the Saturation Isothermal Remanent Magnetisation (SIRM) is reached.
 * Demagnetiser - Removes Natural Remanent Magnetisation (NRM) from samples, for palaeomagnetic dating demagnetisation can be used to remove secondary NRM formed through post-depositional processes and to leave the primary NRM that is used in dating (Walden, 1999b). If the demagnetiser has an ARM (Anhysteretic Remanent Magnetisation) attachment. This can be used to induce ARM, another magnetic characteristic which can beused to study the mineraology.

Data and interpretation
As for magnetic susceptibility the data is corrected for mass to poriduce mass specific values. The IRM/SIRM ratios; this effectively normalises data between samples allowing direct comparison, and it shows the the proportion of a samples remanence that is acquired in relatively high or low magnetic fields (Walden, 1999b).

Data analysis usually involves multivariate statistical techniques such as cluster analysis and factor analysis in order to identify the contribution of different mineral types to the samples magnetic characteristics.

Data interpretation is a skilled task requiring substantial experience. Ensure that the specialist knows what the main archaeological questions are before beginning the analysis so that the final report is tailored to your needs.

Related techniques

 * Magnetic susceptibility

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