Only materials that contain the right magnetic minerals in sufficient quantities can record the Earth's magnetic field, such as clay and stone. They must not have been disturbed since the deposits were formed (i.e. the last time a feature was heated, or the sediments were deposited).
There are several factors that can be used to indicate if the material is likely to represent a 'good' feature for archaeomagnetic dating, which is based on the work of Jeffrey Eighmy (1990).
If a feature appears to be well-fired with no signs of disturbance, then it should be sampled for archaeomagnetic dating. However, it is not possible to determine on site if the material contains the right magnetic minerals or if they are in sufficient quantities to record the Earth's magnetic field. This can only be determined in the laboratory. It should therefore be noted that some features that may look promising cannot record the stable magnetic signature necessary to produce a date.
|Clear evidence of firing||This may be shown through the colour of the material, from red to black, or the presence of heat cracked stones and vitrified material. The structural form and function of the sampled feature may also indicate if it has been heated.|
|Well-fired||The sampled material needs to have been heated over approximately 400°C. There should be evidence to suggest that this has occurred, such as vitrified material and heat-cracked stones. The heat may also have penetrated downwards through the feature, resulting in a thick layer of heated material.|
|Colour||The main magnetic minerals responsible for recording the magnetic field relate to the hematite and magnetite families of minerals. Hematite is red and magnetite is black in colour.|
|Clay content||The magnetic minerals are frequently found in clay. An assessment of the composition of the material to be sampled will indicate the proportion of clay present.|
Water lain deposits
The following aspects need to be considered when sampling a deposited material for archaeomagnetic dating:
|Well stratified deposits||The samples must be collected from a discrete context to ensure that the sampled material was formed at the same time.|
|Clear boundaries||The boundaries separating contexts should be clearly defined. This indicates that the material has not been disturbed since they were deposited.|
|Wetness||The deposits cannot be wet, as high water content in a deposit may allow the magnetic particles to move and therefore realign with the modern magnetic field.|
The deposits selected for dating should show no signs of disturbance and so need to be thoroughly assessed before the samples are collected. It is important to note that even if the deposit appears to be undisturbed it may not return an archaeomagnetic date if the right magnetic minerals are not present in sufficient quantities to record the Earth's magnetic field. This can only be determined in the laboratory.
In order to produce an archaeomagnetic date the material must not have been disturbed, and the magnetic field recorded by the material must not have changed since it was last heated or deposited. The processes that may damage the feature or that may change/affect the magnetism recorded within the sample include:
- If the feature is significantly cracked
- If the feature is visibly slumped
- The activity of plants and animals (bioturbation) near or on the feature may also disturb the material.
- If a feature becomes too dry it can be prone to cracking and result in the general disturbance of the feature.
- If a feature becomes too wet, or even waterlogged, the magnetic particles within a material may be free to realign with the modern magnetic field. It is important to note any changes in the water-table or if the feature shows any signs of being waterlogged in the past.
- If the feature does not contain the right magnetic minerals, or if they are in too low concentrations to record the Earth's magnetic field. This cannot be determined visually in the field, but in the laboratory after the samples have been collected.
It is not always possible to determine if a feature has been disturbed. It is easier to identify the original footprint of formal features, such as hearths or kilns, as the limits are normally defined by a kerb or the kiln wall. It is therefore more obvious if the material has been disturbed in some way in antiquity. Less formal features, such as areas of burning, may not relate to a strict structure and it will therefore be harder to identify if the material has been disturbed.
In order to identify and overcome the possibility that a feature has been disturbed, samples should be collected from all areas of the feature so that the range of directions recorded by the samples across it can be assessed. Anomalous directions may indicate that the material had been disturbed.
The risks of a feature becoming disturbed or drying out can be decreased if it is covered once it has been exposed. This will alert people working on the site to its location, reducing the risk of someone accidentally walking over the material. It also prevents the feature from drying out.