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Frequently asked questions

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This section addresses the questions that are frequently asked by archaeologists about archaeomagnetic dating:

When and where to go for advice

The English Heritage Science Advisors will be able to answer questions and advise on the use of the technique. Information and contact details for the Regional Science Advisors can be found at http://www.english-heritage.org.uk/server/show/nav.001002003006001. Further information can then be obtained from the archaeomagnetic laboratories in the UK:

 Laboratory  Contact  Contact email
Contact phone
 English Heritage
 Paul Linford
 Paul.Linford@english-heritage.org.uk
 02392 856749
 University of Bradford
 Cathy Batt
 c.m.batt@bradford.ac.uk  01274 233533
 GeoQuest Associates
 Mark Noel
 rockside@manx.net  01624 819364
 Museum of London Archaeology
 Sarah Jones
 molas@museumoflondon.org.uk  020 7410 2287
 University of Lancaster
 Mark Hounslow
 m.hounslow@lancaster.ac.uk  01524 510238
 University of Liverpool
 John Shaw
 shaw@liverpool.ac.uk  01517 943463
 University of Liverpool
 Mimi Hill
 m.hill@liverpool.ac.uk  01519 943460
University of Plymouth Don Tarling D.Tarling@plymouth.ac.uk 01752 233102

There are only a few archaeomagnetic laboratories within the UK and none of these can offer a dedicated commercial dating service. It is therefore important to contact the archaeomagnetists as soon as possible to discuss the opportunities for archaeomagnetic dating on a given site. Ideally, the archaeomagnetists should be contacted if potential features are highlighted following the desk-based assessment of the site, following the guidelines set out in 'Archaeological Science at PPG16 Interventions: best practice guidance for curators and commissioning archaeologists' (available at http://www.helm.org.uk/upload/pdf/archaeological_science_at_ppg16.pdf).


The majority of the archaeomagnetic laboratories require between 2 and 5 days notice to come out to a site. This can vary based on the location of the site, the type of features to be sampled and previous work commitments. It is best to contact the laboratory that will carry out the work as soon as the feature is known about, whether this is in the planning stages or during the excavation. It is preferable to inform the archaeomagnetists about any possible features and the timescales of the excavation so that everything can be organised in plenty of time, even if the excavation of the feature shows that it is not suitable for dating. It may not always be possible for a specialist to visit the site at short notice.

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What precision can I expect?

The Earth's magnetic field changes at varying rates through time, and so a general precision cannot be stated. In periods where the change in direction was rapid, it may be possible to produce a more precise age range, but in the periods where the changes are slower the same measurements may result in a larger age range. A related issue is that during the last few millennia the Earth's magnetic poles appear at times to have reoccupied the same position on more than one occasion (for instance during the first centuries BC and AD). This can cause ambiguity as more than one date range is then possible and independent evidence is require to determine which is correct.

The precisions likely to be achieved for archaeomagnetic dates calibrated with the Clark  et al. (1988) and the more recent RenCurve (Lanos 2004) calibration curves have been summarised in the 'Precision' section of the 'Secular variation and calibration curves' page.

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How much does it cost?

The cost of producing an archaeomagnetic date will vary between the laboratories that carry out the work. An attempt has been made here to present the costs that each of the laboratories charge, but it is important to note that the costs may vary depending on the types of features that are sampled, and the number of samples that are collected from a single site. Some laboratories offer reduced costs if the features help with current research interests or with student projects; check the 'Current projects'  page for further information.

University of Bradford

The University of Bradford divides the cost of archaeomagnetic dating into three sections: sample collection, pilot study and full analysis. This allows the client to have more control over the work that is carried out. For example, if it is revealed after the pilot study that the feature is unlikely to return a good archaeomagnetic date, the clients will be informed and can chose to stop the work at this stage. The cost of the work has been calculated on the basis of an hourly rate of pay and so this may vary depending on the complexity of the features sampled. The costs are summarised for an average feature and are correct as of November 2009:

 Stage  Description  Approximate cost
 Site visit and sample collection
 Consultation, 1 day on site, and consumables. Travel expenses need to be added on to this price
 £230
 Pilot study
 Sample recording, the preparation and consolidation of the samples, the initial measurement and report writing (approximately 2 days work)
 £450
 Full analysis
 Detailed measurements of the samples, resulting in the production of an archaeomagnetic date, and the final report (approximately 3 days work)
 £800

It is important to note that this quote is based on an average estimate. Some features may be quicker to sample and process, and will cost less to process. Other features may be more complicated and require more work, therefore costing more. For a more specific quote, please contact Cathy Batt (archaeomagnetism@bradford.ac.uk) for more details.

University of Liverpool

The Geomagnetism Laboratory at the University of Liverpool offers commercial archaeomagnetic dating using both direction and/or intensity measurements. The work is dependent on the time frame of the work requested and the availability of staff. The figures provided here are based on current estimated costs:

Technique
Approximate cost
 Archaeo-directional dating
 £600 plus 40p a mile travel to the site to collect the samples
 Archaeo-intensity dating
 £900

Archaeointensity experiments are more labour intensive and therefore are more expensive, but have the advantage that can be used on unoriented material and for authenticity testing. Dating errors are in general greater for archaeointensity dating than for directional dating. The best approach is to use the complete vector (directions and intensity).

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How much notice is needed for a site visit?

The majority of the archaeomagnetic laboratories require between 2 and 5 days notice to come out to a site. This can vary based on the location of the site, the type of features to be sampled and previous work commitments. It is best to contact the laboratory that will carry out the work as soon as the feature is known about, whether this is in the planning stages or during the excavation. It is preferable to inform the archaeomagnetists about any possible features and the timescales of the excavation so that everything can be organised in plenty of time, even if the excavation of the feature shows that it is not suitable for dating. It may not always be possible for a specialist to visit the site at short notice.

When contacting the archaeomagnetic specialist it is important to be able to answer some important questions about the potential feature:

  • What type of features require sampling?
  • Is it hard fired, friable or soft?
  • What is the expected date of the feature?
  • How long has the feature been exposed for?
  • How big is the feature?

If it is possible, a photograph of the feature should also be supplied for assessment. The archaeomagnetists can then advise on the likely potential of the feature for archaeomagnetic dating, how long it would take to sample and the level of precision that could be expected from the resulting date.

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How long will it take to collect the samples?

The time required to sample a feature depends on several factors, such as:

  • The type of feature
  • How hard or soft the material is
  • How many features need to be sampled
  • The weather on the day of the site visit

It may be possible to sample two or three features per day, which will include the full recording and planning of the feature. Softer features that are sampled using the tube method generally take less time than harder features that require the use of an epoxy resin glue.

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How long will it take to process the samples?

Once the samples have been brought back to the laboratory they need to be recorded, consolidated and prepared for measurement. The samples can then be measured and a report of the results and calibrated age range can be produced. The length of time that this takes depends on several factors:

Factor
Affect
 Hardness of the sampled material
This will affect how easy it is to trim the material into the correct size and for measurement
 Method used to collect the samples
Samples collected using the tube method generally require less sample preparation time than samples collected using the disc method
 The number of samples/ specimens collected
The greater the number of samples/specimens collected, the longer it will take to prepare and measure the samples
 Anomalous specimens
The identification of any anomalies in the measured samples/specimens requires the cause of the discrepancy to be identified. For example, was the feature disturbed or does the anomaly relate to the underlying magnetic mineraolgy of the sampled material?

In general, it can take between 3 to 5 days to process a feature depending on the factors described above. A report can then be written in 1 day. More complicated features may take slightly longer, but this can be discussed with the archaeomagnetic specialists.

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What sort of features can be sampled?

A range of features can be sampled for archaeomagnetic dating but can be generally grouped into heated deposits and sediments. The materials must contain magnetic minerals to record the Earth's magnetic field - in archaeological contexts typically clays and most types of rock. The material must not be disturbed and be in situ. Examples of what should and should not be sampled have been summarised in the 'What can be sampled' section.

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