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Measurement in the laboratory

The laboratory measurements of the samples are usually carried out using a spinner magnetometer, which determines the direction of the magnetic field recorded within the material. The measurement process can be divided into three stages:

  • The initial measurement of the samples determines the natural remanent magnetisation (NRM). This relates to the archaeological signal plus the signal held by less stable magnetic particles, referred to as the viscous component. The less stable component needs to be removed to produce an accurate date for the archaeological event of interest.
  • The pilot demagnetisation of a subset of the samples determines information about the stability of the magnetic signal recorded within the material, and identifies the point at which the viscous point is removed from the samples. This is carried out using one of two methods: thermal or alternating field (a.f) demagnetisation.
  • The partial demagnetisation of the remaining samples uses the information produced during the pilot study to remove the viscous component from the samples to leave the archaeological signal of interest. The samples can then be re-measured to determine the direction of the archaeological signal recorded by the samples.

This provides the minimum information needed to produce an archaeomagnetic date.