Information for archaeologists

Archaeomagnetic dating can be done using either the direction or the intensity of magnetisation (or better both) of burnt materials. When these were originally heated in antiquity, they acquired a direction of magnetisation (magnetic remanence) in the same direction as the Earth’s magnetic field (also called geomagnetic field) at that site at that time. This remanence also has an intensity of magnetisation that is proportional to the strength of the magnetic field at that time. As the Earth’s magnetic field gradually changes both, direction and intensity, the direction and intensity of samples from the site can be dated by comparison with known direction and field intensity records for past times at that locality.

    Such magnetic dating therefore depends on both the reliability of the sample observations and that of the known recorded direction and intensity of the Earth’s magnetic field during archaeological times. Observatory measurements of the direction of the Earth’s magnetic field only commenced around 1600 AD, while intensity records only started in 1835 AD. Consequently older dates depend on having available archaeomagnetic directional and intensity records from previous studies of well-dated archaeological sites. Understandably, archaeologists usually request magnetic dating for sites that cannot be adequately dated by other means! One of the major requirements for archaeomagnetic dating is therefore to obtain more observations from well-dated archaeological sites. Fortunately, such well-dated sites can come from a wide region – within some 600 km of the site being investigated. This is because the Earth’s magnetic field is fairly uniform (and therefore predictable) over an area of some 1 000 000 km². It also makes archaeomagnetic dating unique in that, the more data that becomes available, the more precise the record becomes and hence this dating method is continually increasing in precision.

    Another important consideration is that this technique is extremely good at testing the synchroneity of “magnetic” events within this 1 000 000 km² region. Two sites, e.g. burnt destruction levels several hundred km apart, that were fired at the same time will have identical directions and ancient field intensities. That is, of course, identical within the limits of the technique. However, these can be quite precise and are independent of the actual age. For example, Minoan destruction sites (LMIB) in central Crete have identical directions and ancient intensities as the ‘Minoan’ ash levels on Santorini, 120 km to the North, enabling the synchroneity of the events to be established within some 10-20 years some 3 500 years ago. If the corresponding directions and ancient field intensities can be established within the Egyptian chronology in Lower Egypt or ancient Greece, then absolute dating could be determined within the Egyptian or pre-Hellenic chronologies.

Here you find an example of an archaeomagnetic dating

Archaeomagnetic reconstructions/restorations are based entirely on the directional magnetic properties of samples from a site. They are based on the structure or object having been magnetised in a relatively uniform Earth’s magnetic field at the time that it was heated. Each sample, e.g. a potsherd, will have a direction of magnetisation; all samples directions were originally aligned when the original pot was fired and so the pot, in this case, can be re-assembled by re-aligning each shard magnetisation. (Needs illustration - coming soon). Similar samples from a floor would all have preserved the direction of magnetic North when they cooled after being heated (e.g. in a kiln or burnt structure), or samples from a wall will all have the same direction of North and the same magnetic inclination (Needs illustration - coming soon).

Shards from a spherical pot can therefore be uniquely reassembled. Samples from floors and walls can be at least oriented in their original positions. (It should be stated that such magnetic methods only supplement the standard methods of reconstruction and are usually only practical when, for example, only a few non-contiguous shards are available; these may well enable the shape of the pot to be assessed.) This technique has particular significance in determining the original orientation of key elements in a structure, e.g. the location of wedges in a smelting system.

Another example is that when most Roman coins were hot-dipped (silvered), this was apparently undertaken while the plane of the coin was vertical. Similarly, the orientation of statues, when originally cast, can be assessed (but usually using samples from their burnt clay core).

Geophysical surveying

Archaeomagnetism & geophysical surveying are interlinked as the sources of magnetic anomalies, in particular, are directly related to areas that have been heated and hence magnetised in the past magnetic field. Anomalies associated with such magnetisation have different orientations compared with the magnetisation induced in these materials by the present-day geomagnetic field at the time of the survey. The separation of such anomalies is therefore vital for more realistic interpretations of the anomaly patterns and may even allow an estimation of the possible magnetic age of some structures under ideal conditions.

Environmental analysis

Archaeomagnetism & environmental analysis are similarly interlinked as the magnetic properties of soils, for example, are highly dependent on the oxidation conditions to which they have been subjected. Soils that have been burnt during deliberate or accidental firing of the vegetation have distinctly different properties, enabling the soil wash from such area to be identified in drainage areas, etc.

The example below shows coercivity spectra (of magnetic remanence) for one and the same material, but baked at different temperatures and under different oxidation conditions. The samples originate from the wall of the combustion chamber of a Roman pottery kiln near Bruyelle (Belgium), which was dug into loess. The blue graph represents the spectrum of non baked loess (source material).

When the source material is baked under oxidising conditions, as met in the interior of the wall (~ 65 mm away from the inner side of the combustion chamber), chemical reactions occur, altering the magnetic mineral assemblages present in the sample. Consequently, the coercivity spectrum changes considerably, as it is displayed by the red graph. The remanent magnetisation is about 14 times stronger than those of the source material (blue graph). Laboratory re-heating experiments indicate that this sample had not been heated above 450 °C.

Much higher temperatures (often around 1000 °C or above) are reached inside the combustion chamber of a kiln. The surface of the kiln wall is in contact with the fuel, and a mainly reducing atmosphere is present. Under these conditions another type of magnetic mineral assemblages (see black graph) is produced during baking. The remanent magnetisation is about 170 times higher that those of the source material.

Figure D: Coercivity spectra of unbaked loess (blue), of baked loess under oxidising conditions (red) and of baked loess under reducing conditions (black). The spectra are displayed normalised (= the area below each graph equals 1) and on a logarithmic field scale. The thickness of the graphs corresponds to the measurement error. Data from Spassov and Hus (2005).