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Archaeomagnetic Analysis of a Roman(?) Kiln/Drying Oven, F159, Nosterfield
Summary
A total of 13 samples of fired stone and 1 of fired clay were removed from F159 for the purpose of archaeomagnetic analysis and dating. Specimens were oriented in situ using the button method, combined with spirit levels and a sun compass. Demagnetisation tests showed that the magnetisation in the material is highly stable. The mean archaeomagnetic vector in the samples was compared with the UK Master Curve to suggest that last firing occurred in the date range 100-170A.D.
Magnetic Dating Report
| SITE NAME: | Nosterfield Tarmac Quarry | LOCATION: | Nosterfield, N. Yorks. |
| SITE CODE: | N0S02 | COORDINATES: | 54.2°N 1.6°W |
| SAMPLING DATE: | 10/6/02 | SITE CONTACT: | Annette Rowe |
| CONTEXT: | F159 Fired stone & clay | FEATURE TYPE: | Roman(?) kiln/dryer |
Site/Context Description
Feature was discovered during a watching brief by Field Archaeology Specialists during topsoil stripping of a proposed extension to the Tarmac aggregate quarry at Nosterfield. It comprises a kiln or corn drying oven, about 1.2m long by about 0.8m across, constructed of stone mortared with fired clay. A disuse fill which dished into the feature contained sherds of probable Roman date, thus providing a terminus ante quem for the feature.
Analytical Methods
Sampling via button method with orientation by sun compass. Archaeomagnetic remanence measured using a Molspin fluxgate spinner magnetometer and stability assessed using stepwise, alternating field demagnetisation. Secondary components of magnetisation removed by partial demagnetisation. Mean of selected vectors computed (with unit weights) and corrected to Meriden. Comparison then made to the UK Master Curve to obtain a last -firing date. Further details of technical methods are contained in the Appendix.
Results
| SAMPLE | J | D | I | >A.F. | D | I | Comment |
|---|---|---|---|---|---|---|---|
| NOS1 | 209.7 | 356.4 | 55 8 | 2 5 | 352.6 | 56.3 | Stone |
| NOS2 | 48.1 | 11.6 | 690 | 25 | 12.7 | 68.3 | Stone |
| NOS3 | 145.3 | 349.2 | 61 6 | 2 5 | 347.9 | 62.6 | Stone |
| NOS4 | 82.6 | 353.3 | 67 5 | 2 5 | 358.3 | 67.0 | Stone |
| NOS5 | 123.0 | 353.2 | 79 0 | 2 5 | 354.7 | 80.2 | Stone |
| NOS6 | 77.6 | 350.0 | 61.3 | 2.5 | 350.4 | 60.3 | Stone |
| NOS7 | 247.3 | 8.0 | 66.5 | 2.5 | 6.0 | 66.1 | Stone |
| NOS8 | 1.8 | 348.3 | 60 3 | 2 5 | 346.7 | 63.6 | Stone |
| NOS9 | 34.5 | 12.2 | 711 | 2 5 | 10.9 | 69.9 | Stone |
| NOS10 | 25.1 | 20.0 | 76 3 | 2 5 | 16.3 | 74.1 | Clay |
| NOS11 | 63.1 | 15.2 | 67.1 | 2.5 | 15.5 | 67.1 | Stone |
| NOS12 | 84.9 | 354.1 | 672 | 2.5 | 351.4 | 66.9 | Stone |
| NOS13 | 58.5 | 356.9 | 602 | 2.5 | 356.8 | 61.4 | Stone |
| NOS14 | 129.5 | 354.8 | 629 | 2.5 | 335.3 | 62.8 | Stone |
| MEAN K=110.7 Alpha95=3.8 c.s.e. =2.1 | 357.0 | 66.6 | |||||
| MERIDEN | 357.1 | 65.3 | |||||
D=declination, I=inclination, J=intensity in units of mAm-1x10-3. A.F.=peak alternating demagnetising field in milliTesla. K=precision parameter, c.s.e.=circular standard error, alpha95=semi-angle of the 95% cone of confidence.
Estimated date range for last firing: 100 A.D. - 170 A.D.
FIGURE 1: Directions of natural remanent magnetisation
in samples from F159 at Nosterfield, shown on an equal area stereogram.
In this representation, declination increases clockwise while inclination
increases from zero at the equator to 90° at the centre of the projection.
FIGURE 2: Changes in the direction and intensity of remanent magnetisation in pilot sample N0S4 during stepwise demagnetisation in alternating magnetic fields.
FIGURE 3: Directions of natural remanent magnetisation in samples from Fl59 after partial demagnetisation in an alternating field of 2.5mT.
FIGURE 4: Comparison between the mean archaeomagnetic
vector in the feature with the UK Master Curve 1000 B.C. to 600 A.D.
Numbers refer to the date in centuries.
The error bar is based on the circular standard deviation given in Table 1.
Appendix: Principles of Magnetic Dating
Magnetic dating is based on comparing the remanent magnetisation in an archaeological structure with a calibrated reference curve for the geomagnetic secular variation. Two distinct methods have evolved. The intensity technique relies on obtaining estimates of the past strength of the Earth's magnetic field while directional magnetic dating uses archaeomagnetic measurements to derive the orientation of the geomagnetic vector in antiquity. Intensity dating can only be applied to fired materials which have acquired a thermoremanent magnetisation upon cooling from high temperatures (>600°C) while the directional method enables the age of a broader range of archaeological materials to be determined. For example, sediments and soils may have acquired a dateable 'detrital remanence' if magnetic grains had been aligned by the ambient field during deposition. The growth of magnetic minerals during diagenesis or as a result of manufacturing processes can also give rise to a magnetisation which may enable materials such as iron-rich mortars, for example, to be dated. However hearths, kilns and other fired structures are the most common features selected for magnetic dating primarily because their thermoremanence is generally strong, stable and sufficiently homogeneous that the ancient field can be determined with sufficient precision from a small set of specimens. An analysis of dated archaeomagnetic directions, largely from fired structures, together with lake sediment and observatory records has enabled a master curve for the UK region to be synthesised for the period 2000 B.C. to the present (Clark, Tarling and Noel 1988).
For directional magnetic dating it is essential to obtain specimens of undisturbed archaeological material whose orientation with respect to a geographic coordinate frame is known. A number of sampling strategies have evolved, enabling specimens to be recovered from a range of archaeological materials with orientations being recorded relative to topographic features, the direction of the sun, magnetic or geographic north. For this feature the miniaturised 'button method', illustrated overleaf, was employed (Clark et al, 1988). Modern archaeomagnetic magnetometers are sufficiently sensitive that only small volumes of material (--1 ml) are required for an accurate remanence measurement (Molyneux 1971). This has the advantage of reducing the impact of sampling on archaeological features - of particular significance if they are scheduled for conservation and display. For dating, all archaeomagnetic vectors are transposed to Meriden, the reference location for the UK Master Curve (Noel and Batt 1990).
Bibliography
Clark, A.J., Tarling, D.H. & Noel, M., 1988. 'Developments in archaeomagnetic dating in Britain' Archaeometty, 15: 645-667.
Molyneux, L., 1971. 'A complete result magnetometer for measuring the remanent magnetisation of rocks' Geophys. J. R. astr. Soc., 24: 429-433.
Noel, M. & Batt, C.M., 1990. 'A method for correcting geographically separated remanence directions for the purpose of archaeomagnetic dating,' Geophys. J. R. astr. Soc., 102: 753-756.