Blue Bridge Lane
&
Fishergate House
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A total of 12 samples of hard fired clay - sand were removed from Context 2136 for the purpose of archaeomagnetic analysis and dating. Specimens were orientated in situ using the button method, combined with spirit levels and a fluxgate magnetic compass. Demagnetisation tests showed that the magnetisation in the material is rather unstable with an initial rise in intensity upon demagnetisation. One of the samples yielded an archaeomagnetic vector that was clearly anomalous, almost certainly due to movement within the feature during abandonment and burial. The mean archaeomagnetic vector in the remaining 11 samples were compared with the UK Master Curve to suggest last firing occurred either in the date range 1360-1430 A.D. or 1320-1340 AD. This ambiguity has arisen due to the need to rotate the mean archaeomagnetic vector, by an unknown amount, westwards to correct for a strong anomaly in the local magnetic variation. This anomaly almost certainly arose due to steel railings adjacent to the feature, causing an error in the sample orientation.
Open area excavation by Field Archaeology Specialists of a site within the monastic precinct of St Andrew's priory has revealed the base of a medieval pottery kiln. This kiln is situated about 3m north of the precinct ditched and palisaded boundary. The kiln was possible involved in the production of both pots and tile and was found to be heavily truncated by later medieval activity. The age of context 2136 is presumed to be 13th to 15th century.
Sampling via button method with orientation by fluxgate magnetic compass. Local magnetic variation within the hearth was estimated by comparing readings of the magnetic compass with the sun compass during a brief moment of sun's visibility. 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.
| SAMPLE | J | D | I | A.F | D | I | Comment |
|---|---|---|---|---|---|---|---|
| BBL1 | 145.3 | 19.5 | 50.7 | 20 | 19.6 | 56.3 | |
| BBL2 | 2.7 | 18.9 | 53.6 | 20 | 17.3 | 60.2 | |
| BBL3 | 95.2 | 12.1 | 44.6 | 20 | 15.6 | 59.9 | |
| BBL4 | 39.7 | 18.5 | 60.9 | 20 | 20.7 | 61.2 | |
| BBL5 | 98.7 | 19.7 | 32.9 | 20 | 24.4 | 53.5 | |
| BBL6 | 153.3 | 24 | 61.6 | 20 | 22.8 | 59.7 | |
| BBL7 | 22.6 | 19.2 | 44.2 | 20 | 22.5 | 53.4 | |
| BBL8 | 183 | 41.6 | 29.4 | 20 | 42.1 | 38.6 | REJECT |
| BBL9 | 3.5 | 27.6 | 71.5 | 20 | 21.3 | 64.1 | |
| BBL10 | 186.6 | 21.9 | 54.9 | 20 | 27.6 | 59.4 | |
| BBL11 | 237.3 | 24.4 | 55 | 20 | 25 | 60.6 | |
| BBL12 | 0.6 | 29.7 | 60.4 | 20 | 16.1 | 62.7 | |
| MEAN | 21.3 | 59.2 | |||||
| MERIDEN | 20.7 | 57.9 |
MEAN K=421.3 Alpha95=2.2 c.s.e.=1.2
D=declination, I=inclination, J=intensity in units of mAm-ox10-5. A.F.=peak alternating demagnetisation 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:
1360 A.D. B 1430 A.D. or 1320 A.D. - 1340 A.D.
Directions of natural remanent magnetisation in samples from Context 2136 at Blue Bridge Lane, York, shown on an equal area stereogram. In this representation, declination increases clockwise while inclination increases from zero at the equator to 901 at the centre of the projection.
Changes in the direction and intensity of remanent magnetisation in pilot sample BBL3 during stepwise demagnetisation in alternating magnetic fields.
Directions of natural remanent magnetisation in samples from Context 2136 after partial demagnetisation in an alternating field of 20mT. The vector associated with sample 8 is clearly anomalous and was therefore rejected in the final dating analysis.
Comparison between the mean archaeomagnetic vector in the kiln base with the UK Master Curve 600 A.D. to present. Numbers refer to the date in centuries. The error bar is based on the circular standard deviation given in Table 1. Clearly, the vector (red cross) is rotated too far east to intersect the curve: this is almost certainly due to a strong local anomaly in the magnetic variation which has therefore led to an error in the samples' orientation. The red dashed lines correspond to a westward rotation which is necessary to comply with the curve. An ambiguity in the dating then arises, leading to the pair of suggested dates given in this report. It may be possible to resolve this ambiguity by reference to artefactual evidence.
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 geometric vector in antiquity. Intensity dating can only be applied to fired materials which have acquired a thermoremanent magnetisation upon cooling form high temperatures (>6001C) while 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 homogenous 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 synthesized for the period 2000 B.C. to the present (Clark, Tarling & 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 topographical features, the direction of the sun, magnetic or geographic north. For this feature the miniaturised 'button method', illustrated below, was employed (Clark et al, 1988). Modern archaeomagnetic magnetometers are sufficiently sensitive that only small volumes of material (~1ml) are required for an accurate remanence measurement (Molyneux, 1971). This has the advantage of reducing the impact of sampling on archaeological features B 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 & Batt, 1990).
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