Magnetic investigations and datings of a brick kiln at Veldbæk near Esbjerg (Denmark)

Phys. Chem. Earth, Vol. 23, No. 9-10, pp. 1015-1019. t998

Pergamon

© 1998 Elsevier Science Ltd All rights reserved 0079-1946/98/S--see front matter PII: S0079-1946(98)00135-9

Magnetic Investigations and Datings of a Brick Kiln at Veldba k near Esbjerg (Denmark) N. Abrahamsen 1, U. Jacobsen 1, V. Mejdahl I and U. M e j d a h l 2 lDepartment of Earth Sciences, Aarhus University, Finlandsgade 8, DK-8200 Aarhus N, Denmark 2Esbjerg Museum, Ncrregade 25, DK-6700 Esbjerg, Denmark

Received 6 December 1997; accepted 11 June 1998

In relation to the magnetic sampling, magnetic mapping using the total magnetic field as well as the susceptibility were also made to evaluate the potential use of these methods for other archaeological brick structures.

Abstract. Detailed magnetic total field and gradient measurements over remnants of a tile kiln at Veldb~ek (55.46°N, 8.50°E) near Esbjerg in SWJutland (Denmark) have showed marked magnetic anomalies of up to 200 nT, revealing the rectangular shape of the kiln remains. The bulk susceptibility also showed markedly increased values of the kilnarea as compared to the surrounding unheated smeltwater sands. Investigations of the remanent magnetisation of 39 cored specimens from the kiln floor gave a directionally very stable remanence with mean direction (Din, Im)=(331°, 73.7O), k= 131, ags=2.0°. Comparing with the magnetic master curve for the area, the last time of heating for the kiln may be dated to c. AD 1790 ± 40 y. This age is confirmed by TL-datings and historical evidence of increased building activities using baked bricks at this time in the area. © 1998 Elsevier Science Ltd. All rights reserved.

2 Magnetic survey The magnetic anomalies are generated by a complex interplay between the Earth's magnetic field, the magnetic remanence and magnetic susceptibility of the nearby anomalous materials (in casu the kiln), and the geometric shape of these materials. The magnetic susceptibility was measured directly at the cleaned surface by a kappameter (Goofyzika Bmo Model KT-5), whereas the magnetic survey was made by a proton gradient magnetometer (Geometries G856), using two sensors on a vertical rod. The lower sensor was situated 0.3 m above the surface of the excavation (the exposed surface of the kiln floor), and the top sensor was 1.4 m above the bottom sensor. Measurements were automatically stored and later corrected for daily variations by means of repeated measurements on a local base station.

1 Introduction During roadwork in 1995 Esbjerg Museum excavated remnants of an old brick kiln at Veldb~ek ca. 3 km east of Esbjerg (Denmark). Only part of the kiln floor and minor parts of the walls were preserved (Abrahamsen et al. 1997). The age was unknown, but as the structure was undistufl~ it was decided to use the thermoluminescent as well as the magnetic method for dating, as magnetic dating has earlier given fair results for Danish kilng (Abrahamsen 1988, 1991; Abrahamsen & Skaarup 1994).

2.1 Magnetic anomaly Measurements were made along profiles directed towards the North in a grid of 0.5 m x 0.5 m. An area of 16 m x 12 m was surveyed (Figs. 1 & 2). The total magnetic field "It at the top sensor is shown in Fig. 1, the sensor elevation being 1.7 m above the floor. Even at this height the area of burned stones are well depicted; the rectangular shape of the kiln is also detectable, but no detailed structures are seen from this elevation.

Correspondence to: Niels Abrahamsen

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1. MagnetictotalfieldTt at top xmor 1.7m abovekilnfloor. The total magnetic field Tb at the bottom sensor is more sensitive to detailed features and so is the gradient, or more correct the "vertical difference" AT=(Tt-Tb)/Ah (cf. Koppolt et al. 1996), which is shown in Fig. 2. At this close distance from the magnetic sources the rectangular shape of the remnants of the kiln floor is ve~ well recorded, and by comparisons with the plan of excavation there is a good correspondence between excavated structures and the local magnetic anomalies. Especially three clusters of baked natural boulders and the baked clay bricks are magnetically well marked, these parts being the best preserved remnants of the structure. Using the magnetic gradient in stead of only the bottom sensor has the advantage, that possible uncertainties caused by rapid daily magnetic variations or nearby traffic passing by are eliminated even without control by base station corrections.

Fill. Z Magnet imdient a.onudy AT=(TrTb)/L~u~

2.2 Magnetic susceptibility The magnetic susceptibility was measured along 5 profiles(x=5mto9m) inagridof¼mx 1m across the structure, as illustrated in profiles (Fig. 3) and in a circular plot (Fig. 4). The susceptibility varies between 0.1 and 30 x 10"3 SI. Although the susceptibility measurements are of a more local nature than the magnetic anomalies, the limit of the kiln area is well delimited also by the susceptibility. It appears that the contrast in susceptibility between the kiln remnants (typically between I and 10 x 10-3 SI) and the magnetically neutral soil (mostly smeltwater quartz sand with low susceptibility around or below 10"4 SI) roughly varies as the magnetic anomalies from the previously baked kiln material. 12

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3 Remanent magnetization and magnetic dating

The remanence of 39 specimens was investigated in details. The Q-ratio varies between 3.6 and 25; the natural remanent magnetization thus strongly dominates the induced magnetization. Using thermal as well as alternating-field (AF) demagnetization, these experiments show the direction of the remanence to be very stable. All specimens have Curie-points around 5800 C, indicating the dominant magnetic carrier to be pure magnetite, which is also supported by the bell-shaped decay-curve by the AF demagnetization; the median destructive field is around 40 mT Also micromagnetic experiments were made, as shown in Figs. 5 and 6. The hysteresis curves of three differently coloured samples show a range of the Jrs/Js ratio. A red sample (Vb3a) has Jrs/Js= 0.19, indicating mostly multi domain particles, a medium grayish sample (Vb4h) has Jrs/Js= 0.28, indicating pseudo-single domains, and a light gray sample (Vbla) has Jrs/Js= 0.40, close to pure single domain particles). Sample Vb3a is wasp-shaped, whereas Vbla and Vb4h are normal shaped. Using Fisher statistics (e.g. Butler 1992) on the cleaned directions, a mean direction of (Din, Im) = (331 °, 73.7°) is found, with o~5ffi2.0° (radius of the 95% significance circle), precision parameter k=- 131, and the sum of unit vectors R=38.7103. By comparison with the magnetic master curve (Fig. 7) for the geomagnetic secular variation in the area (Abrahamsen 1972, 1996), the kiln may be magnetically dated. The magnetic age (being the age for the last cooling of the heated kiln material) is seen to be around AD 1790 ± 40 y.

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N. Abrahamsen et al,: Magnetic Investigations and Datings of Brick Kiln

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Fig. 7. Magneticdatingof the Veldbeekkiln.The curvesshowtwo versionsof the geomagneticsecularvariationin SW Jutland(Denmark).The boldcurveis a dipole-transfmmationof historicdatafromthe geomagneticobservatoryin RudeSkovnearCopenhagen250 km E of Veldbeek.A somewhatbetter agreementwiththe strongwesterlydeclinationin the kiln is foundwhencomparingwith a centraldipole-transformationof the Bfiti"""~lhistoricand ~ c secularvariationcurve(thinline),tramformedfrom Meridenin UK (Clark et al. 1988),which is situated some 700 km WSW of Veldlmsk(Abrahamsen1996). The numbersindicatecenturies,and the oval showthe 95% significancearea aroundthe magneticmeandirectionof the Veldlnekkiln.Witha magneticdirectionof(D~ I~) = (331°, 73.7°) a magneticage ofc. AD 1790:t:40 y is likely.

4 Discussion and conclusions

The age of the Veldb~ek kiln was originally expected, for archaeological reasons, to be around AD 15001600. If so, the magnetic direction found could only be obtained by a physical disturbance in form of a counterclockwise rotation of some 30-400 of the whole kiln, which was difficult to understand and accept. If (for some unknown reason) the magnetic declination was not reliable, the magnetic inclination might still have been used for magnetic dating. In this case two dates would be possible for the Veldb~ek kiln, either second half of the 16th century, or the late 18th century, when the inclination was around 740 (cf. Fig. 7). Older dates between AD 500 and 1000 would not have been relevant in this case, as the technique of using fired tile bricks were not applied in Denmark before around AD 1160 (Brendsted 1961, Abrahamsen 1992). Fortunately, the Veldbeek kiln was also sampled for TL-dating. The TL-measurements have given a TLage of AD 1793 ± 20 y (Abrahamsen et al. 1997). The two dating results therefore support each other very well, and the initial archaeological expectation of an older age could be questioned.

Renewed studies of local literature (Mejdahl 1996) have recently confirmed, that in the SW part of Jutland there was a boom in the building activities in two periods, between AD 1792-1810 and AD 186070. During these two periods it became much more common than earlier to use fired bricks for buildings in the area. The elder of these two historically evidenced periods thus is in excellent harmony with both the magnetic and the TL date. We conclude that a magnetic remanence determination as well as magnetic surface measurements (total field surveying over the structure and also susceptibility measurements directly on the surface) are useful tools for both dating, detecting and mapping of archaeological structures, which contain baked clays, stones, slag and other magnetic objects. By surveying it is important to use a configuration of the magnetic sensor which is suitably arranged according to the expected dimensions and depth of the magnetic target (Abrahamsen et al. 1996, Koppelt et al. 1996). Acknowledgements. The hysteresisexperimentswerekindlymadeby

Peter Riisager. We also acknowledgeconstructivesuggestionsfrom Uwe Koppoltand LanriPesouen.

N. Abrahamsen et al.: Magnetic Investigations and Datings of Brick Kiln

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