Achaeometrical Research in Hungary II., 1988

PROSPECTING and DATING - Péter MÁRTON: Magnetic directional data for Hungary and their application for archaeomagnetic dating

Archaeomagnetic dating relies on the knowledge of the SV of the local geomagnetic field pentaining to those times when the archaeological feature to be dated may have been abandoned. A fundamental assumption is that the feature was sufficiently baked and has remained in place since its last use. The technique involves the sampling of the feature at the excavation site, preparation of the samples for magnetic measurement, the measurements themselves which include some magnetic cleaning to remove spurious magnetic components and the statistical evaluation. This process reveals the archaeomagnetic direction of the feature sampled, which is eventually interpreted as the direction of the geomagnetic field during the last cooling of the feature in antiquity. In this way each archaeomagnetic directional measurement yields one declination and one inclination value with their respective statistical parameters that characterise the reliability of the result. The dating proper means finding the date on the horizontal axes of the SV curves at which both the measured declination and inclination best match the respective values of the master curves. This will be identified as the date for the feature's last use. The precision of the archaeomagnetic directional dating depends firstly on how well the applied SV-model reproduces the true SV of the geomagnetic field. Therefore the importance of having well dated archaeomagnetic results for the construction of the master curves cannot be overemphasised. The SV itself also controls the precision of dating by its variability in that good precision can be attained during periods of fast variation while the precision will be poor during periods of slow variation. An equally important factor is the precision of the direction to be dated. This depends to a great extent on the distortions of the magnetisation that leads to a greater scatter (less precise definition of the magnetisation) or to some non-parallelity of the magnetisation to the local geomagnetic field during cooling or both. Three examples were selected to demonstrate the application of the archaeomagnetic dating technique. Explanations of symbols are as follows: N is the number of useful archaeomagnetic samples collected, D and I are the mean declination and inclination (with 95% confidence intervals) of the direction of magnetisation, к is the precision parameter (Fisher, 1953). 1./ Lime kiln from Baj-Kecskehegy. Excavation by S. Petényi, Tata Museum, 1996. Estimated age: Recent (S. P.). Floor samples. Primary magnetisation defined after thermal cleaning between 200 and 500°C: N=9, D = -9.4 ±7.0°, 1=68.2 ±2.6°, k = 394. D indicates an age after AD 1600, I indicates one before 1800 and the date at which both D and I match their respective curves in Fig. 1 simultaneously is AD 1700 (most probable age). 2.1 Four ovens from Visegrád in approximately the same stratigraphie position. Excavations by G. Búzás, M. Szőke and A. Pálóczi-Horváth, Visegrád Museum (G. В. and M. Sz.) and Hungarian Agricultural Museum, Budapest (A. P. H.), 1995-96. Estimated age: AD 13

th - 14

th centuries (G. В. and M. Sz.). Floor samples. Primary magnetisations defined after thermal cleaning as indicated at each structure: N = 7, D= 16.3 ±2.9°, 1 = 59.1 ±1.5°, к = 874, ( 300 °C ), 6, 10.7 ±4.9°, 59.5 ±2.5°, 713, (130°C), 7, 11.2 ±7.9°, 61.3+3.8°, 255, ( 400 °C ), 9, 9.9 ±8.9°, 57.5 ±4.8°, 116, ( 500 °C ). 72