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A record of about 450 years of geomagnetic secular variation is presented from a single archaeological site in Lübeck (Germany) where a sequence of 25 bread oven floors has been preserved in a bakery from medieval times until today. The age dating of the oven‐floor sequence is based on historical documents, 14C‐dating and thermoluminescence dating. It confines the time interval from about 1300 to 1800 A.D. Paleomagnetic directions have been determined from each oven floor by means of 198 oriented hand samples. After alternating field as well as thermal demagnetization experiments, the characteristic remanent magnetization direction was obtained using principal component analysis. The mean directions of 24 oven floors are characterized by high Fisherian precision parameters (>146) and small α95 confidence limits (1.2°–4.6°). For obtaining a smooth curve of geomagnetic secular variation for Lübeck, a spherical spline function was fitted to the data using a Bayesian approach, which considers not only the obtained ages, but also stratigraphic order. Correlation with historical magnetic records suggests that the age estimation for the upper 10 layers was too young and must date from the end of the sixteenth to the mid of the eighteenth century. For the lowermost 14 layers, dating is reliable and provides a secular variation curve for Germany. The inclination shows a minimum in the fourteenth century and then increases by more than 10°. Declination shows a local minimum around 1400 A.D. followed by a maximum in the seventeenth century. This is followed by the movement of declination about 30° to western directions.

Abstract:Quartz is the datable component in mortar. Although its luminescence properties are very well studied, the problem of mortar dating arises from a low level of optical bleaching. In order to reduce the time consuming efforts for dating lime mortar in monuments by optically stimulated luminescence, we investigated the most suitable methods of dose and dose rate determination, and we explored tests which are suitable to predict the datability of a given material. Reverting to the large number of publications on sedimentary partially bleached quartz we found linear modulation techniques especially useful, equally we also recommend the determination of the level of optical depletion before starting analyses. Single grain analysis is regarded as an ultimate but infallible tool to date very poorly bleached quartz from mortar. Dose rate measurements need to take into account the radioactive equilibrium of the uranium decay and the short range inhomogeneity of the environmental gamma radiation field: gamma spectrometry and on-site TLD measurements are best suited.

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Thermoluminescence (TL) is the name given to an effect observed when certain minerals give off light created by natural radiation. Some of these minerals are contained in clay, and the effect occurs upon firing of the clay. The time elapsed between such firings can therefore be measured and serves as a reliable dating method. It is well established in the fields of archaeology (where it is used extensively for authenticity testing) and geology. Until recently this method has not been used in the discipline of architectural history. Here we propose specific adaptations of this method to this discipline and demonstrate the inherent limits in its accuracy. We examined a number of Venetian villas dating from the 15th to the 17th centuries, with three goals in mind: 1. To establish this method in architectural history by cross-checking it against well dated buildings. 2. To try and date buildings of unknown age. 3. To try and distinguish individual building phases of one and the same building. The results obtained confirmed all hopes placed in the method. In this paper we discuss in detail, some of these results exemplifying all three goals cited above; and we give a brief explanation of the scientific background and the sampling procedure.