Absolute dating of historical buildings: the contribution of thermoluminescence (TL)

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1 Journal of Neutron Research, Vol. 14, No. 1, March 2006, Absolute dating of historical buildings: the contribution of thermoluminescence (TL) M. MARTINI* and E. SIBILIA INFN and Università degli Studi di Milano Bicocca, via R. Cozzi 53, Milano, Italy Thermoluminescence (TL) dating of ceramic materials is nowadays a consolidated and powerful archaeometric technique. The Luminescence Dating Laboratory of the Department of Material Science (Milano Bicocca University) started its activity in this field in 1980 and since then several thousands of samples have been successfully dated. In recent years, TL dating has been extensively and systematically applied to bricks sampled from historical buildings spanning from Roman period to the late XIX century. This particular application of TL dating technique is presented, and its potential and specificity are illustrated and discussed. Keywords: Archaeometry; Thermoluminescence dating; Building archaeology; Ceramic materials 1. Thermoluminescence dating The possibility of dating ceramics objects by the use of their thermoluminescence (TL) properties was firstly proposed by Grogler et al. [1] and Kennedy and Knopff [2] and then extensively studied by the group headed in Oxford University by Martin Aitken [3,4]. We refer to Aitken s books [5,6] for a thorough analysis of TL dating. Here, we aim to present the basis of TL dating, limiting this to the extent that it should enlighten the reader to its potential, its limits and the sources of uncertainty, specifically focusing on architectural dating. Dating by TL is a particular application of TL dosimetry in which there is a source of constant irradiation, the natural radioactivity of the ceramics and the surrounding environment, the activity of which can be independently determined. The duration of irradiation is taken to be the same as the age of the ceramics, and proportional to the amount of the TL signal. Of course, it is essential to have an initial zeroing of the TL signal, this generally being provided by the making of the ceramics itself, the high temperature reached by the furnace during the manufacture of the object erasing any previous TL signal by emptying all of the electronic traps. We stress this point because in some cases ceramics, during their manufacture, were not cooked at temperature high enough for trap emptying, as in for instance by sun heating or by placement over a fire. In these cases the period of irradiation does not coincide with the age of the ceramics and the results of TL dating will be incorrect. In the case of TL dating, ceramics can be considered to consist of a number of crystalline inclusions (mainly quartz and feldspar) embedded in the ceramic matrix. The inclusions act as dosimeters of the irradiation which arises principally from the natural radioactivity (U and *Corresponding author. m.martini@unimib.it Journal of Neutron Research ISSN print/issn online q 2006 Taylor & Francis DOI: /

2 70 M. Martini and E. Sibilia Th series, and 40 K) of the ceramic material. This natural radioactivity is the source of the internal dose, given by alpha and beta irradiation, which, in a typical ceramics, is responsible for around the 70 80% of the total absorbed dose. The remaining external dose is given by the environmental gamma rays and by cosmic rays. Being the decay times of the natural radioactivity extremely long compared to the archaeological times, both the internal and external doses are practically constant over historic periods. It is then possible to determine the total amount of dose absorbed by year by measuring the concentrations of natural radioactivity of the ceramics and the level of environmental irradiation. Conventional techniques for radioactivity measurements and new dosimetric techniques, specifically designed for TL dating are used [5,6]. The total dose absorbed since the making of the ceramics is obtained through TL dosimetry on the crystalline inclusions. Various methods of measurement of the total absorbed dose have been studied, of which the main ones, the so-called fine-grain [7] and inclusion [8] techniques, consider grains of different sizes extracted from the ceramics. In these measurements, important dosimetric aspects relative to the absorption of radiation of different kinds (alpha vs. beta and gamma) and the attenuation due to water content of the sample are to be taken into account [5]. A further aspect to be considered is the presence of spurious TL, non dose-dependent emission, frequently associated to the presence of carbonates [9]. Another adverse phenomenon sometimes observed is the anomalous fading [10], an irregular depletion of deep traps population at room temperature. The method of obtaining TL ages is represented by the simple age equation: Age ¼ palaeodose dose rate where the palaeodose is the total absorbed dose since the last heating at high temperature. It is calculated from a comparison between the natural TL, that has been produced by the irradiation due to natural radioactivity, and the artificial TL induced by laboratory irradiation with calibrated artificial sources. From what has been presented, a few conclusions can be drawn regarding the possible applications of TL dating: a relatively large number of measurements are necessary (TL dosimetry of crystalline inclusions, artificial irradiation, radioactivity and dosimetric measurements). As a consequence, to reduce statistical uncertainties, it is convenient to date a large number of coeval items (anyway, not less than three); and even if most of the irradiation comes from the ceramic itself, the contribution from the environment must be known are to minimise uncertainties. The overall error is given by the contributions of the uncertainties in all the measurements of the palaeodose and the dose-rates, leading to a Gaussian distribution and to the use of the standard deviation s. TL dating is performed on at least three coeval samples, and the precision of the result is given as the standard deviation of the mean. The present accuracy of TL dating is between ^5 and ^10% of the age; only in limited favourable cases it can be as low as ^4% [5]. 2. Archaeometric dating in architecture Since the extension to architecture of the stratigraphic techniques specifically developed for archaeology, the relative internal sequence of the various building phases of a monument can

3 Absolute dating of historical buildings 71 be usually precisely determined. Their absolute chronology is, however, sometimes problematic or controversial. In such cases, resorting to absolute dating techniques like chronotypology or mensiochronology is often useful and decisive. Nevertheless, situations in which a whole building or parts of it cannot be surely dated are not infrequent, and the contribution of the main archaeometric dating techniques like radiocarbon, dendrochronology and TL could be extremely useful. It must be remembered, however, that care has to be taken when associating the archaeometric age of a sample to that of the structure it belongs to [11]. These techniques in fact do not date the building of a monument: dendrochronology gives the year when a tree was cut, radiocarbon the death of a living organism and TL the firing of a brick. The results, therefore, refer to events which surely happened before the construction. If the mean life of the objects prior their utilisation is approximately known, longer times cannot be excluded, due to economical or political reasons. Moreover, accidental anthropic actions can distort the natural clocks of the archaeometric dating techniques. Fire (in case of TL dating) and organic contamination of wood and charcoal (in case of radiocarbon dating) lead to an age underestimation. Voluntary human actions (rebuilding, transformation, decay and restoration) can modify the position of datable artefacts (beams and bricks) in the stratigraphic sequence of a building. The reuse was normal and obvious, especially during the middle age, and it must be remembered that taking care of churches, palaces, public buildings and private houses was a regular and promoted practice in the past, much more than we do nowadays. In case of reuse of material from demolitions, dates are older than the building; in case of upkeep or mimetic restoration, dates are younger than the building. 3. TL dating in architecture Having stated that the previous considerations must be always kept in mind when interpreting any dating results, the contribution of archaeometric techniques to the study of ancient buildings is in any case very important, particularly for what concerns TL dating of brick structures. The main advantages of this kind of application are: 1. the availability of large quantities of material; 2. the homogeneity of environmental radioactivity; and 3. the low influence of environmental humidity fluctuation. 1. In case of large brick structures, an abundant sampling is generally possible, both in quantity and number. As a consequence, the good thermoluminescent components of the clay can be selected, increasing the TL signal vs. noise ratio. Moreover, any additional repetition of the analyses can be easily performed, and the number of dated samples can be as high as wanted, enhancing precision and reliability of the results. 2. In the case of historical buildings, the structures are usually made up of bricks purposely manufactured: the results is a remarkable homogeneity of the radioactive local field, that can be more precisely evaluated than in case of buried samples, with a reduction of the overall error. 3. In an open-air brick structure, at least at height unaffected by the drawing up of water from the foundations, the humidity of the samples depends only on the humidity of the air, and seasonal variations are appreciable only in the first 10 cm of depth. If the core of the bricks is sampled for dating, any present and past humidity fluctuation can be practically neglected, again reducing the error.

72 M. Martini and E. Sibilia These considerations lead to the conclusion that TL dating in architecture should give higher precision than in excavation archaeology.

4 72 M. Martini and E. Sibilia These considerations lead to the conclusion that TL dating in architecture should give higher precision than in excavation archaeology. This result has been confirmed by a statistical analysis performed on about 1300 ceramic samples submitted to our laboratory for dating over the last 10 years [12], whose distribution according to typology is shown in figure 1. It could be appreciated that overall errors lower than 6% are much more frequent when dating buildings than excavated samples, as shown in figure 2. In almost all cases, TL data allowed the solution of vexed or problematic questions relative to the age or to the internal chronology of a monument. We have sometimes had the opportunity to compare our results with those obtained on coeval material independently dated by radiocarbon or dendrochronology and to measure samples already well dated on archaeological or historical grounds. As a consequence of this comparison and calibration, the precision and accuracy of our experimental procedures were regularly checked and improved, and the reliability of TL dating techniques itself was confirmed once more [13]. We briefly present the results obtained on five Italian historical buildings. All samples were dated using the fine-grain technique [7]. The Baptistry of Novara. This valuable example of palaeochristian architecture in the Po area was probably built during V century (documents are contradictory). Five bricks certainly related to the first building phase and another four possibly associated to a later earlymedieval addition have been measured. The bricks are dated, with high precision to V century AD (455 ^ 80 AD, the overall error being 5% of the age), all belonging therefore to the original building phase. The results and some experimental details are listed in table 1. The Asinelli Tower in Bologna. The time of construction of the Asinelli Tower, a symbol of the town together with the Garisenda one, was a vexed question. Forty bricks, collected at different levels from the external side of the tower, even with horizontal carrots, have been dated to ascertain the original building age and to remove doubts about the complex restoration history of the monument. The older bricks of the tower are dated to the second half of the XI century, which represents the probable original building phase (1070 ^ 60 AD). The upper part is about two centuries younger. The dates of the central sector (1420 ^ 40 AD), meaningless at first sight, were very likely related to a big fire which occurred in 1412 and which destroyed the wooden balcony that formerly linked the two towers. The Abbey of Polirone (S. Benedetto Po). The monastery of San Benedetto was founded in 1007 over the remains of an abandoned Roman and early medieval settlement. It soon became an important religious and cultural diffusion centre, enlarged and altered several times until the decline followed the plague that spread and raged over Northern Italy during During recent excavations in the main church, important remains of medieval brick structures have been found. Dating them should have ascertained the age of the former abse and of a wall, both surely preceding the building of the monastery itself. All samples turned Figure 1. Distribution of samples dated according to typology.

5 Absolute dating of historical buildings 73 Figure 2. Error distributions for samples from excavation sites (a) and historical buildings (b). out to be reused, and specifically Roman (II IV century AD): the question put by scholars could therefore not be answered. The Abbey of Pomposa (Codigoro). Built during VIII century, the Abbey of Pomposa had its maximum development in the IX century. The cloister has a later polymaterials decoration with wonderful coloured panels whose dimensions are close to those of the II century Roman bricks locally used. This, therefore, made the hypothesis reasonable that the ornamentations were carved on Roman reused bricks. Quite unexpectedly, the seven bricks analysed, all reused, were not Roman, but dated from VI to XIII century [14]. This suggested the existence of kilns located nearby, active in a period when the local economy was instead supposed to be on the wane. Table 1. Novara Baptistry, TL dating. Mean and single results. Samples TL date (AD) Palaeodose (Gy) Doserate (mgy/year) Mi99-No1(a g) 455 ^ 80 Mi99-No1a 445 ^ ^ ^ 0.26 Mi99-No1b 465 ^ ^ ^ 0.22 Mi99-No1c 450 ^ ^ ^ 0.23 Mi99-No1d 435 ^ ^ ^ 0.23 Mi99-No1e 455 ^ ^ ^ 0.22 Mi99-No1f 460 ^ ^ ^ 0.28 Mi99-No1 g 460 ^ ^ ^ 0.24

6 74 M. Martini and E. Sibilia The Basilica of S. Lorenzo in Milano. Rising opposite sixteen Corinthian Roman columns remaining of an old III century temple, is the best-preserved witness of the deep-seated Roman and early Christian tradition of the city. It represents a mixture of styles and elements, result of restorations and overlapping of different styles. Lacking of precise epigraphic and literary sources, the monument puts a number of interpretative questions regarding both its original building and the chronology of the later phases. In the frame of an interdisciplinary research program particularly focussed on the definition of the absolute building chronology, an intensive TL dating project has been performed on samples directly and surely associated with specific architectural contexts. Whenever possible, radiocarbon dating was performed on organic samples found in the same structures selected for TL dating. We could, therefore, obtain highly reliable chronological information for all the buildings of the complex [15], sometimes obtaining unexpected results, not always totally in agreement with the Milanese archaeological and literary tradition. 4. Conclusions Building archaeology has demonstrated that, even in the best conserved monuments, records and sure traces of frequent interventions do exist, and that it is possible to analyse them stratigraphically. Is it, therefore, possible to establish the relative internal chronology of a building that is the necessary starting point of any further investigation. This allows to identify the main dating problems and, among them, those possibly solvable by the different archaeometric techniques. TL dating to be identified is demonstrated to be a useful and powerful tool, provided that its results are correctly considered, interpreted and used. When dating bricks from a building, the availability of large quantities of material, the homogeneity of environmental radioactivity and the low extent of humidity fluctuation often result in a remarkable reduction of the experimental error. These circumstances allow the distinction between structures or restoring phases even chronologically relatively close to be made. The systematic comparison of TL dating results with those obtained on coeval material independently dated and the measure of samples already well dated on archaeological or historical grounds is highly recommended, in order to check and improve the precision and the accuracy of laboratory experimental procedures. References [1] N. Grogler, F. Houtermans and H. Stauffer, Helv. Phys. Acta. 33 (1960). [2] G.C. Kennedy and L. Knopff, Archaeology 13 (1960). [3] M.J. Aitken, M.S. Tite and J. Reid, Thermoluminescent dating of ancient ceramics, Nature 202 (1964). [4] M.J. Aitken, D.W. Zimmermann and S.J. Fleming, Nature 219 (1968). [5] M.J. Aitken, Thermoluminescence Dating (Academic Press, London, 1985). [6] M.J. Aitken, Science-based Dating in Archaeology (Longman, London and New York, 1990). [7] D.W. Zimmermann, Archaeometry 13 (1971). [8] S.J. Fleming, Archaeometry 12 (1970). [9] M. Martini, E. Sibilia, T. Calderon and F. Di Renzo, Nucl. Tracks 14 (1988). [10] A.G. Wintle, Nature 245 (1973). [11] N. Gallo, M. Martini and E. Sibilia, Proceedings of the Third International Symposium on 14 C and Archaeology Lyon, France, 6 10 April (1998). [12] M. Martini, E. Sibilia and L. Ferraro, Proceedings of the Third International Conference on Science and Technology for the Safeguard of Cultural Heritage in the Mediterranean Basin, Alcalà de Henares, 9 14 July (2001). [13] M. Martini and E. Sibilia, Radiat. Phys. Chem. 61 (2001). [14] F. Bevilacqua, C. Chiavari, C. Di Francesco, E. Migliorini, M. Martini and E. Sibilia, Proceedings of the International Conference on Non Destructive Testings and Microanalysis for the Diagnostics and Conservation of the Cultural and Environmental Heritage Rome, May (1999). [15] A. Galli, M. Martini, C. Montanari and E. Sibilia, Biblioteca. Archeologia. dell Architettura. 2 (2004).

4/4/17. Dating Methods. Dating in Archaeology. These things date to 2500 B.C. (but there are no completion dates written anywhere on or near them)

4/4/17. Dating Methods. Dating in Archaeology. These things date to 2500 B.C. (but there are no completion dates written anywhere on or near them) Dating in Archaeology These things date to 2500 B.C. (but there are no completion dates written anywhere on or near them) Dating Methods One of the biggest questions in archaeology is WHEN? Successfully