Elsevier

Journal of Cultural Heritage

Volume 10, Issue 1, January–March 2009, Pages 144-151
Journal of Cultural Heritage

Case study
Secondary phosphates in the ceramic materials from Frattesina (Rovigo, North-Eastern Italy)

https://doi.org/10.1016/j.culher.2008.01.008Get rights and content

Abstract

The pervasive crystallization of secondary phosphates in pores and fractures of numerous potsherds from the archaeological site of Frattesina (Fratta Polesine, Rovigo – North-East Italy) indicates that contamination occurred after burial. The chemical composition of these phases, which are Mg-rich vivianite and mitridatite, shows that sources of phosphorus, calcium, iron and magnesium were locally available and that the precipitation and diagenesis of these minerals were strongly influenced by micro-environmental conditions within the archaeological deposit.

Introduction

The presence of secondary mineral phases forming after burial is an important factor in archaeometrical studies of pottery, since it gives evidence of chemical–physical alteration of potsherds, often in relation to contamination processes. This has important consequences on the definition of reference groups [1], [2] for the possible contamination effect on original chemical composition, and on the characterisation of environmental conditions after burial [3]. Therefore, in the study of provenance and production technology of archaeological ceramic materials, physical modifications and chemical alteration after burial have to be verified.

Recent archaeometrical analysis of pottery production of Final Bronze Age–beginning Iron Age (corresponding to XI century B.C.–early IX century B.C., according to traditional chronology) from Frattesina (Fratta Polesine, Rovigo, North-East Italy) [4], [5] revealed the presence of phosphates in many potsherds belonging to various ceramic classes. The nature and origin of these mineral phases can provide important information on production technology, the nature of the raw materials and/or environmental conditions after burial. One primary source for phosphorus may be bone fragments, which were in some cases added to the clay by the potter. The use of bone is attested, for instance, in the Iron Age ceramics from Scandinavia [6], so that the high content of phosphorus in these artefacts is an important source for the formation of secondary phosphates after burial, and does not indicate environmental geochemical contamination. Recently, secondary phosphate aggregates have been observed and interpreted as the product of the interaction between primary bone fragments and groundwater in specific pH and Eh conditions [3]. Phosphorus may also be present in the burial environment, as derived from human wastes. In these cases, analyses of phosphorous content in the soil may identify areas of human activity in archaeological prospecting, and the presence of secondary phosphate in the pottery may indicate geochemical contamination and provide information on the pH and Eh conditions of the archaeological deposit.

Although concentrations of specific components have often been used to identify and model geochemical contamination in pottery [1], [7], [8], [9], [10], [11], [12], these criteria are somewhat subjective, and an approach which combines microstructural study in optical and electron microscopy, microchemical analysis and X-ray diffraction, can therefore provide better evidence of secondary precipitation and/or alteration of specific mineral phases, and of elemental retention and contamination after burial [2], [3], [13], [14], [15].

In the present paper, we determine the composition, nature and origin of the phosphates in the pottery repertoire of Frattesina, studying eight samples representing the various ceramic classes attested at the site from microstructural, microchemical and minero-petrographic points of view.

Section snippets

Geo-archaeological framework

Frattesina is a proto-urban settlement located in the eastern Po Valley (Fig. 1), 16 km south-west of Rovigo, 10 km north of the river Po, 32 km west of the present Adriatic coast line and 6 m above sea-level. This area is characterised by sandy–silty and silty–clayey alluvial deposits, and fluvial and glacial deposits covered by a thick layer of clayey surface alteration. In protohistoric times, the settlement was located along the Po di Adria, an old branch of the river Po [16], [17],

Analytical procedures

All samples were analysed by polarised light microscopy and scanning electronic microscopy (SEM) on a Camscan MX 2500 electron microscope at the Dipartimento di Geoscienze (DG), University of Padova. Petrographic analysis was carried out following the terminology and descriptive scheme proposed by Whitbread (1989) [26]. Textural elements were estimated by means of suitable comparative charts [27] (Müller, 1964). Mineral assemblages were determined by X-ray diffraction (XRPD) on a PANalytical

Results

Under an optical microscope, the samples show various structural features and differences in the lithological and/or mineral composition of the inclusions (Table 1). The groups listed in Table 1 and described here below refer to the various ceramic paste types described by Saracino et al. [4], to whom we refer for complete characterisation of the petrographic features.

Group (A) is characterised by an anisotropic groundmass, oriented parallel to the potsherd walls, bearing relatively rare pores,

Discussion and conclusions

Interpretation of the origin of phosphorus in potsherds often leads to contrasting deductions on the representativeness of bulk chemical data. One source for phosphorus is bone fragments of which are either found in sediments, which may be solubilised in the local acidic conditions of the burial environment [29], or as primary inclusions in pottery, where it may recrystallise into an aggregate of hydroxylapatite during firing over 600 °C [30], [31], [32], [33]. Alternatively, phosphorus may

Acknowledgements

We are very grateful to Prof. Anna Maria Bietti Sestieri (Dipartimento di Beni Culturali, Università di Lecce), to Dr. Simonetta Bonomi and Dr. Luciano Salzani (Soprintendenza per i Beni Archeologici del Veneto) for providing the samples studied here, and to Dr. Paolo Bellintani (Soprintendenza per i Beni Archeologici della Provincia Autonoma di Trento) for useful information about the excavation at Frattesina. We also thank G. Walton, who revised the English text. This study was carried out

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