Studio archeometrico di ceramiche rinvenute a Pompei (scavi I.E. 1980-81): provenienza e tecnologia di produzione

Abstract

The following work focuses on the archaeometric characterization of different pottery classes found in Pompeii during the I.E. excavations in 1980-1981. These investigations, carried out in the western part of the Forum, during the installation of the electric cables (I.E.= impianto elettrico), revealed numerous useful materials for the reconstruction of the first stages of the city. The ceramic, is certainly the most attested material and its study, allows to obtain archaeological information on many aspects of the past, as chronology, trade and technology. The analyzed ceramics, were selected by three major groups: common ware, black-glaze pottery and votive material. The choice of the samples was not random, because these ceramics classes were widespread in everyday life and in the ancient Mediterranean trades. Archaeometric study aimed to investigate ceramic samples to obtain information related to tthe Pompeii production, its social organization and the possible exchanges of finished objects. The results allow us to reconstruct ancient trades, in which Pompeii was great protagonist. The common ware, including cooking ware (jars and pots) and plain ware (jars and bottles), is certainly the most attested production. The great variability of shapes and fabric, in particular for cooking ware, suggested a wide diffusion, in Pompeii, of products, not only of local production. The first part of the study involved the petrographic, mineralogical and geochemical characterization of local cooking ware and dated in a wide time span (IV-I century BC). The petrographic comparison between two groups, dating to IV-III century BC and II-I century BC, evidenced, the presence of same mineral phases and fragments of volcanic rocks, related to Somma-Vesuvius activity. The differences, between two groups, were related to higher content of the coarse fraction for younger products (II-I century BC), suggesting changes in the production technology. We could hypothesize the use of non-calcareous clay with the addition of temper for the production of the oldest ceramic and the employment of weathered volcanic deposits for the later typology. Technological differences related to firing temperatures, are also showed. The presence of micas and hematite, detected by XRD analysis, indicates firing temperatures ranging between 800-900°C for both groups (Riccardi et al., 1999 and Cultrone et al., 2001). A greater variability was found in the II-I century BC group in which the absence of micas, in some samples, allowed to indicate slightly higher temperatures. The second part of the research focused on the comparison of II-I century BC pottery with two coeval groups of pottery (called group c and group d), characterized by the same ceramic form but with a different fabric. The petrographic analysis distinguished a volcanic fabric for a group c and a granitic fabric for the group d. For the latter one, considering the different geological characteristics of the Campanian area, it’s possible to indicate as possible provenance area, Calabria and Sardinia. However further analysis are necessary to confirm these hypotheses. From the mineralogical point of view, group c showed a temper similar to the local group (group b). Plagioclase, sanidine, clinopyroxenes, amphibole, biotite and a large amount of volcanic rock fragments, were observed in both groups. The differences are related to the increased presence and variability of clinopyroxenes in the group b and on the contrary of higher content of sialic minerals (plagioclase, sanidine and quartz) in group c. In group b, the quartz was detected only by XRD analysis. On the basis of petrographic characteristics, it is not possible to exclude the local origin for the group c, while the different chemical composition detected by XRF analysis does not allow to advance other hypotheses. In general, the mineralogical association, plagioclase, sanidine and pyroxene, is very common in volcanic areas, both in the Campanian magmatic Province (Somma-Vesuvius, Campi Flegrei and Roccamonfina) and in the Roman co-magmatic Province (Vulsini, Vico and Sabatini) characterized by similar volcanism (KS and HKS). The Alban hills are excluded because the products do not contain plagioclase and sanidine (Gaeta et al., 2006). Anyway, the traditional comparison with local raw materials, to have indications of provenance, is particularly difficult and not very useful, considered the coarser fabric of the ceramics. Therefore micro-analysis on individual volcanic phases (clinopyroxenes), were performed on the pottery wares. This method was considered as a good discriminant tool in recent works (Comodi et al., 2006; De Rosa et al., 2007, Barone et al., 2010). The clinopyroxenes of the ceramics in groups b and c were compared with clinopyroxenes of igneous rocks of the Campanian volcanic districts (Somma-Vesuvius and Roccamonfina) and Roman districts (Vulsini, Vico and Sabatini). The pyroxenes analyzed in both ceramics and rocks, were classified on the basis of Morimoto et al.,1988, as diopside. Some trace elements: V, Sr, Zr, Y and REE (rare earth), detected by LA-ICP-MS, were selected to discriminate each group of ceramics, the pyroxenes of the Roman co-magmatic Province and of Campanian magmatic Province. The comparison between the pyroxenes of ceramics with those contained in the rocks, by statistical processing (binary diagrams and multivariate analysis, PCA) confirms the compositional similarity between the pottery of group b (considered local production) with pyroxenes of Somma- Vesuvius and Roccamonfina, and the similarity of the group c with pyroxenes of Roman districts. Therefore this method, was perfectly valid in this study, because allowed us to confirm the locally provenance for the group b and to exclude it for the group c. Moreover, a possible area of origin for the group c was also suggested. The black glaze pottery is dated in a wide time span (IV -I century BC) and it was considered on the basis of archaeological evidence (presence of misfired potteries and furnace spacers) of local production. The possible furnace of production is not been found yet and the archeological attribution of the provenance, on the basis of the very fine fabric, did not give sure information. For this reason, archaeometric analyzes were performed in order to investigate local productions and to clarify the technological changes for over three centuries. Two different groups of ceramics dated to the IV-III century BC and II-I century BC were selected for the analysis. No compositional differences were detected by petrographic and chemical (XRF) analysis. Conversely, the study on black glazes by SEM/EDS and by Raman microscopy (performed at the Department of Chemistry, University College London, UCL), evidenced mineralogical and morphological variations clearly due to a different control of the firing conditions. The early ceramic samples (4th–3rd century) are characterized by a sintered coating in which small crystals of iron oxide content are widespread. Magnetite, hematite and hercynite represent the mineralogical phases recognized by Raman microscopy. The later production is characterized by a completely vitrified glaze containing only magnetite. The differences are due, probably, to the different firing practice, in particular as regards the reducing step, that is, the values of the temperature and period of duration of this phase and the cooling rate (Scarpelli et al., 2014). Finally, in order to confirm the local origin, the possible raw materials used for the production of this ceramic class, were sampled and compared chemically by binary diagrams. On the basis of archaeological indications (Peña and McCallum, 2009) and the geological characteristics of the studied area, clayey materials were individuated in the Salerno province, at Ogliara and at Montecorvino Rovella. These calcareous clayey sediments (15% of CaO), belonging to the Mio- Pliocene sedimentary sequence of the basin of Salerno, show a large compositional similarity with the analyzed ceramics, confirming the hypothesis of Peña and McCallum. Petrographic comparison with the experimental tests of clay fired at 900 °C, confirms these similarities and a greater depuration for the oldest group of ceramics. These results allowed to attest, for the first time, a Pompeii production of black glaze ceramic that, could be considered as reference group; secondly these suggested some considerations relating to the management of ancient quarries in the area of Salerno, which were probably used by potters in a quite extensive area. The study on the votive material is in a preliminary phase yet. Thymitheria, figurines and miniature vessels, characterized by different forms and fabric, were analyzed. On the basis of archaeological assumptions these materials could be considered as local production of workshops (not yet identified) for devotees. Petrographic analyzes have confirmed the compositional differences between the three groups but also inside the same group, and allowed us to indicate the employment of more raw materials (fine clays, clays and volcanic temper, weathered volcanic deposits) for these products. These results could ascertain the presence of more ceramic ateliers specialized in the production of different potteries or, simply, verify the supplying of clayey materials from different sources in the Vesuvian area.