Assessment of clayey raw material suitability for ceramic production in Northern Peloponnese : an archaeometric approach

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Ξανθοπούλου, Βάια
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The aim of this study is to examine possible sources of clayey raw materials, outcropping in the territory of the northern Peloponnese, which were potentially used for ceramic production during antiquity and also to provide a detailed characterization and comparison as to their compositional and technological properties. This was accomplished by the systematic sampling of Holocene, Plio-Pleistocene and Pliocene clayey raw materials (59 samples) from a wide area (31 sites) spanning all over the northern Peloponnese, extending from the region of Aigialeia (northwestern) to the region of Corinth (northeastern). In the case of the ceramic material, our selection was constrained by the recent important discoveries in the site of ancient Helike. The Early Bronze Age material unearthed therein highlights its significant role since then and possibly indicates one of the earliest ceramic productions attested in the area. Coeval archaeological sites do also exist in proximity to Ancient Helike, such as Aigion, or further to the East, such as Ancient Aigeira and Derveni. The ceramic analysis aims to provide a characterization mean for their comparison with the clayey raw materials sampled throughout the area and will help us unveil the early ceramic technology in the area and to make provenance inferences. In order to identify clay materials behavior during firing conditions and to access the eventual mineral phase transformations and use them subsequently as a mean for comparison with the archaeological ceramics, experimental briquettes from the studied raw material were manufactured. The experimental briquettes were fired at three temperatures 700, 900 and 1050oC under oxidizing conditions. In an attempt to characterize fully the raw materials, the experimental briquettes and the ancient ceramics, the integrated approach by using a series of analytical techniques, such as optical microscopy, scanning electron microscopy, cathodoluminescence, thermal analysis and geochemical analyses was indispensable. Clayey raw materials were further examined in terms of technological and physical properties, such as plasticity, grain size distribution and activity. The interpretation and elaboration of the whole data led us to the characterization of the raw materials as calcareous and calcareous-dolomitic clayey sediments and siliceous (only 2 samples). According to their clay minerals content and based on the sampling location were characterized as; Achaean samples: Illitic-chloritic and illitic-chloritic with mixed-phases presence; Corinthian samples: illitic and less chloritic with mixed-phases presence; Xylokastro samples: illitic and less chloritic and smectite with mixed-phases presence. Geochemical analyses revealed no compositional differentiation revealed among the various areas whereas the REE study suggests possible similar sources of origin. Plasticity results allowed us to characterize the studied materials as low and/or moderate plastic materials or silts of low plasticity, a fact that comes in in accordance with the low content of clay fraction and the abundant presence of illite. These data enabled us to infer that they cannot be used untreated as proper ceramic raw materials, but they may be used for large vessels or roofing tiles. Experimental briquettes showed that calcareous fine group and siliceous group resist over time, whereas the experimental briquettes of the calcareous coarse samples collapsed. The archaeometric approach led us to identify 5 main petrographic groups and various small groups and eventually infer about their local provenance. Furthermore the obtained data helps us to unveil the ancient technological processes i.e. levigation or sieving; the technique of tempering (rock fragments or/and grog). The aplastic inclusions as well as the composition (in terms of calcium content) of the clay paste were related with the local lithology, apart from a significant number of the ceramic samples, which even though contained the same inclusions encountered in the rest of the samples, they showed a different composition (non-calcareous). As far as it concerns the firing choices, it is observed the application of low firing temperature through open fires for the large vessels, whereas a wide range of firing temperatures was chosen in the case of tableware. The extensive number of the small groups and loners suggested the household production. We deduced that Helike potters used the same clayey raw material source for about half of the studies samples, but the query raised was about the source of those ceramic samples with the different composition. The characterization of the raw materials showed only one sampling location as potential source for those ceramics and close to Helike site. Another assumption is that the non-calcareous source was possibly depleted through time (naturally or by human activities). In summary the technological features of Helike pottery revealed a skillful pottery craftsmanship in Helike settlement, the existence of different workshops wherein were using different technology and different recipes.
Clayey raw materials, Northern Peloponnese, Ceramic production, Early Bronge Age, Archaeometric research, Early Helladic period, Ancient Helike, Experimental archaeology, Mineralogical analysis, Geochemical study, Ceramic petrography, Ceramic analysis