Heat-induced Modification of Marine Shells Used as Personal Ornaments at the Prehistoric Site of Franchthi Cave, Greece: First Results of a Multianalytical Approach

The prehistoric site of Franchthi Cave located in the south-eastern Argolid in Greece yielded an exceptionally rich collection of personal ornaments. A reassessment of this category of material culture by two of us (CP and MV) led to the hypothesis that one type of personal ornaments, i.e. marine shell beads belonging to the species Cyclope neritea, were intentionally heated to change their natural whitish colour to black. In order to identify possible diagnostic criteria for intentional heating based on the physical properties of the shells we submitted two modern Cyclope neritea shells, one of which unmodified, the other experimentally blackened through heating, and two archaeological specimens, one presenting a natural whitish colour, the other black, to complementary analytical methods: optical (OM) and scanning electron microscopy coupled with an X-ray analysing system (SEM-EDX), infrared (FT-IR) and microRaman spectroscopy as well as differential scanning calorimetry (DSC). Results show that heated shells can be identified based on their microstructure and chemical composition and that the black archaeological shell positively matches the modern shell blackened through heating. Identification of the limited conditions, in which blackening through heating occurs, further supports a special heat-treatment for Cyclope neritea shells at Franchthi Cave. INTRODUCTION Franchthi Cave is a prehistoric site located in the South-eastern Argolid which yielded an over 11 meters stratified sequence of deposits spanning from the early Upper Palaeolithic to the Final Neolithic [2, 4]. One of the materials used as personal ornaments at this site is composed of seashells belonging to the species Cyclope neritea. The shell of this species has a diameter of ca. 1 cm, the dorsal side is orange-pastel-coloured and the ventral side ivorycoloured. An ongoing reassessment of the personal ornaments of Franchthi Cave by two of us (CP and MV) reveals that besides naturally bright coloured archaeological shells (fig. 1b), other specimens display a much darker colour (fig. 1c,d) suggesting they might have been blackened through heating [5]. The aim of this study is to identify possible diagnostic criteria for intentional heating of the shells through the use of a variety of analytical methods that allow characterising their micromorphology, structure and chemical composition by limited sampling. MATERIAL AND METHODS In order to identify possible diagnostic criteria for intentional heating based on the physical properties of the shells we submitted two modern Cyclope neritea shells, one of which unmodified (fig. 1a), the other experimentally blackened through heating (fig. 1c) and two archaeological specimens, one presenting its natural whitish colour (fig. 1b), the other black (fig. 1d), to complementary analytical methods. 9th International Conference on NDT of Art, Jerusalem Israel, 25-30 May 2008 For more papers of this publication click: www.ndt.net/search/docs.php3?MainSource=65