An archaeometric contribution to the interpretation of blue-green glass beads from Iron age Central Italy

Abstract

Several types of (mostly) blue-green glass beads from Iron-Age archaeological sites in Central Italy were studied using a range of spectroscopic techniques: portable X-Ray Fluorescence spectrometry, Fibre Optics Reflectance Spectroscopy, Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectrometry, micro-Raman spectroscopy and Laser Ablation Inductively Coupled Plasma Mass Spectrometry. Complementary information was gathered from each technique and discussed in the frame of the archaeological typology of the objects. The systematic evaluation of the results allowed us to draw some conclusions on the raw materials employed for primary production and to highlight some provenance indicators in the glass. Some of the beads found in the Iron Age (IA) contexts were preliminarily attributed to the Final Bronze Age (FBA) production based on their typology, and the compositional data obtained in this work confirmed that they were low magnesium high potassium (LMHK) glass, typical of FBA in the Italian peninsula. Other beads were assigned to low magnesium glass (LMG) or high magnesium glass (HMG), thus giving further information on the fluxing agents employed in the Early Iron Age (EIA) and beyond. Colour variations among the beads reflected their chemical composition, with different bead typologies coloured in a specific way. In some instances, it was possible to establish different origins for the colouring raw materials. The provenance of the samples was difficult to place, but the chemical evidence suggested a subdivision within the raw glass used to produce the beads: for one set of samples, a local origin of the glass could be hypothesised, whereas several production sites in the Near East were suggested for most of the beads considered in this study. Some preliminary clues for the local working of imported glass were also highlighted for one typological group.

The Non-Invasive Characterization of Iron Age Glass Finds from the “Gaetano Chierici” Collection in Reggio Emilia (Italy)

“…The relative concentrations calculated for several key elements allowed some conclusions to be drawn about the origin of the raw materials used to introduce the colorants, distinguishing them from those found in the Late Bronze Age and in other Early Iron Age glasses, for which a comparison was possible thanks to the common analytical procedures and equipment [43,44]. For example, the set of elements that were found to be in association with cobalt (partially iron, copper, and lead) indicates a specific raw material source common to many coeval Mediterranean glasses.…”

“…Cobalt is the main colorant in the objects included in this study (Groups 1, 2, and most of the objects in Groups 3, 5, 8, and 9), and compositional inferences point to the same raw materials as those used in coeval or later Mediterranean glasses [45,46]. They are different from those used in the Late Bronze Age and Early Iron Age, which have a different chemical signature [43,44]. However, the set of samples considered here also showed alternative ways of producing blue glass, such as by combining copper with cobalt.…”

“…Samples with elevated K 2 O content are present in virtually all groups without forming any obvious pattern. For these samples, the K 2 O content is within 2%, which does not allow us to exclude the possibility that an evaporitic flux source was used, although plant ash glasses are known from the Early Iron Age in central Italy [43]. In fact, plant ash is not the only source of potassium in glass, as this element can also come from the silica source, glass alteration [58], or glass recycling [59].…”

“…The Iranian and European Erzgebirge/Krušnohoří mining regions are excluded as potential cobalt sources because the samples do not show arsenic and nickel associated with cobalt [41,42]. Also, the cobalt source employed in LMHK glass production in the Late Bronze Age in Italy, already documented in the Early Iron Age contexts [43], does not fit the compositional data due to the absence of the same type of association. It should be noted that the compositional approach used here, with the same p-XRF equipment and measurement conditions, has already allowed us to establish the origin of the color in another (and older) set of blue eye beads in Egypt [44].…”

Design and Use of Portable X-ray Fluorescence Devices for the Analysis of Heritage Materials

“…These are several types of (mostly) blue-green glass beads dating back to the Iron Age and coming from archaeological sites in Central Italy, and which were studied and investigated using several spectroscopic techniques: fiber optics reflectance spectroscopy [13], micro-Raman spectroscopy and laser ablation inductively coupled plasma mass spectrometry [14], scanning electron microscopy with energy-dispersive X-ray spectrometry [15], and portable XRF spectrometry. A detailed discussion of the materials can be found in [16]. Each technique provided complementary information, which was then discussed in the frame of the artefacts' archaeological typology.…”