A completely non-invasive characterisation and imaging of ancient Roman “glass-gems” from Aquileia using on-site instrumentations integrated with ion beam

Since Antiquity, glass has also been used to imitate natural gemstones, leading to the development of new recipes for producing opaque and coloured glass-gems. Ancient Roman glassmakers mastered glass production, controlling both the colour and transparency. Opacity could be obtained by specific firing technology inducing microcrystalline phase formations, whereas the colour hues were influenced by specific elements with chromophore properties during glass production, such as transition metals. Proof of this expertise is the widespread of glassmaking workshops across the Roman Empire and related findings of archaeological glass artefacts still well preserved in numerous museums. The National Archaeological Museum of Aquileia (Italy) houses a rich collection of more than 6000 gems, including 1300 glass-based 'glass-gems', and not yet fully investigated. Iconographic and glyptic studies date the glass-gems from the II century BCE to the III century CE. Archaeological information on the production period, manufacturing processes and provenance can be explored by investigating the overall composition of a glass specimen. A completely non-invasive study has been carried out on a selection of glass-gems through the combination of X-ray fluorescence imaging and particle-induced X-ray/gamma emission to disclose the elemental composition, the relative distribution and gather new hints on their connection with the appearance, colour and glassmaking technology. Micro-Raman spectroscopy has been applied on opaque glass-gems to investigate the crystalline phases acting as opacifiers. Combining different non-invasive methods enables a comprehensive identification and characterisation of such glass-gems, crossing their elemental composition in terms of crystalline and glassy phases and associated key elements.