Once upon a glass mosaic in the apse of S. Sabina’s Basilica in Rome. Interdisciplinary study of a Late Antique/Medieval lost decoration

• The only evidence of the apse glass mosaic in the S. Sabina Basilica under study. • Green glass tesserae only preserved, and a selection chosen for analyses. • Interdisciplinary approach supports a 5th-9th cent dating of the apse mosaic. • Some tesserae reused/recycled; others newly produced for S. Sabina apse mosaic. • Comparisons among all the green tesserae from the mosaics of Rome performed. The present interdisciplinary study examines the glass tesserae from the apse mosaic of S. Sabina , part of a wider research project on Early Medieval wall paintings and mosaics in Rome. The tesserae in different green shades (pale green, green and dark green), the sole remnants of the original mosaic now lost and replaced by 16th-century frescoes, were analysed using various techniques (OM, SEM-EDS, EPMA, XRPD) to identify raw materials and production technologies, aiming to date the mosaic (5th, 9th, or 12th century AD). The analyses identified various glassy matrices, comparable to known compositional groups such as 'Roman', 'HIMT', 'Foy 2.1′, and 'Foy 3.2′, as well as opacifiers/pigments like lead antimonate and lead stannate. These results indicate that the green and most pale green tesserae were specifically produced for this mosaic. The dark green tesserae show greater variability, suggesting the reuse and recycling of pre-existing materials. The archaeometric analyses support a dating between the 5th and 9th centuries AD, aligning with art-historical assessments. The comparison with the 5th-century AD counter-façade mosaic of S. Sabina revealed significant differences in the composition of the glass matrices and opacifiers/pigments, indicating that the counter-façade mosaic was primarily made from reused tesserae. Comparisons with green tesserae of other Roman mosaics highlighted a general trend of recycling and reusing tesserae, except for the 7th-century AD S. Agnese fuori le mura mosaic, where newly produced and reused tesserae coexisted. These data reinforce the hypothesis of a 5th to 9th-century AD dating for the S. Sabina apse mosaic, providing new insights into the production and use of glass tesserae in Late Antique and Early Medieval Rome.