Obsidian and Obsidian-like Glass Tesserae: A Multidisciplinary Approach to Study the Dedication Wall Mosaic in the Church of St. Mary of the Admiral in Palermo (12th Century)

Abstract

Pliny the Elder testifies that roman workshops used volcanic glass (obsidian), but also produced and used a dark glass (obsidian-like glass) quite similar to the natural one. In the context of the study on medieval mosaics, the use of the obsidian and obsidian-like tesserae is a challenging research topic. In this paper, we present the results of a multidisciplinary study carried out on the Dedication wall mosaic, realized by a byzantine workshop in the 12th century in the Church of St. Mary of the Admiral in Palermo, and where numerous black-appearing tesserae, supposed to be composed of obsidian by naked-eyes observation, are present. Historical documents, multispectral imaging of the wall mosaic, and some analytical methods (SEM-EDS and XRPD) applied to a sample of black tesserae, concur in identifying here the presence of obsidian and different obsidian-like glass tesserae. This evidence, although related to the apparent tampering and restoration, could open a new scenario in the use of obsidian and obsidian-like glass tesserae during the Byzantine period in Sicily and in the reconstruction of multiple restoration phases carried out between 12th and 20th century AD on the mosaics of St. Mary of the Admiral.

The Medieval Glass Mosaic of S. Agnese fuori le mura in Rome: Multispectral Imaging for Preliminary Identification of Original Tesserae

“…These analytical techniques, which have been shown to be particularly suitable for the characterization of glass tesserae, were previously applied to other in situ glass mosaics, such as the paleo-Christian mosaic of S. Maria Mater Domini in Vicenza and the Byzantine Dedication wall mosaic of St. Mary of the Admiral in Palermo [25,27].…”

“…Studies of detachments, moisture, and structural inhomogeneities in wall mosaics using non-invasive methods are documented in the scientific literature, including contact and no-contact techniques [19][20][21][22][23]. Less well known and more recent among these non-invasive, nocontact methods, but very promising in this field, is multispectral imaging [24,25]. This technique, well known as a diagnostic tool in the field of paintings, especially in the identification of preparatory drawings, pentimenti, and traces of restoration and remaking, has only recently shown its great potential to support the finding of inhomogeneities affecting decorated surfaces [26] and specifically wall mosaics [25].…”

“…Less well known and more recent among these non-invasive, nocontact methods, but very promising in this field, is multispectral imaging [24,25]. This technique, well known as a diagnostic tool in the field of paintings, especially in the identification of preparatory drawings, pentimenti, and traces of restoration and remaking, has only recently shown its great potential to support the finding of inhomogeneities affecting decorated surfaces [26] and specifically wall mosaics [25]. In particular, the use of multispectral imaging to combine and process the VIS (visible) and IR (infrared) bands is extremely useful in the preliminary identification of possible different materials and traces of restorations, reuses, or replacement of mosaic parts.…”

Joint Investigation with Ground Penetrating Radar and Infrared Thermography as a Diagnostic Support for the Restoration of Two Wall Mosaics in the Church of St. Mary of the Admiral in Palermo, Italy

“…Infrared thermography (IRT) is a non-invasive method widely applied to cultural heritage studies [39][40][41][42][43][44][45][46], with several cases of applications to wall mosaics [39,46]. This telemetry technique determines the temperature of a surface by measuring the "black body" radiation emitted by each object as a function of its own temperature.…”

“…The various vicissitudes of the church of St. Mary of the Admiral in Palermo, not always documented, have caused great difficulties in interpreting the origin of the tesserae and the various alteration and restoration interventions. Recently, diagnostic studies were carried out with non-invasive techniques and laboratory analyses to distinguish modern from medieval materials [38] with the aim of contributing to the reconstruction of the restoration phases of these mosaics, carried out between the 12th and the 20th century [39].…”