From Crystals to Color: A Compendium of Multi-Analytical Data on Mineralogical Phases in Opaque Colored Glass Mosaic Tesserae

Abstract

<jats:p>This study aimed at laying the groundwork for a compendium on mineralogical phases responsible for the colors and opacity of ancient glasses, with specific reference to mosaic tesserae. Based on the awareness that a comprehensive database of these phases is currently lacking in the available literature, this compendium foresees two main objectives. The first scope was to set the basis for a well-structured database, as a reference point for scholars from different backgrounds for comparative and methodological purposes. The second goal was to provide insights on analytical methods that could be profitably used for achieving an in-depth characterization of coloring and opacifying inclusions; a tailored multi-analytical approach based on easily accessible and widespread techniques like OM, SEM-EDS, μ-Raman, and XRPD is proposed here. Micro-structural and compositional features of glass tesserae, where different types of crystalline phases were detected (Sb-based, Sn-based, Cu-based, Ca-phosphate-based) are presented in well-structured synoptic tables, following a summary on historical–archaeological information on their use.</jats:p>

A Tale of Two Legacies: Byzantine and Egyptian Influences in the Manufacture and Supply of Glass Tesserae under the Umayyad Caliphate (661–750 AD)

“…XRPD exactly characterised crystalline phases as metallic copper, responsible for the red hue as well as for the opacity (Figure 7e). Features of opaque red tesserae are consistent with so-called "dullish red" glass, a low-lead (PbO 1-2 wt%), low-copper (CuO about 2-3 wt%) glass with nanometric rounded particles of metallic copper dispersed into the matrix [71,[73][74][75][76]. ).…”

“…As it has been discussed more in-depth elsewhere [57,71,72], the exact classification of the colours of glass tesserae on an entirely objective basis stands as the first issue to be addressed, to avoid any subjective description and to provide a reliable criterion for the selection of tesserae to be analysed and, at a later stage, compared.…”

“…Green/red banded tesserae are not unattested in the literature: in Roman mosaics from Pordenone, Trento and Aquileia [73,77], several green/red-banded and orange/red-banded tesserae were found, all coloured by the addition of copper-based phases. It is, however, still unclear whether the bi-coloured pattern could always be interpreted as an accidental occurrence (like a failed attempt to produce red glass) or if a deliberate combination of two different kinds of glass took place [71]. (c,d) BSE images showing nonhomogenous, zoned vitreous matrix; (e) BSE detail of the area where green and red glass were juxtaposed (note the nanometric rounded particles); (f,g) EDS spectra acquired on red and green stripes of glass.…”

Multi‐technique characterization of glass mosaic tesserae from Villa di Teodorico in Galeata (Italy)

“…Starting from fourth century AD, antimony-based crystalline phases (calcium and lead antimonate) have been progressively substituted by tin-based phases, [12] maybe because of either the interruption in the supply of antimony or the beginning of closer relations between the Roman Empire and India. [18][19][20] Nevertheless, lead antimonate was again extensively used from 15th century to achieve an opaque yellow color in glasses and glazed ceramics and as a pigment for painting. [21] Tin-based phases are used as colorants and opacifiers in white glasses but only as opacifiers in glasses colored by metallic oxides (like copper or cobalt).…”

“…[18] Lead-tin oxides were employed for opaque yellow and green glasses in Eastern Mediterranean between the fifth and the ninth century AD and in Italy between the 6th and the 16th century AD, especially during Late Antiquity. [18] In G2 and G5, in addition to cassiterite, calcium phosphate Ca 3 (PO 4 ) 2 acts as opacifier. Its use was indirectly confirmed by the Raman spectrum of some whitish inclusions showing the signature of β-rhenanite (β-NaCaPO 4 ) [23,24] with main peaks at 429, 449, 583, 966, 1013, 1025, and 1047 cm À1 (Figure 9).…”