The palaeo-Christian glass mosaic of St. Prosdocimus (Padova, Italy): archaeometric characterisation of ‘gold’ tesserae

The palaeo-Christian glass mosaic of St. Prosdocimus (Padova, Italy): archaeometric characterisation of tesserae with antimony- or phosphorus-based opacifiers

“…Of the 110 opaque, coloured tesserae, selected for archaeometric analyses and subdivided into seven glass types and 20 chromatic groups (see Table 1 in Silvestri et al, 2011b), tesserae belonging to glass types "White" (chromatic group: Opaque White e BIOP), "Blue" (chromatic groups: Blue (B), Opaque Blue (BO), Dark Blue (BS), Pale Blue (CE), and Azure (AZ)), "Yellow" (chromatic group: Opaque Yellow (GSO)) and "Green" (all samples of chromatic group, Dark Green (VS) and the tessera VG1, Yellow-Green in chromatic group), characterised by antimony-based opacifiers, and all tesserae belonging to chromatic groups "Pale Green" (VCH), and "Dark Brown/Amber" (NS), and tessera VG3, "Yellow-Green" in chromatic group, characterised by phosphorus-based opacifier, are considered here, for a total of 55 samples (Table 1). Exceptions are samples VG2, VG4 and GSO1 which, although they belong to chromatic groups "Yellow-Green" and "Opaque Yellow", are characterised by other types of opacifiers, and will be discussed elsewhere.…”

“…1) and this may suggest, in first approximation, the coexistence of different compositional groups due to different Table 2 Mean chemical compositions and standard deviations for groups identified among Paduan coloured and opaque tesserae (in bold). Also reported: comparisons with chemical compositions of identified groups among Paduan "gold" tesserae (MSG1b and MSG2b, data from Silvestri et al, 2011b) and those of natron glass identified in Western Mediterranean area (data from Foy et al, 2003 production technologies. In order to explain this peculiar compositional signature and to set the glassy matrix of Paduan natron tesserae in a wider cultural and geographical context, they were compared with compositional natron groups from the 4th to 9th centuries, known in the literature and mostly found in the Mediterranean area, i.e., Levantine I, Levantine II, Egypt I, Egypt II, HIMT, (Brill, 1988;Freestone, 1994;Freestone et al, 2000Freestone et al, , 2002Foy et al, 2003;Foster and Jackson, 2009) and Groups 1, 2 and 3 of Foy et al (2003) and with Roman glass of the 1ste3rd centuries AD (Nenna et al, 2000;Silvestri, 2008).…”

“…In order to explain this peculiar compositional signature and to set the glassy matrix of Paduan natron tesserae in a wider cultural and geographical context, they were compared with compositional natron groups from the 4th to 9th centuries, known in the literature and mostly found in the Mediterranean area, i.e., Levantine I, Levantine II, Egypt I, Egypt II, HIMT, (Brill, 1988;Freestone, 1994;Freestone et al, 2000Freestone et al, , 2002Foy et al, 2003;Foster and Jackson, 2009) and Groups 1, 2 and 3 of Foy et al (2003) and with Roman glass of the 1ste3rd centuries AD (Nenna et al, 2000;Silvestri, 2008). Comparisons were also made with compositional groups identified in the "gold" tesserae of the same mosaic (Silvestri et al, 2011b), in the hypothesis that the same 'base composition' of the glass was used to produce both transparent and opaque tesserae. It should be noted here that, although the chemical composition of these glass tesserae is more complex than that of transparent and not intentionally coloured glass which composed the above "reference" groups, because of the addition to the basic mixture of opacifiers and colourants, which may change the relative ratio among the various elements, a deliberate choice was made to compare the Paduan tesserae with these groups and not with the chemical compositions of other mosaic tesserae.…”

“…The other relies on the introduction of suitable raw materials which lead to in situ crystallisation of opacifying agents through separation from the melt, and is called secondary (Verità, 2000;Lahlil et al, 2010b). In this context, the opaque coloured glass tesserae from the chapel of St. Prosdocimus (Padova, Italy), dated to the 6th century AD (for more details on the historical background of the mosaic, see Silvestri et al, 2011b, and references therein), allowed us to carry out careful analytical study on the production technology of palaeo-Christian glass mosaic in a period known to be as crucial, due to the technological transition between the use of antimonybased opacifiers and that of tin-or phosphorus-based ones.…”

“…A previous paper focused on the "gold" tesserae, i.e., those with metal foil, usually of gold, and characterised by transparent glass, not deliberately coloured or decolourised (Silvestri et al, 2011b). The present paper focuses on the opaque tesserae with antimony-or phosphorus-based opacifiers and deliberately coloured, with the aim of characterising the glass composition, with particular attention to identification of opacifiers and colourants, and their correlation with the chromatic and optic appearance of the samples.…”