Florence baptistery: chemical and mineralogical investigation of glass mosaic tesserae
Introduction
The Florence Baptistery, dedicated to St. John the Baptist, patron saint of the city, is located in the religious centre of Florence, facing the Cathedral of Santa Maria del Fiore. This majestic sacred building, created to house the cathedral font, has a diameter of 25.6 m. The building needs to be so big because it has to accommodate the crowds of religious believers receiving baptism on just two fixed dates per year.
The founding date of the Baptistery is still uncertain, but it is thought to have been built between the fourth and fifth centuries AD. The building became the official city baptistery only in 1128. The Baptistery interior is decorated with a magnificent and very complex decorative composition of mosaics (Giusti, 1994a, Giusti, 1994b, Giusti, 1994c, Hueck, 1994), aiming to represent the Christian faith and the entire cosmos of the salvation story from the Creation to the Final Judgment, with particular attention to the role of John the Baptist, patron saint of the city. The mosaic decorations cover the top of the “scarsella” (the rectangular apse built in 1202), the whole dome (composed of eight segments), part of the women’s gallery and the parapets.
The first mosaic decorations were those realized in the scarsella between 1260 and 1270 and those of the dome realized by Jacopo Torriti, Coppo di Marcovaldo, and Cimabue between 1270 and the beginning of 1300. The mosaics of the women’s gallery belong to the last phase, subsequent to the decoration of the dome.
Mosaics have been deeply studied in terms of stylistic interpretation, but only in the recent years it has been paid attention to the raw materials and to the technique employed for the tesserae production (Antonelli et al., 1989). Now a considerable amount of information is available regarding the techniques and the raw materials employed for the mosaic production, especially referring to the ancient treatises of recipes. Albeit a lot is known regarding glassmaking in Venice and Orvieto, details concerning glassmaking in Rome, Florence and other Italian centers are rather scarce as regards that period of time.
Twenty-three glass tesserae were sampled from the fourth panel on the south-east portion of the women’s gallery parapets (the Nehemiah panel), where the prophets and other characters identified by writings and scrolls with Bible verses are represented. This panel represents the prophet Nehemiah, accompanied by a scroll with the verse: CONVERSIQUE SUNT ET CLAMAVERUNT AD TE; TU AUTEM (DE CAELO EXAUDISTI ET LIBERASTI EOS IN MISERICORDIIS TUIS MULTIS TEMPORIBUS) [2 Es 9:28] (the part in brackets is not considered as included in the mosaic and is further reduced by numerous abbreviations).
The samples object of this study were drawn during the restoration work realized by the Opificio delle Pietre Dure of Florence and concluded in the year 2008.
In this work the results of the chemical and mineralogical characterization of a suite of mosaic tesserae coming from Florence baptistery will be presented with the aim to investigate the production technology employed for their production. The data will be discussed and compared with other analyses relative to the glass tesserae of the Florence Baptistery analyzed by Verità (2004). The results obtained will be discussed and compared with the historical sources, to establish possible relationships with local glass production known in this period.
In addition to the chemical–physical analyses for the determination of chemical composition, non-invasive spectroscopic measurements by using optical fibres (FORS) were performed on these samples in order to evaluate the potentialities of this technique. FORS has been applied for industrial purposes since the early seventies, while their first application to the field of works of art was made at the National Gallery in the late seventies (Bullock, 1978) and successively during the restoration works in the Brancacci Chapel, Chiesa del Carmine, Florence, Italy (Bacci et al., 1991) and at the Victoria and Albert Museum (Martin and Pretzel, 1991). So far this technique has not been extensively applied to studies of historical and archaeological glass materials as well as ancient stained windows. In this context, we had the opportunity to exploit the potentialities and drawbacks of this technique for an in situ application. Even if this technique does not allow a complete characterization of glass, it can be used extensively, being not invasive, to give important information that can be useful for reducing and addressing sampling for further and more exhaustive analyses.
The relevance of this work is undoubtedly linked to the extraordinary importance of one of the Italian Renaissance architectural masterpieces and it represents a fundamental contribution to the mosaic glassmaking technique understanding, in a scarcely documented period of time.
Section snippets
Experimental
The chemical composition of the samples was obtained by Electron Microprobe Analyses, while the identification of dispersed opacifier particles was performed with X-ray Powder Diffraction and Scanning Electron Microscopy investigations.
In addition, spectra in the Vis-NIR range (380–1700 nm) were acquired in reflectance mode (FORS).
Chemical data
The results of the chemical analyses are reported in Table 1 and in the bi-plots of Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5. Fig. 1, reporting the Na2O and K2O levels of the analyzed samples, provides clues about the fluxing agents employed for the glass production. A clear subdivision of the mosaic tesserae into two groups appears from the plot: i) a high potassium group (BF8, BF9, BF11, BF14, BF15, BF19, BF30) – (with K2O from 12.82% to 25.37% and Na2O ranging from 1.78% to 4.74, and ii) a
FORS
The non-invasive spectroscopic investigations conducted on the bulk of the tesserae gave further insight into the composition of the mosaic tesserae.
Unfortunately, the high noise levels present in the deep brown and black tesserae (BF1, BF2, and BF4) did not permit definite identification of chromophores. Instead, good spectra were obtained for the blue tesserae (BF8, BF9, BF11, and BF14) for which all the spectra presented signals of Co(II) together with Fe(II) and Fe(III) (Fig. 8, see BF14).
Base raw materials
The samples analyzed can be divided into groups on the basis of both chemistry of major elements, and variety of colors and degrees of opacity, which, in turn, reflect the chemical data. The study of the opaque and/or deeply colored glass was more difficult than the study of transparent common glass. In opaque and/or deeply colored glass the deliberate addition of different components to obtain the desired color nuance complicates the chemistry of the resulting glass.
The samples analyzed can
Conclusions
The chemical analyses performed on the tesserae from the Baptistery of Florence make it possible to hypothesize the use of three different recipes, employing different raw materials for the production of tesserae of different colors. In particular, a sodium bearing flux (probably plant ash) was added to a feldspatic sand to produce brown, red, white and Fe-colored green samples, and to a purer silica source to produce (mainly) dark green Cu-colored tesserae; a potassium bearing flux along with
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2020, Journal of Archaeological Science: ReportsCitation Excerpt :Using this approach, we seek to identify elements that might be commonly problematic in pXRF analyses of similar assemblages as well as those that have the greatest potential to be accurately measured. Recommendations for an appropriate methodological approach will be made and the potential usefulness of employing pXRF to study an assemblage containing lead-rich glass such as mosaic tesserae will be discussed (Arletti et al., 2011; Boschetti et al., 2016; Silvestri et al., 2014; Wypyski, 2005). Studies of mosaic tesserae stand to benefit enormously from pXRF analysis as it could be used to rapidly analyse hundreds of tesserae, even while in situ, and to identify groups of tesserae that may relate to different glass sources, recycling and/or later restoration campaigns, and the data may be used for further sampling decisions.
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2013, Spectrochimica Acta - Part B Atomic SpectroscopyCitation Excerpt :It was founded in 25 BC and was the head capital of the Roman administrative province of Lusitania. Bulk chemical composition, surface degradation pathologies (dealkalinization layers and deposits), chromophores, and opacifying elements are commonly analyzed in historical glasses by conventional techniques such as X-ray fluorescence spectrometry (XRF), scanning electron microscopy coupled with energy dispersive X-ray spectrometry (SEM/EDS), and ultraviolet–visible spectrophotometry (UV–vis) [17–21]. Nevertheless, they have the disadvantage of being techniques which produce partial or almost total destruction of valuable samples of historical glasses.
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2011, Journal of Archaeological ScienceCitation Excerpt :Some authors have addressed the problem of classifying mosaic glass (Ruffini et al., 1999a, 1999b; Verità, 2000; Brill, 2002; Fiori et al., 2004; Andreescu-Treadgold et al., 2006; Vandini et al., 2006) and identified some important differences on chronological and geographical bases, starting with the use of various raw materials, colourants and opacifiers. However, a systematic and extensive study on glass mosaics has not yet been undertaken; those carried out so far (e.g., Freestone et al., 1990; Brill, 1999; Verità, 2000; Verità et al., 2002; Verità and Vallotto, 2003; Henderson, 2004; Verità and Zecchin, 2007; Boschetti et al., 2008; Arletti et al., 2008a, 2008b; 2011; van der Werf et al., 2009) mainly focus on a few samples coming from the same site, probably due to the value and availability of the material, often still in situ, and cover various periods of time, often not contiguous with one another. The possibilities offered by the specific case under consideration here – of working on more than 3000 loose tesserae characterised by a full range of colours – is an unique occasion for careful and precise analytical study on palaeo-Christian glass mosaics.