Florence baptistery: chemical and mineralogical investigation of glass mosaic tesserae

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Abstract

A recent restoration of the mosaics of the Baptistery in Florence enabled the collection and analysis of a group of glass tesserae. Twenty-three tesserae, dated between the end of the 13th century and the first half of the 14th century, showing different colors were sampled and analyzed to identify the raw materials, colorants and opacifiers employed in their production.

Chemical analyses of major and minor elements, performed by WDS-EMPA, showed that the sample set can be subdivided into two groups on the basis of the flux employed during production: i) a group of seven samples with high levels of K2O, and ii) a group of sixteen Na-rich samples. Analyses of the main components of the glass, yielded by the vitrifying portion, suggest that two different typologies of raw materials – with different proportions of carbonatic and feldspatic components – were employed for the production of these tesserae. Scanning electron microscopy observations, coupled with EDS qualitative analyses and X-ray powder diffraction experiments, revealed the presence of SnO2 crystals dispersed in a high lead matrix in all the opaque samples, with higher concentrations of particles in the white samples. Spectroscopic analyses using Fibre Optic Reflectance Spectroscopy (FORS) for the identification of chromophores were also conducted on the mosaic tesserae with the aim of acquiring data for integration with information collected in other analyses. This technique was applied in situ, being non-invasive, simple and inexpensive.

The data here obtained are compared with the historical sources available (glass recipe books) to identify the raw materials and the glassmaking techniques employed in view to establish relationships between the existing local production and the analyzed mosaic glass.

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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