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Figure 1- La Vierge adorant l’Enfant en présence de Saint Jean-Baptiste, Andrea Della Robbia, 1485-1515

©2006 Musée du Louvre, Pierre Philibert

Tin-oxide particles were first used for glaze opacification around the 8th century in the Middle East, the technique then spreaded in Europe around the 13th century [1,2]. Tin-opacified glazed ceramics were used extensively during the Renaissance period, for example in Italy. Present study focuses on one family of artists: Della Robbia’s family. Luca Della Robbia created a bottega near Florence in the mid-15th century and then three generations followed him as head of this sculpture workshop first in Italy and then in France until the first part of the 16th century. They were sculptors and adapted the glazed ceramics techniques for great sculpture (Figure 1). They produced a lot of pieces with reproducible methods and adapted the recipe of tin opacified glazes hiding the natural coloration of the underlying ceramic paste to obtain a deep colours palette (white, blue, green and yellow). This opacification is obtained thanks to the use of a lead-tin “calx” blended with a finely ground glass frit and deposited on the surface of ceramic paste. This mix produces, after firing, the final opacified glaze [3]. The calx used here is an intimate mixture of oxides and defined compounds obtained by the simultaneous calcination of lead and tin metals.

The ceramic paste used by the Della Robbia family is rich in calcium and the glass that forms the final glaze is mainly composed of SiO2, PbO, Na2O, K2O... SnO2 crystals are dispersed in this glass to opacify it and make the ceramic substrate invisible (Figure 2). The manufacturing processes used by these Renaissance artists to produce their tin-opacified glazes are not well understood. That’s why this work tries to answer this question using an experimental approach coupled with the study of millimetric samples of the Della Robbia family’s production. 

Figure 2 - SEM observation (back-scattered electrons) of a glazed ceramic shard from Della Robbia’s artwork and a zoom on SnO2 crystals dispersed in the glaze.

This work has been done with Léa Cutard, Gilles Wallez, Anne Bouquillon and Daniel Caurant from PCMTH team in IRCP and C2RMF and has been presented in a poster at a glass conference (from the USTV) in November 2024 and won the best poster award. This presentation explores the reactivity of calx.

For calx synthesis, various proportions of lead and tin metals were calcined at different temperatures (between 600°C and 900°C) and the final mix was analysed by XRD. These analyses allowed to quantify the relative proportions of three produced components (PbO, SnO2, Pb2SnO4) (Figure 3). It appears that Pb2SnO4 becomes the main phase above 600°C and this composition does not evolve up to 900°C, temperature above which the lead-stannate seems to be starting to decompose. So, to produce mainly Pb2SnO4, the calcination temperature does not need to be precisely defined. To better understand the formation process of the micrometric SnO2 particles present in the Della Robbia glazes, we investigated the reactivity between the calx and a glass frit based on Della Robbia’s glazes composition analysis by EDS.

 Figure 3 - Lead-tin mixture before and after calcination under air (left) and phases quantification determined by Rietveld analysis from XRD patterns for firing temperatures ranging from 600 to 1000°C (right).

Pb2SnO4-rich calx was firstly heated with silicate glass at temperatures between 800°C and 1100°C. This investigation showed that the Pb2SnO4 phase is not stable in contact with the glass particles even at 800°C and that a small amount the PbSnO3 yellow phase is only present up to 900°C which then totally decomposed above this temperature to form SnO2 (Figure 4). These results combined with the analysis of Della Robbia’s tin-opacified glazes indicate that the artists used a firing temperature above 900°C, probably around 1000°C.

This work coupling archaeometry and experimental investigations opens several perspectives to complete the study of the Della Robbia tin-opacified glazes. For instance, the reactivity between the calx and glass frit mixture deposited on a ceramic substrate remains to be explored. This would lead to a full recreation of Della Robbia’s ceramic system.

Figure 4 - Changes on calx-silica mix aspect (left) and opacified glass obtained by mixing calx and a glass frit at various temperatures (right). The yellow heterogeneities observed on the optical microscopy images at 800 and 900°C correspond to the PbSnO3 phase.

[1] M. Matin, “Tin-based opacifiers in archaeological glass and ceramic glazes: a review and new perspectives,” Archaeological and Anthropological Sciences, vol. 11, pp. 1155–1167, 2019.

[2] J. Rosen, La faïence en France du XIIIe au XIXe siècle : technique et histoire. Artehis, 2018.

[3] A. Zucchiatti, A. Bouquillon, I. Katona, and A. d’Alessandro, “The ‘Della Robbia blue’: a case study for the use of cobalt pigments in ceramics during the Italian renaissance,” Archaeometry, vol. 48, no. 1, pp. 131–152, 2006.

Lise Boutenègre

ENS de Lyon au département de Chimie.

Agrégée préparatrice à l’ENS et doctorat au sein de l’équipe PCMTH