Thèse de Gianluca Gariani, lundi 20 mai 2019, Paris

Sujet Unravelling materials, provenance, and serial manufacturing of the «Madonne di gesso». A first technical study on stucco devotional reliefs from Italian Renaissance Masters
Date lundi 20 mai 2019 à 13h30
Lieu C2RMF, 14 Quai François Mitterrand, 75001 Paris- Palais du Louvre, Amphithéâtre Palissy

Résumé
The production of devotional reliefs flourished in 15th century Florence, where original marble models from Renaissance Masters were moulded to be reproduced in series. Among the materials mostly used to this purpose was the so-called “stucco”. Despite the wide diffusion of stucco reliefs, details concerning their constitutive materials and manufacturing processes are fragmentary and poorly studied. In the framework of a multidisciplinary project fostered by the Louvre Museum, this thesis explored three research lines in order to: 1) identify and characterise stucco mineral constituents; 2) determine raw material provenance; 3) investigate manufacturing techniques and serial casting processes.
A multi-analytical approach was devised to investigate composition (SEM-EDX, PIXE, LA-ICP-MS) and structure (XRD, FEG- SEM, SμCT) of micro samples obtained from 30 representative artworks attributed to 7 Florentine Masters.
Results showed that all the artworks are made of a gypsum based stucco (40-44 wt% CaO and 52-56 wt% SO3) with secondary mineral phases (anhydrite, calcite, dolomite, celestine and clay minerals) in few weight percentage maximum.
PIXE and LA-ICP-MS trace elements analyses, as well as the study of W/P ratio and pores morphology by SEM-EDX and SμCT, pointed out that about 20 reliefs present similar compositional and structural parameters. This confirms the hypothesis of common practices and of a common production centre according to the Florentine attribution of the artworks.
A geological survey was performed in 3 quarries in the surroundings of Florence and the provenance of the raw gypsum was investigated by means of geochemical and isotopic composition. In particular synsedimentary clay minerals with variable amounts of K, Mg and Fe, related trace elements (e.g. Ti and REE), and multi isotopic fingerprint (S, O, Sr) of gypsum have been tested as provenance markers. Usefulness and pitfalls of the approach were verified through a first promising comparison with data obtained on stucco reliefs. Miocene quarries seems to have compatible signature with the ones measured on artworks.
Nonetheless, small differences related both to accessory minerals proportions, to trace element contents, and to pores microstructures suggest that some of the stucco reliefs after models of Donatello and A. Rossellino might have been produced following different practices or particularly by using different raw materials. These evidences, corroborated by the activity of these artists in many Italian regions, raise the hypothesis of the production of these casts outside the Florentine area.
In parallel, THz-TDI, Xray radiography and Computed Tomography were tested for the non-invasive investigation of the internal stratigraphy of plaster layers possibly revealing different manufacturing techniques.
Finally, serial manufacturing techniques were investigated on 3 groups of replicas belonging to 3 models after Donatello & B. Bellano, D. da Settignano and A. Rossellino. Structured light 3D scanner and mesh comparison of the 3D models obtained on stucco casts proved to be a very useful tool. The method allowed to measure millimetric differences of shapes and topography among reliefs stored at the Louvre, Bargello and Bardini Museum. This helped to find direct correlations between casts, to verify which ones were most likely produced with the same mould, and to propose a chronology within the same casting series.
Results obtained disclosed new and important insights on the practices of 15th century Florentine botteghe and on the manufacturing of these artworks. This work and the large set of data produced will also provide solid basis for further investigations and will hopefully prompt interest on this topic worth to be studied with increased attention.

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