The Research
To determine the various components in the wall paintings, both the original and that added during subsequent conservation work, the scientific laboratories of the Opificio delle Pietre Dure (OPD), which coordinated the diagnostic investigations with the Superintendents of Historic, Artistic and Ethno anthropological Patrimony and of Architecture and Landscape for the Provinces of Florence, Pistoia and Prato, carried out chemical testing (sampling and cross sections), and physical and optical measurements (infrared reflectography, UV fluorescence). One particular type of UV fluorescence (FLIM) was tested by the Center for National Research (CNR) Centro Gino Bozza and the Physics Department at the Polytechnic University of Milan, with coordination by the OPD.

The UV light induced fluorescence has shown traces of original binding material remaining on the surface of the plaster and in portions painted with tempera (a secco) but almost completely lost (such as the details of the evangelists in the vault), as well as vast areas that have been retouched and in deposits of non-original material. This description illustrates the complexity of the conservation work and the necessity to develop specific intervention procedures for every portion of the painting surface, while remaining a single conservation intervention. One of the principal problems to be addressed was the presence of synthetic resins used by Leonetto Tintori for consolidating the painting surface, following the methodology of the time, and with which he himself experimented. These resins caused large areas to appear shiny and to be impermeable.

After the identification of the type of resin through chemical and stratigraphical analysis, and following testing with traditional solvents, which did not yield satisfying results, a micro-emulsion was developed made with organic solvents in a mixture of water and surfactant to be applied as a poultice with cellulose pulp and silica over Japanese paper. This work was completed by the synergy of the Opificio delle Pietre Dure and the Department of Chemistry of the University of Florence.
Following the application on the vault, in particular on the figure of Saint John, it was clear that a significant thinning of the added resin could be achieved. This partial extraction of the resin, repeated in other areas, allowed the surface to regain its permeability without weakening the paint layer.
The cleaning procedures (view images) were carried out gradually and progressively over a period of time to allow surface drying and to evaluate the optical equilibrium and relationship (view images) between different scenes and pictorial layers.

The results obtained, through the removal of sulphates which interfered with the wavelength of light, and the removal of atmospheric particulate and soot, returned the luminous and spacious aspects of the colors (view images).

Reintegration, carried out by toning down the plaster abrasions, with glazes tuned to the original colors in areas of paint layer abrasion, and with chromatic selection in the areas of filled losses, returned the unity of the image and reduced the multiple discontinuity of the painted surface.
This conservation program, therefore, gives back to visitors and to scholars alike a restored pictorial cycle, freed for the most part from inadequate added materials. Although not returned to its “original splendour” (consider, for example, the tarnished decoration in gilded wax view images), it is now legible again with recovered continuity, luminosity and chromatic values which allow the full appreciation of this masterpiece   (view images).

Text by Cristina Gnoni Mavarelli e Isabella Lapi Ballerini