Non-invasive in situ analytical techniques working in synergy: The application on graduals held in the Certosa di Pavia

The old library of the Certosa di Pavia still holds thirteen graduals, choir-books with lyrics and scores. Their rich decorations were made by various artist-monks during the fiftieth century. In 2009, the gradual numbered 814 underwent a conservative restoration intervention under the supervision of the Certosa Museum Director, Letizia Lodi, to be on display in Verona, and later it was also included in the exhibition of Arcimboldo works in Milan (2011). Non-invasive in situ analyses were performed on seven books of this collection, considering fourteen illuminated pages in total, painted by four authors: they showed different technical features, but the same typical materials. Pigments and binders were examined exploiting the synergy between four complementary techniques, namely, EDXRF (energy dispersive X-ray fluorescence), FORS (fibre optics reflectance spectroscopy), FTIR (Fourier transform infrared spectroscopy) and micro-Raman spectroscopy. Summarizing the palette, the blue pigments used are ultramarine, azurite and, in one case, smalt blue; the red and pink hues are given by red ochre, vermillion and lakes; the yellow ones by orpiment, lead-tin yellow and yellow ochre. The white used is lead white, both in mixtures and as underpainting layer. The application of techniques with different penetration depths allowed to detect the stratigraphic sequences without sampling, opening to the interesting application to off-limits masterpieces. Intriguing is the case of green pigments, recognized as malachite and basic copper sulphates, but possibly also given as a mixture of copper-based and yellow pigments. The different kinds of gold and silver gildings were considered and investigated, accounting for different executing techniques for the different authors. The closing challenge will be determining the binders without any sampling, but exploiting FTIR spectra for the detection of organic binders.