Non-destructive pigment characterization in the painting Little Madonna of Foligno by X-ray Powder Diffraction

X-ray Powder Diffraction (XRPD) is one of the non-destructive techniques habitually used for the structural analysis of crystalline solids. This technique can therefore be used to differentiate between crystalline and vitreous or amorphous solids such as Smalt Cobalt Blue or the binders used in paints. The “final” pigments used in the artist's palette are frequently composed of a complex mixture of inorganic and organic compounds, causing different interferences and matrix effects, such as filtering and signal overlapping and background noise increase. These effects can make difficult pigment identification and quantification contributing to mischaracterization. Nevertheless, when the experimental difficulties can be overcome, the results obtained are precise, definitive and useful. In the present research, the limitations were avoided or minimized by using an algorithm for data analysis like the one used by the XPowder™ software. In addition, most of the natural pigments used in the painting The Little Madonna of Foligno are identified and quantified. The preparation layers are composed of white lead, along with a homogeneous mix of several arsenic sulfides and small amounts of roselite. The color layer contains lapis lazuli and minor azurite for the blue colors, and cinnabar for the red ones. Other pigments found were: lead‑tin yellow, iron(III) oxy-hydroxides (red and yellow ochers), celadonite (green earth), and possibly olivine, diopside and other ferromagnesian minerals in the form of pyroxenes and amphiboles, whose diffraction lines also appear in the diffractograms, although with weak intensities and precise quantifications are not allowed. Crystalline Thenard's blue was not found.