Multi-Scale X-ray Imaging of the Pigment Discoloration Processes Triggered by Chlorine Compounds in the Upper Basilica of Saint Francis of Assisi

Abstract

In this paper, the chromatic alteration of various types of paints, present on mural painting fragments derived from the vaults of The Upper Basilica of Saint Francis of Assisi in Italy (12th–13th century), is studied using synchrotron radiation. Six painted mural fragments, several square centimeters in size, were available for analysis, originating from the ceiling paintings attributed to Cimabue and Giotto; they correspond to originally white, blue/green, and brown/yellow/orange areas showing discoloration. As well as collecting macroscopic X-ray fluorescence and diffraction maps from the entire fragments in the laboratory and at the SOLEIL synchrotron, corresponding paint cross-sections were also analyzed using microscopic X-ray fluorescence and powder diffraction mapping at the PETRA-III synchrotron. Numerous secondary products were observed on the painted surfaces, such as (a) copper tri-hydroxychloride in green/blue areas; (b) corderoite and calomel in vermillion red/cinnabar-rich paints; (c) plattnerite and/or scrutinyite assumed to be oxidation products of (hydro)cerussite (2PbCO3·Pb(OH)2) in the white areas, and (d) the calcium oxalates whewellite and weddellite. An extensive presence of chlorinated metal salts points to the central role of chlorine-containing compounds during the degradation of the 800-year-old paint, leading to, among other things, the formation of the rare mineral cumengeite (21PbCl2·20Cu(OH)2·6H2O).

Advanced mapping of inorganic treatments on porous carbonate stones by combined synchrotron radiation high lateral μXRPD and μXRF

“…This technique, when complemented with the elemental information of micro-X-ray fluorescence (μXRF) mapping, becomes particularly well-suited for the micro-analysis of highly heterogeneous systems, especially when the new phases are present in minor fractions if compared to the substrate’s minerals or are poorly crystalline. These techniques, although well-known and already used in many fields over the years 31 – 35 , in CH materials are mainly used in the examination of oil paintings 22 – 39 and wall paintings 40 , 41 , ceramics 42 , 44 , paper 45 , and metals 46 , 47 , but, to the best of our knowledge, have not been employed to study the reaction products of inorganic consolidation solutions on porous carbonate stones in CH.…”