1. Courtauld Institute of Art, Somerset House, Strand, London, WC2R 0RN UK (G.V., S.C.); The British Museum, Great Russell Street, London, WC1B 3DG UK (G.V.); SMAArt, Department of Chemistry, University of Perugia, via Elce di Sotto 8, 06123 Perugia Italy (C.C.); ULTRAS-CNR-INFM and IFN-CNR, Politecnico di Milano, Dipartimento di Fisica, Piazza Leonardo da Vinci, 32, 20133 Milan, Italy (D.C.); and The Getty Conservation Institute, 1200 Getty Center Drive, Los Angeles, California 90049 (F.P.)