Molecular and structural characterization of some violet phosphate pigments for their non-invasive identification in modern paintings

Abstract

A complete non-invasive characterization by XRF, XRD, near-FTIR and UV–Vis reflectance spectroscopy has been performed on some commercially available violet pigments as well as on pure violet Co-salts also known to be used as pigments. The obtained results show that, after a preliminary elemental characterization, the studied pigments can be easily identified by near-FTIR and UV–Vis spectroscopies since they exhibit peculiar spectral bands in these regions. Among the analyzed samples emerged that the pigment 45350 - “Manganviolett” from Kremer consists of two α- and β-NH4MnP2O7 polymorphs, being α-NH4MnP2O7 the most abundant one; furthermore we found that the pigment R1215D -“Cobalt violet” by Winsor & Newton (no longer available since 2006) displays spectral features that match exactly those of 45820-“Kobaltviolett hell” from Kremer and both are composed by cobalt ammonium phosphate hydrate. Such non-invasive study allowed for the identification of “Manganese Violet” (α-NH4MnP2O7) and anhydrous cobalt phosphate (Co3(PO4)2) on some Boccioni's paintings during MOLAB in situ measurements at the Museo del Novecento (Milano).