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

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 alpha- and beta-NH4MnP2O7 polymorphs, being alpha-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 (alpha-NH4MnP2O7) and anhydrous cobalt phosphate (Co-3(PO4)(2)) on some Boccioni's paintings during MOLAB in situ measurements at the Museo del Novecento (Milano). (C) 2016 Elsevier B.V. All rights reserved.