Gold nanoparticles in painted miniatures on ivory: Non-invasive evidence and characterisation

Abstract

To document the use of gold nanoparticles (also known as “Purple of Cassius” pigment) in miniature painting, seven 19th century portraits from Czech collections containing purple colour were studied. The research was performed non-invasively, as it was not possible to sample the small-sized and delicate paintings. Large area X-ray fluorescence scanning indicated the presence of Purple of Cassius in four paintings, while in the three of them, the desired shade was achieved by mixing red and blue pigments, for example vermillion and Prussian blue. The detected gold nanoparticles were characterised in detail by X-ray powder diffraction and via innovative use of environmental scanning electron microscopy. The obtained diffraction patterns enabled the calculation of average size of the particles, which equalled to 8–14 nm. Based on available historical recipes, the pigment preparation was replicated in laboratory, yielding analogous pigment with similar particle size. For the first time, Purple of Cassius was identified and characterised in portrait miniatures.