Assessing the quality of natural ultramarine blue pigments in old master paintings

Natural ultramarine blue is a well-studied pigment, yet the assessment of its quality and purity in works of art remains challenging. This work addresses this issue by establishing an innovative analytical protocol based on micro-Raman mapping to assess detailed compositional information and enable the complete statistical evaluation of the composition of ultramarine blue in paint stratigraphic samples. The experimentation has involved the analysis of several standards of natural ultramarine pigment powders, with different purity grades used to train the Raman technique in mineral identification. The study focused on the identification of the mineral lazurite, which is responsible for the blue colour, and on the associated silicates, carbonates, oxides, phosphates and sulphides associated with the mineral from which the pigment derives. This holistic approach has allowed the selection of diopside as a purity marker of the pigment. Significant differences in the statistical molecular analysis related to the purity grade of the pigment have allowed for the first time to make considerations regarding the quality of the blue pigment in some paint cross-sections from old master paintings from the Courtauld Gallery in London. These samples were also analysed using Synchrotron-based micro-X-Ray Powder Diffraction (SR μ-XRPD) mapping, confirming compositional information gained from the vibrational analysis. The results of this study provide a novel method for the characterisation of different grades of ultramarine blue pigment used in works of art. This data, together with context provided by historical and social information, will aid authentication attribution and inform conservation interventions based on a thorough knowledge of its molecular composition.