This paper presents results from the examination of a set of experimental samples using fibre optic reflectance spectroscopy (FORS) and diffuse reflectance imaging spectroscopy in the short-wave infrared (SWIR) range, combined with statistical analysis of the data for the discrimination and mapping of poppy and linseed oil. The aim was to evaluate the efficacy of this non-invasive approach for the study of paint samples with a view to the application of the method for characterisation of the two drying oils in painted art. The approach allowed discrimination between the two drying oils based on FORS spectra and the hyperspectral cube, indicating the influence of the spectral region around 1700 nm on the statistical discrimination based on the anti-symmetric and symmetric first overtone stretching of methylenic CH₂ groups. This method is being studied as a potential non-invasive method of organic analysis of oil types that have formerly been studied using gas chromatography-mass spectrometry (GC-MS), which requires micro-samples.

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.