Raman identification of pigments and opacifiers: Interest and limitation of multivariate analysis by comparison with solid state spectroscopical approach—II. Arsenic-based opacifiers and relation with cobalt ores

Abstract

Raman analysis is a noninvasive technique that can be conducted on-site via a mobile system. However, the presence in variable quantity of different crystalline or amorphous phases (glassy matrix, opacifiers and pigments) in the enamel leads to the partial observation of Raman signatures, signatures often not being characterized in the literature. We study here the phases observed in the white, blue and yellow/green enamels of French and Chinese objects from the 18th century and compare the effectiveness of a solid-state spectroscopic approach (identification of the parameters determining the vibrational modes in relation to the structures) and the multivariate chemometric approach (PCA and Hierarchical Clustering), carried out on the global or partial spectra, or on the parameters extracted from the first approach. The incorporation of the spectra of neighbouring or similar phases from the literature into the corpus of processed data, in particular those obtained for phases that are not initially structurally characterized, is decisive for the quality of the results. At least four different lead arsenates are identified. The combination of visual observation of the peaks and multivariate analysis shows that the shape, position and area of the band associated with the main As-O stretching vibration around 820 cm−1 vary according to several criteria related to production techniques specific to certain workshops. The contribution of multivariate analysis based on spectra alone appears to be very limited compared to the examination of spectroscopic parameters.