Investigation of layered glazes coloured with carbon-based pigments using micro spatially-offset Raman spectroscopy

Non-invasive and non-destructive Raman spectroscopic techniques have been used to detect the stratigraphic variation of tints in semi-translucent films, comprising stacked layers with varying low concentrations of carbon-black pigment dispersed in a painting varnish imitating historical artist materials, as in the ‘Sfumato’ technique often associated with Leonardo da Vinci's works of art. Understanding the structures behind this effect could help to conserve such paintings. Micro-Raman spectroscopy is one of the analytical techniques usually applied to painting materials that has proved suitable for tackling the challenging detection and characterization of carbon-based pigments in organic-rich paintings. Model transparent samples have been fabricated following a selected recipe from historical sources and analysed using Raman-scattering-based experimental techniques, including micro-spatially offset Raman spectroscopy (micro-SORS). Single glaze layers spanning a range of concentrations, and multilayer systems mimicking the resulting stratigraphy of the Sfumato glaze technique, have been studied. Raman-spectroscopy performance to detect the pigment was assessed on the single layers; the spectral behaviour was characterized according to the concentration of pigment. The micro-SORS technique was tested on the multilayer systems and appears to be able to distinguish between different stratigraphic samples, varying in concentration of the same carbon-based pigment, and the order of layers. This proof-of-concept observation is promising. It calls for further studies to be undertaken to obtain comprehensive results about an increased number of model painting materials, especially for carbon-based materials mixed with other pigments.