Offset Raman Spectroscopy Research Articles

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

AbstractNon‐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.

Non‐destructive analysis of concentration profiles in turbid media using micro‐spatially offset Raman spectroscopy: A physical model

AbstractWe introduce an analytical physical description of micro‐spatially offset Raman spectroscopy (micro‐SORS) experiments in turbid samples with a special emphasis on its use for studying the penetration depth of chemical species (agents) into a matrix. Such samples exhibit varying concentration gradients with depth and reflect typical situations encountered in Cultural Heritage framework. The experiments aim at yielding the variations (trends) of the Raman intensity ratio of the agent and matrix marker bands, , with increasing micro‐SORS defocusing distance . The physical model shows that can be expressed as a function of the concentration profiles of the analytes. The analysis makes use of Monte Carlo simulation results of the propagation of photons in turbid media. Despite the introduced approximations, the obtained formulas provide an insightful tool that allows justifying the observed trends and facilitates the interpretation of micro‐SORS data and systematic comparison among different samples. Basic comparison with previously published experimental micro‐SORS data indicates good consistency. The obtained relationships could also be useful for a semi‐quantitative estimate of concentration profiles in unknown samples.

Non‐invasive and in situ investigation of layers sequence in panel paintings by portable micro‐spatially offset Raman spectroscopy

AbstractNowadays, the research in conservation science is focused on developing advanced portable analytical techniques able to achieve subsurface data in situ and in a non‐invasive way. Micro‐spatially offset Raman spectroscopy (micro‐SORS) is an emerging analytical technique developed to address the need of non‐invasively retrieving the molecular composition of the subsurface compounds; however, so far, micro‐SORS studies of artworks have been carried out only with benchtop Raman microscopes, precluding the non‐invasive analysis of large objects and artworks in situ. Here, for the first time, portable micro‐SORS was used for non‐invasive and in situ investigation of art objects. The potential of portable micro‐SORS is demonstrated through the reconstruction of the layer sequences in selected areas of two 16th century panel paintings, providing unequivocal information about the preparation layer spread over the panels and the pigments used in the painted layers and supplying essential information on the artist's technique.