Comparison of Different Non-invasive µ-Raman Set-ups for Studying Pigments in Art Works

2017; 1(1): 06-16 Micro-Raman spectroscopy is widely used for the identification of pigments on art works. Due to the priceless value of artistic objects, non-invasive measurements are strongly preferred over sampling strategies. However, the non-invasive spectral response is often lower if compared to measurements carried out on a sample. In this study, spectral intensity of measurements performed with various non-invasive Raman set-ups was compared with spectral intensity from a traditional micro-Raman set-up,. The comparison was done on pure pigments reference swatches first, to test the methodology. Then a painting was investigated, as case study on a real artefact, directly on its surface and on taken samples under the microscope. For the non-invasive measurements, four horizontal beam set-ups and an external measuring head connected to the spectrometer by fibre optics were used. Two lasers were used: 785 and 532 nm. The results were determined as counts (Raman signal intensity) per second of measurement and per Watt of laser intensity (Cnts/Ws). A method to compare performances among the different experimental conditions is proposed in this work. The horizontal beam configurations showed spectral responses in signal intensity very similar to those of the traditional microscope but the ones from the fibre optics were only at about 10% of that of the microscope. These results show the potential of horizontal set-ups for the analyses of artworks, as their performance is equivalent to the traditional microscope configuration. However, the more challenging positioning for the horizontal beam set-ups can hamper the acquisition of a good spectrum, since either the Raman spectrometer or the artwork has to be moved with μm scale precision to position the laser spot on the area of interest. This often requires improvements, especially regarding the mechanical stability of the analytical set-up as well as the artwork.