Temperature sensing during Raman spectroscopy of lead white films in different purity grades and boundary conditions

In the field of laser-based diagnostics and treatment techniques for artwork conservation it is known that lead white, wide use as art pigment in the past, shows low photothermal stability upon laser irradiation. Thermal alterations of lead white paint films are often observed during Raman spectroscopy, although the origin of heating has not yet been exhaustively explained. Here, we approach the interpretation of this phenomenon through the preparation of high- (analytes) and low-grade (art pigment) lead white films, thorough compositional and structural characterizations, measurement of the optical parameters, and online temperature measurements during Raman spectroscopy excited at 1064 nm. Spectra and associated temperature profiles were achieved using a system equipped with an online thermal sensor. The data collected show the crucial importance of the degree of purity in heat generation upon laser exposure. Due to their higher content of FeO inclusions, low-grade films (150-250 ppm against 50 ppm of high-grade) showed absorption coefficients five times higher than high-grades films, and temperature rises up to 80-100 °C upon 207 W/cm irradiation and 25 s exposure. Optical absorption and heating of low-grade films determined weaker signals and higher backgrounds in the Raman spectra, whose origin was ascribed to luminescence of FeO inclusions. Moreover, the spectral and thermal influence of the substrate was also investigated thus achieving a complete picture of the photothermal behavior of lead-white art pigments during Raman spectroscopy.