Thermally Optimised Raman Spectroscopy for Safe Analysis of Wall Paintings and Application to Lorenzetti's Masterpiece

ABSTRACT This work focuses on developing and testing an optimised Raman approach based on mobile instrumentation with 785 and 1064 nm laser excitation wavelengths to characterise artworks safely (i.e. by preventing possible photothermal alteration effects). A recently developed low‐size portable measuring head for thermally controlled Raman spectroscopy (exc. 785 nm) including confocal Raman, emissivity and surface temperature measurement channels was here integrated with a home‐made Raman system provided with 1064 nm laser excitation wavelength, high‐throughput compact InGaAs spectrometer and custom‐designed graphic user interface. Both, Raman excitation wavelengths were interchangeably adapted to the mentioned multichannel head using commercial fibre optic probes. Then, we conducted a laboratory validation study on a large set of samples simulating fresco and secco‐applied mural paintings to investigate the photothermal behaviour and best operating conditions at both excitation laser wavelengths. Results demonstrated, for the first time, that laser‐induced heating was slightly higher at 785 nm than 1064 nm. Moreover, the proposed thermally controlled two‐wavelength approach also evidenced a more robust and enhanced analytical capability, allowing to gather information on degradation, and conservation issues and support the decision‐making process for defining conservation and surface protection operations. Following a preliminary laboratory optimisation, we applied the proposed approach to the molecular characterisation of the Allegory and Effects of Good and Bad Government, a series of three fresco cycles considered Lorenzetti's ‘undisputed masterpiece’.