Mitigating laser-induced alterations of lead pigments during Raman microscopy

• Safe irradiation protocols were established for Raman microscopy of lead pigments. • Optimal Raman conditions maximize S/N while preventing pigment alteration. • Laser-induced alterations were evidenced through new Raman bands, intensity decrease, and visible darkening of pigment surfaces. • Pigment behavior strongly depended on excitation wavelength, laser power density, exposure time and focusing conditions. • Binding media and varnish mitigate pigment alteration, yet laser power density during Raman analysis should remain below thresholds defined for neat pigments. This study investigates the interaction between laser light and lead-based pigments during Raman spectroscopy analysis. Two pigments, red lead (Pb 3 O 4 ) and massicot (PbO), known since antiquity, have been examined using three Raman spectrometers: a mobile system with a continuous wave (cw) diode laser source emitting at λ exc = 785 nm, and two laboratory microscopes with excitation wavelengths at λ exc = 473 nm and λ exc = 532 nm, respectively. The pigments were analyzed neat, in powder form, and also dispersed in a binder and coated with a layer of varnish (in the case of red lead). Key irradiation parameters were examined including laser power, focusing conditions (using various objective lenses), time of pigment exposure to the laser beam and excitation wavelength. The main objective of this research has been to establish safe irradiation protocols for Raman microscopy when analyzing common pigments widely used by artists. It is of great importance for the analyst to obtain reliable results, maximizing the signal-to-noise (S/N) in the Raman spectra while in parallel preventing pigment damage due to excessive laser exposure. The study revealed that the two lead-based pigments interacted differently with the excitation laser beam, as evidenced either on the basis of new Raman bands which appeared in the spectra indicating chemical transformation of the original pigment or through observation of noticeable decrease in band intensity with increasing laser power. A visible darkening of the pigment surface occurred when laser power exceeded a certain threshold (alteration threshold). Furthermore, the presence of a binding medium in the paint layer and/or a varnish coating reduced the extent of pigment alteration, particularly for red lead. Despite this favorable protective effect offered by the binder and the varnish, it is strongly suggested that during Raman analysis laser power density be kept below the pigment alteration threshold values as defined in the case of the neat pigments.