Raman spectroscopy as a means for the identification of plattnerite (PbO2), of lead pigments and of their degradation products

Abstract

The Raman spectra of plattnerite [lead(IV) oxide, PbO₂] and of the lead pigments red lead (Pb₃O₄), lead monoxide [PbO, litharge (tetragonal) and massicot (orthorhombic)], lead white [basic lead carbonate, 2PbCO₃·Pb(OH)₂] and of their laser-induced degradation products were recorded using a range of different excitation lines, spectrometer systems and experimental conditions. The degradation of PbO₂ is more extensive along the pathway PbO₂ → Pb₃O₄ → PbO (litharge) → PbO (massicot) the shorter the wavelength of the excitation line and the higher its power. The Raman spectrum of PbO₂, which is black and of the rutile structure, is particularly difficult to obtain but three bands, at 653, 515 and 424 cm⁻¹, were identified as arising from the b_2g, a_1g and e_g modes respectively, by analogy with the corresponding modes of isostructural SnO₂ (776, 634 and 475 cm⁻¹). A further oxide was identified, PbO_1.55, the Raman spectrum of which does not correspond to that of any of the laser-induced degradation products of PbO₂ at any of the wavelengths used. The Raman results are critical to the future use of Raman microscopy for the identification of lead pigments on artworks.