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

The Raman spectra of plattnerite [lead(iv) oxide, PbO2] and of the lead pig ments red lead (Pb3O4), lead monoxide [PbO, litharge (tetragonal) and massi cot (orthorhombic)], lead white [basic lead carbonate, 2PbCO(3). Pb(OH)(2)] and of their laser-induced degradation products were recorded using a rang e of different excitation lines, spectrometer systems and experimental cond itions. 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 ob tain but three bands, at 653, 515 and 424 cm(-1), were identified as arisin g from the b(2g), a(1g) and e(g) modes respectively, by analogy with the co rresponding modes of isostructural SnO2 (776, 634 and 475 cm(-1)). A furthe r oxide was identified, PbO1.55, the Raman spectrum of which does not corre spond to that of any of the laser-induced degradation products of PbO2 at a ny of the wavelengths used. The Raman results are critical to the future us e of Raman microscopy for the identification of lead pigments on artworks.