Deterioration of lead based pigments on a fresco: a micro-Raman investigation

Micro-Raman spectroscopy analyses were carried out on some pictorial fragments taken from a portion of the fresco in the Chapel of St. Stephen, situated in Montani (BZ), Val Venosta, Italy and painted around 1430. Within this building, especially on the apse and on the vault, an alteration of lead based pigments is clearly visible, as a dark coating. The lead pigment was presumably mainly white lead, used to create highlights and light and dark effects. Samples were taken from altered blackened areas and from areas cleaned according to the traditional method of conversion of white lead, using a solution of acetic acid and hydrogen peroxide in cellulose pulp. The “cleaned” samples appear in their original colors, yellow and green. The samples taken from cleaned areas show characteristic spectra of lead-tin yellow pigments, both type I and type II, identified predominantly from green samples. Goethite, hematite, celadonite green earth, and lapis lazuli have also been found. The Raman spectra have been taken at very low (< 0.1 mW) laser power due to the complex behaviour of lead oxides for photo-thermal effects induced by the laser excitation (here 632.8 nm) [1]. Fig.1 - Raman spectrum of the black degradation material (a) compared with that of plattnerite (b). (c) and (d) are Raman spectra (massicot and litharge, respectively) obtained by laser photo-degradation of plattnerite. The micro-Raman spectra from blackened degraded samples give no evidence of white lead but show always a structured wide band centered at about 515-520 cm-1 which can be attributed to the presence of plattnerite (PbO2), well known alteration product of lead based pigments, especially in presence of moisture and in strongly alkaline environment. Other features at about 230 cm-1 and 600 cm-1 cannot be attributed to lead oxide phases, litharge or massicot [Fig.1]. The Raman spectra are nearly insensitive of the laser power and very often show PbO (litharge/massicot) features of varying intensities. On the other hand, starting from synthetic plattnerite, lead oxides (red lead Pb3O4, litharge and massicot) are obtained at increasing laser power [Fig.1]. The nature of the dark degradation material is discussed on the basis of Raman and XRD results and on degradation tests on white lead, with different binders. [1] L. Burgio, R. J. H. Clark, S. Firth, Raman spectroscopy as a means for the identification of plattnerite (PbO2), of lead pigments and of their degradation products, Analyst 126 (2001) 222–227.