In Situ Spectroscopic Detection of Lead Sulphide on a Blackened Manuscript Illumination by Raman Microscopy

This study investigates a common pigment degradation process, the darkening of lead white by its conversion to lead sulphide (PbS), that is often assumed to have taken place before restoration treatments are applied. For the first time, Raman spectra similar to those obtained from mineral galena (PbS) have been acquired directly from the surface of a manuscript illumination suffering from lead white discoloration. These data, which were collected using a new generation, highly sensitive Raman microscope, indicate that previously published Raman spectra of PbS on historical materials were the result of laser-induced degradation of that material during the analysis. This new instrumentation can be used routinely for the in situ, non-destructive identification of lead white degradation products prior to restoration treatments and provides a means of studying the effects of conservation treatments on artists' materials.