| Abstrakt: | SUMMARIES. 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.
CONCLUSION. This work demonstrates the value of Raman microscopy as a non-destructive analytical technique for identifying the chemical species responsible for lead white discoloration, one of the most common and conspicuous forms of pigment degradation. In the future it will be possible to identify the presence of PbS on affected artwork by comparing Raman spectra acquired on samples in situ with the reference spectrum shown in Figure 3a without the need for intentional degradation of the sample to provide corroborative evidence. The ability to identify microscopically thin layers of PbS as the offending material on the many thousands of artworks so affected ensures that a suitable conservation treatment can be applied and that the original intent of the artist can be restored, if this is considered appropriate. Furthermore, this work indicates that Raman microscopy is a promising technique for the study of conservation treatments and their effects on pigments in aqueous media on paper, an area not much investigated so far. |
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