On the use of photobleaching to reduce fluorescence background in Raman spectroscopy to improve the reliability of pigment identification on painted textiles

Subjecting a specimen of red lead watercolour paint on silk to photobleaching was demonstrated to be a simple means by which to engineer a reduction in the magnitude of the fluorescent background that was approximately exponential with time, with a corresponding improvement in the signal to noise ratio of the Raman spectrum, thus rendering the characteristic peaks more easily visible and allowing more confident identification of the pigment. However, relative heights of the Raman peaks obtained from the sample were seen to alter progressively as a result of irradiation, indicating that some component of the sample was undergoing degradation that may result in longer-term damage to a fragile historic artefact. It was also shown that crystals of the lead monoxide pigment massicot were present in the samples of red lead on a painted silk artefact dating from 1750. It is concluded that this was either due to deliberate mixing of pigments by the artist, contrary to historic records, or as a result of the roasting techniques used to create red lead pigments at the time and not due to thermal degradation of the pigment during Raman analysis.

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