Raman spectroscopic library of medieval pigments collected with five different wavelengths for investigation of illuminated manuscripts

Abstract

Raman spectroscopy is widely applied in the cultural heritage field to perform non-destructive measurements in situ, in order to identify materials, specifically pigments. The spectra collected can be challenging to interpret because certain source laser wavelengths may be absorbed by specific pigments, leading to large fluorescence backgrounds which obscure the weak Raman signals, or worse cause photodegradation of the sample. Furthermore, the reference spectra for a specific pigment obtained from a particular laser wavelength is not always available and is a crucial step in the detective work of pigment identification, especially when the resonance Raman effect can enhance some signals. As the range of lasers available increases, spectral libraries do not always record spectra acquired with the same wavelength used to carry out the measurements in field. In this work, reference spectra of 32 different compounds, mostly used in mediaeval manuscripts as pigments and inks, are recorded. Five different wavelengths were used as excitation sources. The aim is to provide a useful and more complete reference source to enable better planning of which laser wavelength is the most appropriate to study a specific set of pigments, and to allow comparisons between spectra acquired with the same wavelength, leading to the unequivocal pigment identification in a step by step manner.