Testing of Raman spectroscopy as a non-invasive tool for the investigation of glass-protected pastels

Five French pastels and a sanguine drawing dating from the 17th to the 20th century were studied by Raman spectroscopy. Different operative conditions were used: the pastels were investigated through their protective glass, and the results obtained were compared with those obtained after removing the glass and after sampling a micrometric particle of pigment. Different parameters (wavelengths, powers of excitation and objectives) were tested in order to assess the optimal procedure of analysis for this fragile work of art. The results obtained for black (carbons), yellow (chrome/cobalt yellow), red (lead oxide, vermillion, orpiment), brown (red lead and chrome yellow), blue (Prussian blue, lapis lazuli/ultramarine), green (mixture of above blue and yellow pigments) and white (calcite, lead white, anatase) pigments are presented and the consistency of the pigments' period of use with the dating proposed for each pastel is evaluated. In one of the pastels, the blackening of the carnation colour made of an unstable mixture of lead white, red lead and vermilion was studied. Raman spectroscopy is used to investigate six French pastels dating from the 17th to the 20th century experimenting different operative conditions and parameters, in order to assess the optimal procedure of analysis for this fragile kind of work of art through their protective glass. The results obtained for the different pigments, the relative criteria of dating and the blackening process in one of the pastels are presented. Copyright © 2010 John Wiley & Sons, Ltd.