Raman spectroscopy characterization of colored pigments in archaeological materials

Archaeological artifacts and artworks with colors are being increasingly analyzed in these years. The pigment materials used for their creation are much studied after the development of modern non-destructive micro sampling analytical techniques. Art historians, museum conservators, and archaeological scientists are now much aware of the importance of physicochemical characterization for the attribution of the historical period and genuineness of an item. Ancient technological methods used in the construction of the items may be characterized by spectroscopists with a minimal disturbance to the artifacts or artworks. In this connection the Raman spectroscopy technique must be considered a most elegant method for pigment and materials analysis of relevant museum and archaeological materials. This is done by correlating some bands in the studied pigments with those of well characterized references. The use of Raman spectroscopy can be taken to illustrate this: It provides e.g. information of importance to art restorers and museum conservation scientists in preserving materials and the understanding of deterioration processes. It does so by identification of key components, as shown in Fig. 1. Prior to the advent of modern Raman spectroscopy it was difficult to analyze archaeological materials, due to difficulties arising principally from the generation of fluorescence by shorter-wavelength visible radiation. There was also the real possibility of sample degradation occurring from the use of the relatively high laser powers that years ago were necessary for sufficient sample excitation. The major advantages of Raman spectroscopy over infrared spectroscopy, namely the weak Raman scattering of water, could not be fully exploited. This situation has changed recently, as shown below in this review. The number of research papers on the subject of Raman spectroscopy applied to pigments and art has been growing very fast during the last years. To get a comprehensive overview we refer to three recent theme numbers of Journal of Raman Spectroscopy1, 2, 3 and other dedicated texts such as e.g. Edwards et al.4, 5 To help the reader we start by a short presentation of some technical details in Raman spectroscopy.