Spectrochemical characterization by micro-FTIR spectroscopy of blue pigments in different polychrome works of art

Analytical examination of blue paint samples, taken from different artworks dating from the second century AD until 1960, was carried out using FTIR m icrospectroscopy (both in reflection and transmission mode, the latter with the aid of a diamond cell) supported by micro-Raman spectroscopy and SEM o bservations as well as EDX spectrometry. Pigments examined included azurite , Egyptian blue and ultramarine blue, either natural or artificial. Charact eristic spectra for each pigment are presented and an account for the princ ipal differences observed is given. In particular, the FTIR's capability of probing single pigment grains on the surface of paint cross-sections, when integrated with a microscope, brought to light frequency differences and o rientation behaviour of the bands of the various minerals used as painting materials. Indeed, a striking effect of shifting and variation of intensity of some of the bands of azurite and Egyptian blue was noted and related to polarization phenomena occurring when the incident light stroke differentl y oriented faces of the single pigment crystals. The effects of different m ineralogical composition of the semi-precious stone lapis-lazuli (from whic h ultramarine blue is derived) on the appearance of the spectrum of the pig ment are also discussed. Finally, a means of distinguishing the natural fro m the artificial ultramarine blue pigment, on the basis of their FTIR spect ra, is proposed. (C) 1999 Elsevier Science B.V. All rights reserved.