Samples of blue ultramarine pigments have been investigated by EPR and Raman spectroscopies, as well as by colorimetry. For the first time, correlations between EPR, Raman and colorimetric parameters of these pigments have been established. It is now established that there is not only one blue ultramarine, but different shades of blue depending on the relative concentrations of the blue and the yellow chromophores. The greenish and the reddish shades of blue are characterised by EPR, Raman and colorimetric parameters.
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Gobeltz et al. (1998) studied this question.
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