The blue pigments used on altarpieces in 15th Century in Catalonia and Crown of Aragon are principally the azurite mineral. To a lesser extent lapis lazuli, also of mineral origin and occasionally, in the background areas, outlining the figures, and for the chromatic ground layer, indigo, were used. Data from several altarpieces belonging to well-known artists of the time is presented. Synchrotron radiation X-ray diffraction (SR-XRD), micro-infrared spectroscopy (µ-FTIR), synchrotron radiation micro-infrared spectroscopy (µSR-FTIR), Raman spectroscopy, scanning electron microscopy with X-ray microanalysis (SEM-EDS) are used. X-ray diffraction and infrared spectroscopy in association with synchrotron radiation have shown to be especially interesting due to the micron spot sizes, high brilliance and energy tunability which help to obtain good separation of the signals coming from different phases/substances and their localization in the various paint layers. Examples are presented which illustrate the potential of each analytical technique for the identification of this kind of materials in the 15th century paintings. The natural origin and composition of the pigments and its distribution in the paint layers are determined and some correlations with other contemporary paintings obtained. Finally, the alteration compounds related to blue pigments are determined in each case.

Peer Reviewed