New insights on blue pigments used in 15th century paintings by synchrotron radiation-based micro-FTIR and XRD
The blue pigments used on altarpieces in the 15th century in Catalonia and Crown of Aragon are principally composed of the azurite mineral. To a lesser extent, lapis lazuli, also of mineral origin, was used and occasionally in the background areas and outlining the principal figures, indigo (of vegetal origin) was used for the chromatic preparation layer. Data from several altarpieces belonging to well-known artists of that time are analysed by synchrotron radiation X-ray diffraction (SR-XRD), micro-infrared spectroscopy (μ-FTIR), synchrotron radiation micro-infrared spectroscopy (μSR-FTIR), Raman spectroscopy, and scanning electron microscopy with X-ray microanalysis (SEM-EDS). X-ray diffraction and infrared spectroscopy in association with synchrotron radiation have proven to be especially useful due to the micron-sized spot size, high brilliance and energy tunability, which help to obtain good separation of signals coming from different phases/substances and determine their localization in the various paint layers. The examples presented illustrate the potential of each analytical technique for the identification of the type of material present in the 15th century paintings. Moreover, the natural origin and composition of the pigments and their distribution in the paint layers are determined and some correlations with other contemporary paintings are proposed. Finally, the alteration compounds related to blue pigments are determined in each case.