The polychrome plasterwork decorations of the Room of the Beds in the Royal Bath of Comares of the Alhambra monumental ensemble have been studied using Raman microspectroscopy and complementary techniques. This area keeps the testimony of the controversial restorations carried out in the 19th century in an attempt to imitate the lost original appearance of the authentic Nasrid plasterwork. Raman spectroscopy and energy dispersive X-ray fluorescence have been employed to identify the pigments and extenders. Scanning electron microscopy-energy dispersive X-ray spectroscopy has been used to gain additional information about the morphology of the painting layers. Additionally, infrared microspectroscopy provided insight into the nature of the organic materials employed as binders. Vermillion, synthetic ultramarine blue, hematite, and carbon black were clearly identified in red, blue, brown, and black decorations by Raman spectroscopy. Green decorations were executed with a copper-arsenic pigment that could not be unambiguously identified although the presence of Raman bands typical of arsenate stretching bands could point to alteration processes of copper arsenite pigments. Regarding the execution technique, the pictorial layer was applied over a preparation layer of white lead that also contained barite using a proteinaceous binder.The presence of anglesite and other phases related to hydrocerussite alteration due to humidity and salts was also evidenced. Finally, a comparison of the materials found in this redecoration with those identified in original Nasrid decorations has been performed, revealing noticeable differences in both the materials and the execution technologies.

The Alhambra Monumental Ensemble (Granada, Spain) is a unique well-preserved palatine city from the medieval Islamic period, and it constitutes the best example of Nasrid architecture. In this work, we focus on the study of one of its most unknown decorations: Wall paintings with the appearance of red bricks. These faux-brick decorations are found in many different locations within the Alhambra complex, including both exterior and interior walls, arches and vaults. We have considered locations from different Nasrid reigns to gain information about their characteristics in terms of materials, execution techniques and conservation state. They have been studied combining a non-invasive methodology using portable equipment (X-ray fluorescence (XRF) and Raman spectroscopy) with complementary studies on selected samples (Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDS) and Raman microimaging). In general, those located in the exterior are poorly preserved, in contrast with the good conservation state of the interior motifs. The red rectangles of these decorations were painted over a white finishing layer, which acted also as the edge lines between these false bricks. The red colour was always due to hematite (α-Fe2O3), as revealed by its characteristic Raman bands. The use of a natural red ochre pigment (very abundant in the region) could be hypothesised considering XRF and SEM-EDS results. In general, the white layer was made of lime mortar and the presence of CaCO3 in the painting layers suggests the use of lime-based techniques (either fresco or mezzo fresco). Only in one of the indoor locations, a different execution technique, based on gypsum (CaSO4·2H2O) plaster, was used. The identification of calcium oxalate in this location, in the form of weddellite (CaC2O4·2H2O), can be interpreted as the result of organic binder degradation. Furthermore, superficial contamination with gypsum was always detected in outdoor locations.

Non-invasive techniques (X-ray fluorescence, XRF, and Raman spectroscopy) were used for the study of the Hispano Muslim wall paintings. Principal component analysis (PCA) was performed on the semi-quantitative XRF results directly provided by the in-built factory calibrations with minimum user manipulation. The results obtained were satisfactory and highlighted differences and similarities among the measurement points. In this way, it was possible to differentiate the decorations carried out on gypsum plasterwork and the wall paintings over lime plaster. The color palette, revealed by combining the results from XRF and Raman spectroscopies, comprised the pigments hematite, lapis lazuli, cinnabar (in poor conservation state), and possibly, carbon. Evidence of past interventions was also provided by PCA on XRF data, which detected the presence of Pb, Ba, and Zn in some areas. Furthermore, the preparation layers have been studied in detail on cross-sections of two microsamples. Several layers of lime plaster with a compact microstructure have been observed. The characteristic of the pictorial layer and the identification of calcium oxalate point to the use of a secco-technique. The main alteration identified was a gypsum surface layer covering the painting and signs of plaster deterioration due to gypsum migration to more internal areas. Finally, the comparison with the observations made by restorers in previous interventions on these paintings revealed the importance of the representativeness gained with the in situ study, which enabled the analysis of a high number of areas.

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