Spectroscopic Investigation of Wall Paintings in the Alhambra Monumental Ensemble: Decorations with Red Bricks

Abstract

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.

Characterization of Wall Paintings of the Harem Court in the Alhambra Monumental Ensemble: Advantages and Limitations of In Situ Analysis

“…The identification of this hematite and the elemental composition are compatible with the use of red ochre pigments. These are commonly found in the wall paintings of the Alhambra complex, particularly in the characteristic Nasrid motifs imitating red bricks [ 6 ]. Red ochre has been also identified in other Islamic wall paintings in the Iberian Peninsula [ 35 ].…”

“…Both precedents could have an influence on Hispano-Muslim mural painting. Wall painting in the Alhambra can be found in exterior walls and some other architectural elements like vaults, mainly in the form of faked breaking of bricks in red [ 6 ]. However, it stands out especially in the decoration of the wall socles, in rooms reserved for domestic activities and private places.…”

In situ Raman spectroscopy for cultural heritage studies

“…Barone et al [ 73 ] combined two mobile Raman systems, one dual‐532 and 785‐nm laser system and a 1064‐nm laser spectrometers on the Sala Vaccarini frescoes in Catania, Italy. Arjonilla et al [ 74 ] studied wall decorations mimicking red bricks scattered indoors and outdoors in the Alhambra Monumental Ensemble. Haematite was responsible for the red appearance, while in the white layer, calcite (lime mortar) and in an indoor area gypsum (Paris plaster) were found revealing two different application techniques.…”