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The application of spectroscopic techniques to the study of historic and cultural heritage is a crucial tool to increase the knowledge of a community. It is well established that this knowledge provides social groups to become conscious of the importance of their preservation for future generations, so they will understand what came before them. In this sense, all over the region of Extremadura, a wide and varied manifestation of objects and architecture can be found. This cultural heritage belonging to different civilizations settled there is dedicated to a wide variety of uses. The sacred art has been played an essential role in the community where belongs ever since. One of the greatest exponent of artworks from the catholic religious context are altarpieces, which are composed of sculptures, relieves, panel paintings, and etc. These masterpieces are an unique elements of the world's cultural heritage not only because of their morphology and artistic value but also to their social, economic, and cultural implications. The characterization of pigments, methods, and techniques used by craftsmen are critical in finding solutions to problems of restoration, conservation, dating, and authentication.
Four artworks belonging to differnt altarpieces, and dated from the 13th to the 18th centuries have been considered in this study. The whole group came from three different southern churches of Extremadura. Virgen del Valle and Peana, belong to the central part of a lateral altarpiece located at the Nuestra Señora de Gracia parish. San Juanito is a part of an altarpiece at the sanctuary Cristo de las Misericordias. Finally, Cristo del Perdón comes from the parish of Santa Catalina. Except the last one sculpture whose realization are attributed to the prestigious masters of wood Jorge Fernández Alemán, the rest of pieces are anonymous, and this is the main reason why exact dates of their realizations are missed. The aim of this study was to cast light about their origins, pigments, and techniques used not only in their realization but also in the different restorations suffered across centuries. Up to our knowledge, this kind of analysis is the first time carried out in the Extremadura imagery heritage.
MATERIALS AND METHODS
To perform the analyses, several techniques have been used. Energy‐dispersive X‐ray fluorescence gave the elemental composition of the pigments used for the decoration of the pieces studied. This technique has the advantage that it can be applied in situ, and the transport of the artworks to the laboratory is not necessary.
Another advantage is to be a nondestructive technique: No extraction of sample is needed. Pigment identification was carried out with two portable EDXRF instruments. On one hand, a tailored homemade equipment composed of an Ag‐X‐ray tube, with a voltage and intensity ranging up to 30 kV and 100 μA, respectively, and a Si‐PIN detector, with a resolution of 220 eV (FWHM 55Fe) was used. In this case, the points of interest on the sample surface were located by a system composed of a level laser crossed with a pointer laser to determine the intersection point of the incident X‐ray beam and detector axis. On the other hand, a handheld spectrometer composed by an Rh tube up to 50 kV and 100 μA and a FAST SDD detector was also used. This spectrometer was mainly used in the analyses performed in the Cristo del Perdón sculpture, due to the capacity to access easily to the points of interest.
Stratigraphy of some samples was studied taking fragments of millimeter size from the parts to be analyzed and embedded in polyester resin. These fragments were analyzed by two methods. Firstly, they were observed through an optical microscope with a 10× ocular and 4×, 10×, and 40× objectives. This observation allows distinguish the different layers existing in the analyzed zone, finding out possible previous restorations or repaints. In addition, the same extracted millisamples were observed studying the X‐ray emissions produced in a scanning electron microscope QUANTA 3D FEG equipped with an energy‐dispersive X‐ray analyzer to determine the elemental composition of the sample layers. All the analyses in this last equipment were made in low vacuum using the backscattered‐electron detector
In this work, four religious sculptures from the 13th to 18th centuries have been analyzed. Stratigraphic optical views show different layers which in some cases include two well‐distinguished pictorial coverings. This implies that, at least, a general intervention was carried out on the original polychromies.
The absence of Au and Ag in most of the analyzed golden zones and the massive presence of Al and Si with different metals, such as Fe, Cu, Zn, and Ti, in the EDXRF results suggest the application of brass powder on mica leaves to make false gilding. However, a few golden areas presented Au layers on red bole, especially in the San Juanito sculpture, indicating the traditional gilding technique applied in these cases. In addition, gypsum, and calcite seem to be the main compounds used in the ground layers in most of the samples analyzed. SEM‐EDX studies confirm the EDXRF results of absence of Au at least in the samples removed from the artworks. However, in two cases, an Ag‐layer was detected, although it was not identified with the EDXRF technique. Moreover, different gilding techniques have been detected depending on the area of the sculpture analyzed. However, in order to complete the characterization of these sculptures, the knowledge of organic binders used is required.
In addition, some of the samples analyzed showed a restored layer with different pigments from the original polychromic sculptures. The work presented here show how the application of several spectroscopic techniques can be used to study and characterize the composition of the materials used for creating artworks. Different interventions can be discovered analyzing layers. This study constitutes a very helpful tool not only for the classification of the sculptures, but also these techniques are specially indicated to undertake correct restorations and should be applied before curator actions should be performed.
Note: This work is partly presented at International Conference on Physics on June 27-29, 2016 New Orleans, USA.