Non-destructive and micro-invasive testing techniques for characterizing materials, structures and restoration problems in mural paintings
Graphical abstract
Introduction
The monastery of Santo Spirito in Ocre (L’Aquila, Italy) is a complex of extreme historical and architectural importance as it is the sole example of coenobitic Cistercian architecture substantially intact in the Abruzzo region. The first nucleus of the monastery was built in 1222. Abandoned by the Benedictine order in 1652, it was used as a cemetery from the nineteenth century until 1939. The church stands between the north side of the monastery and the cloister; it consists of a single nave originally covered by an ogival vault, partially replaced by beams after the cave-in, likely occurred in the eighteenth century. The chapel-sacristy is decorated in the lower part with a velarium; above this, frescoes depicting scenes from the Beato Placido’s life are still visible in some areas of the walls. The cycle was supposed to start from the west wall and go further to the south wall. During the 70’s, the detachment of some sixteenth frescoes allowed both the discovery of older layers of confined paintings (indeed, a crucifixion attributed to the thirteenth century is still visible on the southern wall), and of large series of paintings on the eastern wall representing miracles performed by the Beato Placido, not belonging to the cycle and presumably superimposed a few years after the realization of the cycle itself (second half of XIV century) [1]. The historical complex has been the subject of a long and troubled history of interventions for its possible reuse. Unfortunately, an exhaustive outline of the situation regarding the restoration projects performed over the years is lacking to the knowledge of the authors because of the incompleteness of official documents. Since 1978, the site, still unguarded and unused, was the subject of vandalism and theft [2]. In the recent years, the state of preservation of the pictorial cycle appeared rather precarious because of graffiti and a noticeable fallen of plaster and color. In addition, the 2009 earthquake further worsened the situation causing the separation of the northern wall. During the recent restoration campaign completed in 2013 that provided the structural and aesthetic reinstatement of the vestry, diagnostic studies were performed both on the east and on the south walls. Non-destructive and micro-invasive testing techniques were employed with the aim to identify possible structural defects and, simultaneously, to characterize both the original and the subsequent materials applied for restoration purposes.
For the sake of clarity of the readers, a summary of the performed analyses is shown in Fig. 1.
Section snippets
Near-InfraRed reflectography (NIRR)
As an imaging technique, NIRR is a method that allows advanced types of visual analysis [3]. The NIRR method can concentrate its action into narrow-wavelength bands. Among the digital cameras currently in use in order to perform the NIRR method, it exists the Canon 40DH spectrum enhanced with the internal filter named EOS045, wideband spectral and EF55-200 lens of the same company, also equipped with a CMOS sensor 22.2 × 14.8 mm having 10 megapixels (5.7 μm pixel pitch). The latter was used in this
NDT techniques
Fig. 2a shows the right wall where micro-samplings were collected, while the influence of the rising damp can be observed by looking at the bottom part of the wall into the long wave infrared spectrum in Fig. 2b. Indeed, the temperature of the wall surface is affected by the heat exchange (by convection and radiation) with the surrounding environment and by water mass transfer and evaporation. The energy associate with the mass transfer within the wall (water and salts) is less significant than
Conclusions
The diagnostic project of mural paintings was realized combining non-destructive and micro-invasive analytical techniques. In particular, IRT technique identified structural problems, rising damp phenomena, as well as splitting of the painted layer. Instead, NIRR technique improved the visualization of particular pigments. As regards micro-analytical techniques, EDS, FTIR and XRF analyses allowed the identification of pigments as ochres, natural earths used for their relative stability in
Acknowledgements
The authors are grateful to Dr. Michele Nardone, Dr. Lorenzo Arrizza and Ms. Fabiola Ferrante of the University of L'Aquila for their valuable assistance in performing the μ-Raman, SEM-EDS and TG/DTA analyses, respectively. The authors want also to thank the continuous support of the Soprintendenza per i Beni Ambientali, Architettonici, Artistici e Storici per l’Abruzzo (Italy), as well as Ms. Jenny Rolo (restorer) for the kind help during the sample taking procedure.
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