Multi-analytical study of the medieval wall paintings from the rupestrian church Grotta del Crocifisso at Lentini (eastern Sicily): new evidence of the use of woad (Isatis tinctoria)

Abstract

This study presents the results of the examination and characterisation of the wall paintings that decorate the rupestrian church named Grotta del Crocifisso, which is located in the territory of Lentini (eastern Sicily, few tens of kilometres from Catania and Syracuse). The earliest mural paintings in the church date back to the twelfth century AD. A multi-analytical approach was adopted for the characterisation of stone materials, secondary degradation products, and pigments. For this purpose, the following techniques were used: reflected light microscopy (RLM), polarised light microscopy on thin sections (PLM), X-ray powder diffraction (XRPD), mercury intrusion porosimetry (MIP), portable X-ray fluorescence (p-XRF), scanning electron microscopy (SEM-EDS), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy (RS). The lithic substrate and the plaster’s coating layers were thoroughly characterised from compositional and textural points of view, and the use of locally available raw materials was established. Similarly, the newly formed crystalline phases produced by alteration processes of the original materials were recognised. The red, yellow, brown, and green pigments were easily identified by p-XRF and SEM-EDS. The use of “earth pigments” widely available in the surrounding area (various types of ochre) was thus highlighted. The recognition of the dark blue pigment created some additional issues in its identification, making further diagnostic methods necessary. In fact, the use of the most common mineral pigments was categorically excluded by both p-XRF and SEM-EDS, since no chromophore metallic elements were highlighted with the exception of trace amounts of iron. A combination of detailed microscopic observations together with the application of FTIR and RS supported the use of an organic pigment obtained from the maceration of woad (Isatis tinctoria). The green pigment is the result of a mixture between woad and yellow ochre. Woad is even today easily available in Sicily, and some additional experimental tests were carried out on Isatis tinctoria that had been freshly collected in the area (treated with traditional procedures). Over the past centuries, woad was widely used for dyeing fabrics, but its practice for wall paintings has only been sporadically proven. The identification reported in this case study could be considered a novelty at least in the Sicilian panorama.

Current progress on murals: distribution, conservation and utilization

“…The frescoes in this church, which date back to the 9th to tenth century, present a direct derivation from the Byzantine stylistic features [ 26 ]. In the rupestrian church Grotta del Crocifisso (Lentini, Italy), the earliest mural paintings date back to the twelfth century, and these wall paintings constitute the greatest wealth of this church [ 27 ]. Located in Castellammare del Golfo ( Trapani, Italy), Santa Margherita ’s cave is a natural cave, containing the remains of paintings, which can date back to thetwelfth century.…”

“…Using this method, the composition of the pigments in the fifteenth century Thubchen Lakhang monastery was presented, and it was found that the blue pigments were azurite (C 2 H 2 Cu 3 O 8 ) and sometimes with lazurite ((Na, Ca) 8 [(S, Cl, SO 4 , OH) 2 |(Al 6 Si 6 O 24 )]), the red and orange paint layers were orpiment (As 2 S 3 ) and vermilion (HgS), the brown and green color decorations are composed by red ochre (Fe 2 SO 3 ) and malachite (Cu 2 CO 3 (OH) 2 ) [ 89 , 90 ]. For the murals in the rupestrian church Grotta del Crocifisso at Lentini in Italy, X-ray fluorescence and scanning electron microscopy can easily identify the red, yellow, brown, and green pigments [ 27 ]. Using multiple techniques including binocular microscopy, field emission scanning electron microscopy, X-ray spectroscopy, X-ray fluorescence, Fourier transform infrared spectroscopy, thin-film X-ray diffraction, micro-Raman spectroscopy, and Gas chromatography-mass spectrometry, the materials composition in the sixteenth century Orthodox Syrian Church in south India were determined.…”