“Forgotten Colors”. Tracing the Origin of Mineral and Artificial Early Medieval Pigments in Raetia Curiensis.

Cavallo, Giovanni and D'Erme, chiara and Cassitti, Patrick (2023) “Forgotten Colors”. Tracing the Origin of Mineral and Artificial Early Medieval Pigments in Raetia Curiensis. In: 21st Swiss Geoscience Meeting, 17-18.11.2023, Mendrisio. (Unpublished)

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Abstract

Only very few examples of Early Medieval wall paintings survive in Europe, the most well-known are mapped in Fig. 1. Half of the aforementioned sites are located in the historic region of Raetia Curiens corresponding to the present region of the Grisons Canton (Switzerland) and the Western part of South Tyrol (Italy). This situation led to the propose of a research project funded by the Swiss National Science Foundation (SNSF) (https://data.snf.ch/grants/grant/201055) for comparative studies of wall paintings and painted fragments in the ancient Raetia Curiensis region using the same methodological approach at all sites, including non-invasive (Technical Photography, TP; Hand-held X-ray Spectroscopy, HH-XRF; portable Infrared Spectroscopy, p-ATR/FTIR; Fiber Optics Reflectance Spectroscopy, FORS) and micro-invasive analysis (Optical Microscopy, PLM; Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy, SEM/EDX; Laser Ablation - Inductively Coupled Plasma Mass Spectroscopy, LA-ICP-MS; isotope analysis). The scope of the research is not only oriented towards the characterization of the painting materials and pictorial technique but also for understanding the provenance of the raw materials and the technological processes behind the production of artificial pigments. The application of non-invasive analysis (Bläuer & Keller, 2020; Cavallo et al., 2020; 2023) allowed for a preliminary identification of the materials used. In summary, Fe-bearing yellow and red mineral pigments (generally referred to as ochre) and C-based pigments (presumably charcoal black) were detected in all the studied sites. The presence of red lead (Pb3O4) is common though its presence in St. Peter in Mistail and St. Prokulus in Naturns is questionable. Remarkable is its extensive use in the painted stucco fragments from the St. Martin church in Disentis. The presence of lead white [cerussite and/or hydrocerussite, PbCO3 and/or Pb3(CO3)2(OH)2] was confirmed in all the sites with the exception of St. Peter in Mistail and the monastery church of St. John. Blue pigments (natural ultramarine and Egyptian blue) were detected at Müstair and Mals. Green Cu-based pigments were found in Naturns A green Cu-based pigment was found also at Chur whilst the presence of Egyptian blue was clearly detected both on the Eastern wall of the crypt and on painted fragments. The results of the non-invasive campaign allowed for planning a well-disegned sampling strategy at the site of Disentis. About 40 micro-samples were analysed using microscopic techniques (PLM and SEM/EDX) in order to establish the thickness of the paint layer, the relationship with the preparation and/or the ground layer, the (inorganic) binder possibly used, the mix of different pigments. After this first analytical run, a further selection of samples was carried out for in-depth studies (LA-ICP-MS and lead isotope analysis) at the Department of Geosciences, University of Bern. These first tests confirmed most of the results gathered using non-invasive analysis. Extremely important is the presence of a Pb-Sn-(Si)-based pigment (lead-tin yellows type I and II?) always associated with minium in different proportions depending on the analysed micro sample. This is important for better understanding the technical and technological process behind Pb-based pigments manufacture and use. In addition, the presence of V and As associated with ochre in a few samples may be an important marker for starting to establish the geological provenance of the raw materials.

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