Marco Veneranda

In situ X-ray fluorescence-based method to differentiate among red ochre pigments and yellow ochre pigments thermally transformed to red pigments of wall paintings from Pompeii

Most of the magnificent wall paintings from the ancient city of Pompeii are decorated with red and yellow colors coming from the ochre pigments used. The thermal impact of the pyroclastic flow from the eruption of Vesuvius, in AD 79, promoted the transformation of some yellow painted areas to red. In this work, original red ochre, original yellow ochre, and transformed yellow ochre (nowadays showing a red color) of wall paintings from Pompeian houses (House of Marcus Lucretius and House of Gilded Cupids) were analyzed by means of a handheld energy-dispersive X-ray fluorescence spectrometer to develop a fast method that allows chemical differentiation of the original red ochre and the transformed yellow ochre. Principal component analysis of the multivariate obtained data showed that arsenic is the tracer element to distinguish between both red colored ochres. Moreover, Pompeian raw red and yellow ochre pigments recovered from the burial were analyzed in the laboratory with use of a benchtop energy-dispersive X-ray fluorescence spectrometer to confirm the elemental composition and the conclusions drawn from the in situ analysis according to the yellow ochre pigment transformation in real Pompeian wall paintings.

Biodeterioration of Pompeian mural paintings: fungal colonization favoured by the presence of volcanic material residues

This work was focused on the study of the biodegradation processes jeopardizing a mural painting conserved in the basement of Ariadne House (archaeological site of Pompeii, Italy). The fresco stood out for its peculiar state of preservation: the upper part, recovered in 1988, was just barely colonized by microorganisms. On the contrary, the lower part (excavated in 2005) was almost completely covered by extensive biological patinas. The genomic characterization carried out by polymerase chain reaction (PCR) highlighted the presence of seven different fungi strains on the mural surface. Beside, in situ and laboratory analyses were performed with the purpose of identifying the causes of the heterogeneous spatial distribution of the biopatinas. The in situ Raman spectroscopy and energy dispersive X-ray fluorescence (ED-XRF) spectroscopy measurements excluded any link between the heterogeneous colonization and the original materials present in the wall. On the other side, X-ray diffraction (XRD) and scanning electron microscopy (SEM) on microsamples proved the presence of a thin volcanic material layer overlying the lower part of the fresco. Considering that most of the biofilms of the studied mural painting only growth over these residues, it was confirmed the role of volcanic material as a suitable support for biological colonization. Thanks to the obtained results, this research helped to understand more in depth an important degradation pathway threatening the artworks from one of the most important archaeological sites in the world.

Study of the soluble salts formation in a recently restored house of Pompeii by in-situ Raman spectroscopy

The walls and mural paintings of Pompeii exposed directly to the rainfalls are the most impacted in view of the observed decay. However, there are also wall paintings in protected rooms showing evidences of decaying. The aim of this research was to study the salts formed in such protected wall paintings only by non-invasive and in-situ Raman spectroscopy to understand their decaying processes. The perystile of the House of the Gilded Cupids (Regio VI, Insula 16), one of the most important houses of Pompeii was studied. Although an exhaustive restoration was carried out in 2004, a new conservation treatment was needed in 2013 and only two years later, extensive crystallizations of soluble salts were again threatening several of the restored surfaces, thus, the presence of an unsolved degradation pathway was deduced. Thank to the proposed methodology, it was pointed out that the key is the acidified rainfall impact in the non-protected backside of the walls containing the wall paintings. Thus, a new concept in the preservation of the houses of Pompeii is provided, in which the need of the protection of those walls from both sides is suggested to avoid the movement of water through the pores of the walls.

In-situ multianalytical approach to analyze and compare the degradation pathways jeopardizing two murals exposed to different environments (Ariadne House, Pompeii, Italy)

This study aimed at using portable analytical techniques to characterize original and decayed materials from two murals paintings of Ariadne House (archaeological site of Pompeii, Italy) and define the degradation pathways threatening their conservation. The first wall, located in an outdoor environment, has been directly exposed to degradation processes triggered by weathering and atmospheric pollution. The second wall, placed in a basement under the ground floor, has been constantly sheltered from sunlight exposure and drastic temperature fluctuations. The analytical data obtained in-situ by using Raman spectroscopy and Laser Induced Breakdown Spectroscopy (LIBS) correlates the degradation patterns affecting the two surfaces to their environmental context. The deterioration processes detected on the outdoor wall, which entailed the complete loss of the paint layer, were mostly related to leaching and thermal fluctuation phenomena. The mural painting from the basement instead, showed deep degradation issues due to soluble salt infiltration and biological colonization. The results obtained from this unique case of study highlight the indispensable role of in-situ spectroscopic analysis to understand and predict the degradation pathways jeopardizing the cultural heritage and provide to the Archaeological Park of Pompeii important inference to consider in future conservation projects.

Study of corrosion in archaeological gilded irons by Raman imaging and a coupled scanning electron microscope–Raman system

In this work, analytical and chemical imaging tools have been applied to the study of a gilded spur found in the medieval necropolis of Erenozar (Bizkaia, Spain). As a first step, a lot of portable equipment has been used to study the object in a non-invasive way. The hand-held energy-dispersive X-ray fluorescence equipment allowed us to characterize the artefact as a rare example of an iron matrix item decorated by means of a fire gilding technique. On the other hand, the use of a portable Raman system helped us to detect the main degradation compounds affecting the spur. Afterwards, further information was acquired in the laboratory by analysing detached fragments. The molecular images obtained using confocal Raman microscopy permitted us to characterize the stratigraphic succession of iron corrosions. Furthermore, the combined use of this technique with a scanning electron microscope (SEM) was achieved owing to the use of a structural and chemical analyser interface. In this way, the molecular characterization, enhanced by the magnification feature of the SEM, allowed us to identify several micrometric degradation compounds. Finally, the effectiveness of one of the most used desalination baths (NaOH) was evaluated by comparing its effects with those provided by a reference bath (MilliQ). The comparison proved that basic treatment avoided any side effects on the spur decorated by fire gilding, compensating for the lack of bibliographic documentation in this field. This article is part of the themed issue ‘Raman spectroscopy in art and archaeology’.

In-situ multi-analytical characterization of original and decay materials from unique wall mirrors in the House of Gilded Cupids, Pompeii

The House of Gilded Cupids (Regio VI, Insula 16, 7, 38) was unquestionably one of the most important residences of Ancient Pompeii, where important archaeological artefacts such as mural paintings, mosaics, sculptures and lalariums were rediscovered. This work characterizes two wall mirrors that, together with those recovered from the House of Efebo and the Domus of Euplia, represent the only ones found in the archaeological site of Pompeii. The 2015 and 2016 expeditions of the Analytica Pompeiana Universitatis Vasconicae project performed an in situ multi-analytical study, using only portable non-destructive analytical techniques. Molecular data provided by Raman spectroscopy suggested obsidian was the reflective matrix for both mirrors. Elemental data provided by energy dispersive X-ray fluorescence and laser induced breakdown spectroscopy (LIBS) systems were concordant with Raman spectroscopic results, enabling the detection of Ca, K, Al and Na as the main elements included in the Si matrix characteristic of obsidian igneous rocks. The LIBS data confirmed the presence of obsidian hydration layers. All techniques were used to investigate the degraded white and yellow crusts of the mirrors. Gypsum (CaSO4·2H2O) and nitrocalcite (Ca(NO3)2·4H2O) were identified in the white areas and gypsum and goethite (α-FeOOH) in the yellow crusts. LIBS depth profiling on the white crusts found a layer of nitrocalcite on top of the gypsum layer. Gypsum and goethite were proposed to form after partial dissolution of the mortars patches (with high iron contents) used in modern restorations around the mirrors, followed by a reprecipitation on the surface of the mirrors. Nitrocalcite was proposed to form after the attack of atmospheric NOx on gypsum crusts. These results represent the first analytical work focused on the study of these unique mirrors and provide the knowledge needed for defining more adequate conservation treatments.