Signals marked with an asterisk belong to Compton and Rayleigh scatter. Raman spectra collected from the crusts detected on Mirror 1. White crusts a are composed of calcium sulphate, which often display the additional presence of nitrocalcite. The yellow crust b is composed of a mixture of calcium sulphate and goethite.
Smaller white crusts were also found on the surface of Mirror 2 but in this case, the molecular analysis highlighted the presence of gypsum alone see Fig. In the case of the spectra represented in Fig. Lixiviation processes generally occur in the presence of high humidity levels, the atmospheric water is one of the most important factors that has contributed to the degradation process. Research has proved that the pH value of the rainwater in the Vesuvius region is strongly influenced by the dissolution of aerosols originating from maritime, continental and anthropogenic sources [ 51 ].
The abundant atmospheric acid pollutants CO 2 , NO x , SO x can react with the mortars, causing partial dissolution of carbonate and sulphate based compounds mainly of calcium. When this water evaporates, the gypsum reaches a saturation point and the salt precipitates on the obsidian surface or within its cracks. In the lower part of Mirror 1, a yellow crust was also observed see Fig. The XRF spectra in Fig. This hypothesis was confirmed by Raman analysis that identified a mixture of goethite and gypsum see Fig.
Comparison between LIBS spectra collected from restoration mortar and yellow crust respectively. The major presence of Fe in the filling material applied in past conservation works is clearly observable. Considering that the archaeological site of Pompeii is located in a highly polluted area [ 53 ], atmospheric sulphur SO x may also have contributed, together with the process mentioned above, in the development of those crusts.
The elemental compositions of the white crusts on Mirror 1 were investigated using the depth profiling capabilities of the LIBS system. The samples were depth profiled by collecting multiple measurements from the same spot. The LIBS spectra shows the presence of nitrogen, calcium, oxygen and sulphur providing evidence to support the Raman spectroscopic findings. An emission band attributed to potassium is clearly evident, suggesting potassium nitrate KNO 3 is also in the white crust.
The LIBS system has an open-air path between the detector and the point of analysis. Thus, the LIBS spectrum registers the elements present in the sample and in the atmosphere, a constant nitrogen response in samples without nitrogen is the signal from the atmospheric nitrogen. This constant value in this case from 6th to 14th shots is present as a background in all measurements. The response of shots 1—5 indicates that nitrogen, from nitrocalcite, is clearly present in the outer most layer of the analysed area in the white crust, disappearing after the 6th shot.
The LIBS spectrum has no sulphur signal from shots 1—3, but in the spectra 5—7 it is present while in spectra 8—14 it is again absent. Assuming that gypsum is the sulphur source, we must conclude that gypsum is within the layer of nitrocalcite. The LIBS spectra after shot 8 are attributable to the elements present in obsidian, not in the white crust.
The LIBS data provide information about the stratigraphic distribution of degradation on the obsidian surface, suggesting the presence of an inner layer of gypsum covered by an outer layer of nitrocalcite. The possible explanation for the formation of the upper layer of nitrocalcite is a chemical interaction of nitric acid aerosols with gypsum. In a first step, the water of the aerosols and the dissociated nitric acid cause a partial dissolution of gypsum surface layer.
The research summarized in this work clearly underlines the advantages provided by the combined use of elemental and molecular portable analytical techniques in Cultural Heritage for characterization and preservation studies. The complementary use of Raman spectroscopy, ED-XRF and LIBS techniques provided information about the composition of the analysed artefacts, allowing the identification of obsidian as the reflective material used for both Mirrors.
Analysis of the Raman spectra suggested the island of Pantelleria as a possible source of the obsidian used to manufacture the mirrors. The Archaeological Park of Pompeii is currently considering prioritizing the preservation of the mirrors integrity, sampling from these artefacts is at the moment forbidden.
However, it is hoped that the results and prospects of future studies stemming from this research will persuade the institution to undertake studies of provenance, from which numerous historical and commercial inferences could be derived, sacrificing a very small amount of material. The multi-analytical study also identified the major degradation products and we put forward a hypothesis that led to their formation.
By combining the elemental results with the molecular data provided by Raman spectroscopy, three different degradation processes were distinguished. It was proved that the gypsum crusts over both mirrors precipitated after a partial dissolution of the mortars used in past restoration works, which have been probably weakened by the impact of atmospheric agents. Obsidian hydration was not influenced by anthropogenic factors, but represents a natural consequence of the exposure of this vitreous material to humidity.
Finally, the deposition of nitrocalcite layers together with potassium nitrate over the gypsum crusts is indicative of the aggressive attack of the modern atmosphere around Pompeii, where nitric acid aerosols are playing an important role in the conservation of archaeological remains. In addition to providing important data about composition and conservation problems of unique artefacts, the results of this research will be extremely beneficial for conservators, helping them to define future conservation works.
In conclusion, this manuscript represents a clear example of the advantages provided by the crossing of boundaries between Cultural Heritage and analytical research fields. The eruption of Vesuvius of 79 AD and its impact on human environment in Pompei. Pagano M. I primi anni degli scavi di Ercolano, Pompei e Stabiae, Raccolta e studio di documenti e disegni inediti.
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