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Pyroclastic strata have always been thought to protect the archaeological remains of the Vesuvian area (Italy), hence allowing their conservation throughout the centuries. In this work, we demonstrate that they constitute a potential threat for the conservation state of the mural paintings of Pompeii. The ions that could be leached from them and the ion‐rich groundwater coming from the volcanic soil/rocks may contribute to salt crystallisation. Thermodynamic modelling not only allowed to predict which salts can precipitate from such leaching events but also assisted the identification of additional sources of sulfates and alkali metals to explain the formation of the sulfates identified in efflorescences from the mural paintings of Pompeii. For the future, fluorine, mainly related to a volcanic origin, can be proposed as a marker to monitor the extent of the impact in the mural paintings of Pompeii in situ. [ABSTRACT FROM AUTHOR]

Ariadne's house is in the Regio VII of the Archaeological Park of Pompeii (Italy). As it is exposed to external alteration agents (rainfall, water infiltration and atmospheric pollution), the mural paintings located in this important residence clearly show the effects of ongoing degradation processes. This is especially the case of Room 22 (also called the blue room), where the original blue decorations varied towards greyish green hues. In situ and laboratory spectroscopic analyses by means of non‐destructive X‐ray fluorescence, infrared and Raman spectroscopies identified the original pigments and the materials used in the conservation works. Looking in detail at the stratigraphic distribution of the original pigments, using Raman spectroscopy, an inner layer of Egyptian blue mixed with celadonite was detected under a layer of pure Egyptian blue. This discovery proves the presence of a more ancient painting below the one that can be observed today. Considering that celadonite was an inexpensive pigment (thus, probably used to reduce the costs of the paintings), this information strengthens the hypothesis that based on archaeological evidences, there is evidence of an evolution of the socio‐economic status of the ancient owners of the residence. In addition to that, Raman data also proved the occurrence of salt efflorescences; they provided insights about the compromised conservation state of the wall paintings, and they highlighted the action of atmospheric acid gases as an important factor. [ABSTRACT FROM AUTHOR]