Understanding the degradation of the blue colour in the wall paintings of Ariadne's house (Pompeii, Italy) by non‐destructive techniques

Abstract

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.

Elucidation of the Chemical Role of the Pyroclastic Materials on the State of Conservation of Mural Paintings from Pompeii

“…In addition, the ARI leachate solution is almost saturated with respect to niter (KNO 3 ), epsomite (MgSO 4 ⋅7 H 2 O), halite (NaCl), and thenardite (Na 2 SO 4 ) or mirabilite (Na 2 SO 4 ⋅10 H 2 O) [32] . Some of these salts were identified in the efflorescences from the mural paintings considered in this work and in further in situ analyses at the Archaeological Park of Pompeii, [5, 33] and are known to create significant problems in stones and other building materials [34–36] …”

Elucidation of the Chemical Role of the Pyroclastic Materials on the State of Conservation of Mural Paintings from Pompeii

“…Moreover, although the Na+ concentration was also incremented in the thermodynamic calculations to explain the formation of some salts experimentally detected, the lack of supersaturation of mirabilite and thenardite in most of the cases suggests that a much higher concentration of this alkali metal should be present for the precipitation of these salts. Since mirabilite/thenardite have been widely identified in the murals of Pompeii, [5,31] and considering that sodium is not the most concentrated alkali metal of volcanic origin, an additional source should be identified for the formation of these kind of sulfates (e.g. consolidation mortars rich in alkali metals used in past restorations).…”

“…(Na 2 SO 4 •10 H 2 O). [32] Some of these salts were identified in the efflorescences from the mural paintings considered in this work and in further in situ analyses at the Archaeological Park of Pompeii, [5,33] and are known to create significant problems in stones and other building materials. [34][35][36] Special attention should be paid to the formation of sylvite (KCl) in the ARI leachate,w hich suggests ap referential formation of this alkali metal chloride due to the higher concentration of both ions in the modelling.…”

“…Furthermore,although the concentration of Na + was also incremented in the thermodynamic calculations to possibly interpret the formation of some salts experimentally detected, the modelled solution was almost never supersaturated with respect to thenardite and mirabilite.This suggests that amuch higher concentration of Na + must be present (or asubstantial drop of temperature to occur) for the precipitation of these dangerous species.S ince the crystalline phases of Na 2 SO 4 have been widely identified in the mural paintings of Pompeii, [5,33] and considering that sodium is not the volcanogenic alkali metal with the highest content, an additional source should be identified for the formation of these salts. High values of Na + were identified in the groundwater from the archaeological site of Cumae.…”

Applications of Raman spectroscopy in art and archaeology

“…Ongoing degradation caused by external agents (rainfall and atmospheric pollution) is a serious problem in the Archaeological Park of Pompeii (Italy). Prieto‐Taboada et al [ 17 ] studied the special case of a blue colour changing into greyish green hues in mural paintings in Ariadne's house using in situ and laboratory spectroscopic methods. Analyses were performed with a mobile Raman system (785‐nm excitation), a laboratory‐based confocal Raman system (785‐nm excitation), an ATR‐FTIR spectrometer and an energy‐dispersive XRF spectrometer.…”