Characterization of Soluble Salts on the Frescoes by Saturnino Gatti in the Church of San Panﬁlo in Villagrande di Tornimparte (L’Aquila)

: Salt crystallization is one of the most dangerous forms of degradation affecting frescoes. This phenomenon can lead to cracking, ﬂaking and detachment of the pictorial layer, ultimately ruining the work of art. However, the characterization of soluble salts via chemical analysis can be employed to determine the conditions of the artifact and establish the proper restoration and/or conservation strategies to be adopted. In this archaeometric study, a ﬁrst-ever characterization of the soluble salts and related degradation phenomena on the frescoes by Saturnino Gatti in the church of San Panﬁlo in Villagrande di Tornimparte (L’Aquila) was carried out. Sampling was performed in areas with evident detachments, exfoliations and saline crystallization (efﬂorescences). Eleven samples of powder and fragments were taken from different panels of the fresco: nine were taken from Panels A, C, D and E and two from the top part of the vault. Chemical characterizations were performed using two analytical techniques: ion chromatography (IC) and attenuated total reﬂectance Fourier-transform infrared spectroscopy (ATR-FTIR). Ion chromatography was used for the quantiﬁcation of the main ions and to evaluate the presence of soluble salts, whereas infrared spectroscopy was used to characterize the mineralogical phases. The results show efﬂorescence consisting of newly formed gypsum and carbonate in samples taken closer to the ground. Furthermore, a good correlation between sodium and chloride ions was observed, indicating the presence of an efﬂorescence composed of newly formed sodium chloride. Capillary rise and inﬁltration were highlighted as the main sources of soluble salts. This information will be crucial in guiding future restoration or conservation operations.


Introduction
This paper contributes to the Special Issue "Results of the II National Research project of AIAr: archaeometric study of the frescoes by Saturnino Gatti and workshop at the church of San Panfilo in Tornimparte (AQ, Italy)" in which the scientific results of II National Research Project conducted by members of the Italian Association of Archaeometry (AIAr) are discussed and collected. For in-depth details on the aims of the project, see the introduction of the Special Issue [1].
One of the most serious forms of damage affecting porous stone materials used in monuments and other construction materials is directly linked to the physical action exerted by the crystallization of soluble salts [2][3][4][5][6]. This process can cause several degradation phenomena, including flaking, cracks, detachment or staining of the pigment layer, efflorescence, sub-efflorescence and formation of crusts, all of which can lead to irreversible damage of the original material [7,8]. As documented in numerous studies [7,9,10], walls and frescoes suffer greatly from decay induced by salt crystallization, and conservation strategies often need to be put in place in order to salvage the artworks. However, these abundant mineralogical phases. The aim of this research is to characterize the salts in the masonry that caused the degradation observed on the surfaces of the church, both in terms of nature and concentration. A complete characterization of the fresco was performed by sampling areas at different heights, covering the entire structure. Thanks to the chemical analysis, it was possible to trace the sources of the soluble salts. The results of this study will also be useful when conducting the restoration processes of the walls.

Sampling
Eleven samples of powder and fragments were taken from different panels of the fresco in order to characterize the degradation produced by the presence of salts. The samples ( Figure 1 and Table 1) were taken from areas where salt efflorescence and detachment of the plaster surface were visible. Sampling was carried out at different heights: seven fragments were taken close to the ground level (SG_1 to 7), two samples were taken near window splays (SG_8 and SG_29), and two samples were taken from the top of the vault (SG_37A and SG_41B).
is located in the city of L'Aquila, in the Abruzzo region of Italy. Eleven samples were retrieved at different heights along the walls of the church hosting the frescoes and characterized using ion chromatography (IC) and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR). The former was used to determine the ionic composition, in terms of both anions and cations, whereas the latter was employed to identify the most abundant mineralogical phases. The aim of this research is to characterize the salts in the masonry that caused the degradation observed on the surfaces of the church, both in terms of nature and concentration. A complete characterization of the fresco was performed by sampling areas at different heights, covering the entire structure. Thanks to the chemical analysis, it was possible to trace the sources of the soluble salts. The results of this study will also be useful when conducting the restoration processes of the walls.

Sampling
Eleven samples of powder and fragments were taken from different panels of the fresco in order to characterize the degradation produced by the presence of salts. The samples ( Figure 1 and Table 1) were taken from areas where salt efflorescence and detachment of the plaster surface were visible. Sampling was carried out at different heights: seven fragments were taken close to the ground level (SG_1 to 7), two samples were taken near window splays (SG_8 and SG_29), and two samples were taken from the top of the vault (SG_37A and SG_41B).

Ion Chromatography Analysis
Quantification of the main ionic components and evaluation of the presence of soluble salts were performed with ion chromatography (IC). Anions (NO 3 − , SO 4 2− , Cl − ) and cations (Na + , K + , Ca 2+ , Mg 2+ , NH 4 + ) were determined using an HPLC Dionex ICS-1000 Ion Chromatography System (Thermo Scientific, Dionex Corporation, Sunnyvale, CA, USA) equipped with a conductivity detector. Anion analysis was carried out with an Ion Pac AS14A IC Column, using 8 mM Na 2 CO 3 /1 mM NaHCO 3 as the eluent, isocratic elution, a constant flux of 1.5 mL min −1 and an ULTRA anionic self-healing suppression (ASRS-ULTRA), whereas cations were analyzed with an Ion Pac CS12A IC Column, using 20 mM methanesulfonic acid (MSA) as the eluent, isocratic elution, a constant flux of 1.5 mL min −1 and an ULTRA cationic self-healing suppression (CSRS-ULTRA).
In order to prepare the solutions to be analyzed, a small portion of the sample was ground in an agate mortar. Of this powder, around 2 mg was transferred in a plastic test-tube and treated with 10 mL of milli-q water (Merck Millipore Milli-Q, Burlington, MA, USA). The suspensions were immersed in an ultrasonic bath for 1 h, centrifuged at 3000 rpm for 3 min and filtered using 0.45 µm non-sterile hydrophilic membranes (PTFE Millex-14 LCR, 25 mm, Millex ® Syringe Filters, Merck Millipore, Burlington, MA, USA) before injection in the instrument.

ATR-FTIR Analysis
Fourier-transform infrared spectroscopy was carried out in attenuated total reflectance mode (ATR-FTIR) in order to identify the main components and phases of the samples. The instrumentation used to perform the analyses was a Nicolet 380 FTIR spectrometer (Thermo Electron Corporation, Waltham, MA, USA). The detection window used was between 400 cm −1 and 4000 cm −1, and 64 scans with a resolution of 4 cm −1 were performed, along with smoothing operations (15 points).

Ion Chromatography Analysis
The results of the determination of the main ionic species are shown in Table 2. The concentrations of anions (Table 2) varied between the different samples; even between those taken from the same panel, but at different heights. For almost all the samples, sulphates were the species present in highest concentration, with higher values (>700 ppm) found in samples collected near the ground: SG_6 and 7 (Panel A), SG_4 (Panel C), SG_3 (Panel D), SG_1 (Panel E). Overall, it is possible to appreciate a relationship between sulphate concentration and sampling height. Indeed, the samples taken close to the window splays or in the vault showed, on average, lower concentrations of sulphates compared to samples taken at lower heights. A clear trend in terms of sampling height could not be observed for chlorides and nitrates. In this case, the differences between the samples are related to sampling site (panel), with Panels A and E showing the highest average concentrations of both species.
Moving on to cations, almost all the samples showed high concentrations of calcium ions (above 50 ppm, except for SG_37A). A correlation can be observed between the concentrations of calcium and sulphate ions. Indeed, as was the case for sulphates, it is possible to observe, on average, a relationship with sampling height, with calcium content being lower for samples taken at greater heights. All other cations were found in concentrations below 10 ppm, therefore secondary with respect to calcium. In this case, no significant correlations between concentration values and sampling height or sampling site (panel) could be appreciated.

ATR-FTIR Analysis
The samples were also analyzed using attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR). Table 3 summarizes the characteristic bands observed in the different samples. For most of the samples (SG_1, SG_3, SG_4, SG_5; SG_6, SG_7 and SG_29), typical gypsum bands were observed, in agreement with the IC results. Indeed, these samples showed high concentrations of sulphates (above 100 ppm), whereas samples SG_2, SG_8, SG_37A and SG_41B were associated with sulphate concentrations below 30 ppm. Based on the combined results of both IC and ATR-FTIR techniques, it is possible to assume that most of the ionic sulphate concentration derives from gypsum formation. Moreover, the characteristic calcite bands were recognized in all samples, which explains the nature of the high calcium concentrations observed in IC analysis. Finally, samples SG_2, SG_7, SG_8, SG_29, SG_37A and SG_41B also showed silicate bands, probably attributable to the substrate.

Discussion
The investigations carried out on the samples made it possible to characterize the different phases of salt crystallization in the walls of the Church of San Panfilo in Villagrande di Tornimparte. These were often related to the sampling height. General graphs of samples from the same area taken at different heights are shown below. It is interesting to observe how some samples, e.g., SG_1 and SG_2 ( In fact, sample SG_1 shows a good concentration trend between calcium and sulphate ions and lower concentrations of the other ions ( Figure 2). This observation allowed us to hypothesize that the area under investigation is affected by the sulphation process, which leads to the formation of newly formed crystalline gypsum in the form of efflorescence. This hypothesis is confirmed when observing the ATR-FTIR spectrum acquired for this In fact, sample SG_1 shows a good concentration trend between calcium and sulphate ions and lower concentrations of the other ions ( Figure 2). This observation allowed us to hypothesize that the area under investigation is affected by the sulphation process, which leads to the formation of newly formed crystalline gypsum in the form of efflorescence. This hypothesis is confirmed when observing the ATR-FTIR spectrum acquired for this sample ( Figure 3, Table 3). The characteristic peaks of the gypsum centered at 3522, 3400, 1683, 1620, 1108, 667, 597 and 418 cm −1 can all be observed [18,19]. In addition, O-H stretching and bending vibrations of calcium carbonate were found, with peaks at 1412, 872 and 712 cm −1 [19,20]. These peaks can also be attributable to newly formed carbonate salts and not to the substrate, since the samples were taken at salt efflorescence. In contrast, sample SG_2 shows low concentrations of these ions; in particular, sulphate ions ( Figure 2). In fact, the typical bands of gypsum were not observed ( Figure 3, Table 3). Sample SG_2, on the other hand, shows the classic carbonate peaks (Figure 3, Table 3), which can be attributable to efflorescence produced by carbonate salts. Indeed, it is known in the literature [14] that the area closer to the ground is where the relatively less soluble salts, such as calcium carbonate and gypsum, tend to form and precipitate as newly formed crystals, generating salt efflorescence which causes the observed detachment of the painted surface. The tensions generated by the increase in size of the salt crystals in the pores of the material can cause the detachment of the most superficial parts of the plaster and/or colored finishes, or, in the most severe cases, can even damage part of the load-bearing masonry. As was demonstrated by Kilian et.al. (2023), gypsum accumulation can cause hardening of the surface, which tends to decrease the superficial porosity of the material. The formation of such crusts reduces water evaporation from the surface and consequently increases sub-efflorescence formation [21].
Moving on to Panel A, the samples taken from this sampling site at different heights show different concentrations of calcium and sulphate ions (Figure 4). In contrast, sample SG_2 shows low concentrations of these ions; in particular, sulphate ions ( Figure 2). In fact, the typical bands of gypsum were not observed ( Figure 3, Table 3). Sample SG_2, on the other hand, shows the classic carbonate peaks (Figure 3, Table 3), which can be attributable to efflorescence produced by carbonate salts. Indeed, it is known in the literature [14] that the area closer to the ground is where the relatively less soluble salts, such as calcium carbonate and gypsum, tend to form and precipitate as newly formed crystals, generating salt efflorescence which causes the observed detachment of the painted surface. The tensions generated by the increase in size of the salt crystals in the pores of the material can cause the detachment of the most superficial parts of the plaster and/or colored finishes, or, in the most severe cases, can even damage part of the load-bearing masonry. As was demonstrated by Kilian et.al. (2023), gypsum accumulation can cause hardening of the surface, which tends to decrease the superficial porosity of the material. The formation of such crusts reduces water evaporation from the surface and consequently increases sub-efflorescence formation [21].
Moving on to Panel A, the samples taken from this sampling site at different heights show different concentrations of calcium and sulphate ions (Figure 4). In fact, samples SG_6 and SG_7, taken a few cm above ground level, show higher values of these ions than sample SG_8, which was taken at a greater height close to a window splay (Figure 4). This result indicates that the wall of Panel A close to the ground level are affected by efflorescence produced by sulphation, which produces newly formed gypsum crystals that lead to the detachment of the painted surface [22]. For these samples, the presence of gypsum is confirmed by infrared analysis ( Figure 5, Table 3). Typical calcite bands were observed ( Figure 5, Table 3) [19,20], and can be attributed to carbonate salts, which contribute to the degradation of the wall. In fact, samples SG_6 and SG_7, taken a few cm above ground level, show higher values of these ions than sample SG_8, which was taken at a greater height close to a window splay (Figure 4). This result indicates that the wall of Panel A close to the ground level are affected by efflorescence produced by sulphation, which produces newly formed gypsum crystals that lead to the detachment of the painted surface [22]. For these samples, the presence of gypsum is confirmed by infrared analysis ( Figure 5, Table 3). Typical calcite bands were observed ( Figure 5, Table 3) [19,20], and can be attributed to carbonate salts, which contribute to the degradation of the wall.
A similar result was obtained by analyzing samples taken from Panel C. Samples SG_4 and SG_5 were taken at lower heights, near the ground level, whereas sample SG_29 was retrieved at a greater height, near the window splay (Figures 6 and 7, Table 3). In this case, sample SG_4 shows significantly higher concentrations of both calcium and sulphate ions, with respect to samples SG_5 and SG_29. These results indicate an even stronger relationship between sampling height and ionic concentration (calcium and sulphate), once again reaffirming the previous conclusions that gypsum neoformation salts deriving from capillary rise tend to be located closer to the ground, in the vicinity of the walking surface. The pressure exerted by these salts causes the rupture of the porous material, leading to the degradation of the walls and, therefore, serious damage to the work of art, in this case the fresco. Indeed, the sampling site from which sample SG_4 was retrieved appears highly degraded, and most of the pictorial layer is no longer visible (Figure 1). In contrast, the area from which sample SG_5 was taken shows signs of superficial degradation, but the pictorial layer is still intact. Compared to the other samples taken close to the window splays (SG_8) or from the vault (SG_37A, SG_41B), sample SG_29 shows relatively high concentrations of calcium and sulphate. This may be attributable to external infiltrations that are specific to this sampling site. A similar result was obtained by analyzing samples taken from Panel C. Samples SG_4 and SG_5 were taken at lower heights, near the ground level, whereas sample SG_29 was retrieved at a greater height, near the window splay (Figures 6 and 7, Table 3). In this case, sample SG_4 shows significantly higher concentrations of both calcium and sulphate ions, with respect to samples SG_5 and SG_29. These results indicate an even stronger relationship between sampling height and ionic concentration (calcium and sulphate), once again reaffirming the previous conclusions that gypsum neoformation salts deriving from capillary rise tend to be located closer to the ground, in the vicinity of the walking surface. The pressure exerted by these salts causes the rupture of the porous material, leading to the degradation of the walls and, therefore, serious damage to the work of art, in this case the fresco. Indeed, the sampling site from which sample SG_4 was retrieved appears highly degraded, and most of the pictorial layer is no longer visible (Figure 1). In contrast, the area from which sample SG_5 was taken shows signs of superficial degradation, but the pictorial layer is still intact. Compared to the other samples taken close to the window splays (SG_8) or from the vault (SG_37A, SG_41B), sample SG_29 shows relatively high concentrations of calcium and sulphate. This may be attributable to external infiltrations that are specific to this sampling site.    The sample taken from Panel D (SG_3) also showed high concentrations of sulphate and calcium ions (Figure 8a), attributable to the same degradation phenomena, which was also confirmed by observing the spectrum obtained by infrared spectroscopy (Figure 8b,   Figure 7. ATR-FTIR spectra of samples SG_4, SG_5 and SG_229 taken from Panel C. Marker bands of carbonates (*) and sulphates (ˆ) are highlighted.
The sample taken from Panel D (SG_3) also showed high concentrations of sulphate and calcium ions (Figure 8a), attributable to the same degradation phenomena, which was also confirmed by observing the spectrum obtained by infrared spectroscopy (Figure 8b, Table 3). In fact, the sampled area is characterized by the complete loss of the plaster and of the pictorial layer caused by the crystallization cycles of these salts (Figure 1).
Appl. Sci. 2023, 13, x FOR PEER REVIEW 12 of 18 Table 3). In fact, the sampled area is characterized by the complete loss of the plaster and of the pictorial layer caused by the crystallization cycles of these salts (Figure 1).  Based on the results obtained, it is difficult to assume the precipitation of a specific salt. In fact, from the infrared spectra of samples taken from the detached surface ( Figure  10, Table 3), carbonate bands were observed [19,20] that can be attributed either to newly formed carbonate crystals or to the paint substrate.  Based on the results obtained, it is difficult to assume the precipitation of a specific salt. In fact, from the infrared spectra of samples taken from the detached surface ( Figure 10, Table 3), carbonate bands were observed [19,20] that can be attributed either to newly formed carbonate crystals or to the paint substrate. Based on the results obtained, it is difficult to assume the precipitation of a specific salt. In fact, from the infrared spectra of samples taken from the detached surface ( Figure  10, Table 3), carbonate bands were observed [19,20] that can be attributed either to newly formed carbonate crystals or to the paint substrate.  The graph shown in Figure 11a indicates that the calcium and sulphur trends are perfectly aligned for many of the analyzed samples in which efflorescence produced by poorly soluble gypsum salts was assumed. Gypsum has a low solubility and hygroscopicity compared to most salts, and its deliquescence occurs at a very high relative humidity (over 99.9%, according to [12]). Hence, it is stable in a variety of environments, even extremely humid ones. It should be noted that studies in literature [12] show that the decay rate of wall paintings due to salt contamination originating from gypsum crystallization causes significant deterioration, but also that it is 10 to 100 times slower than that caused by more soluble salts. In fact, the areas where newly formed gypsum was found did show efflorescence, but detachment of the paint surface was not always very evident (except for the area where SG_4 and SG_3 was sampled). This is supported by other studies which indicate that the deterioration caused by gypsum in stone and similar building materials is not produced by hydration pressures [12], but is the result of its crystallization within the porous matrix of the material, as with any other non-hydrating salt. Because of these characteristics, once gypsum is deposited in the pores of a building material or on a pictorial surface, such as a fresco, it will tend to accumulate over time. Its extremely low mobility also explains why desalination of materials contaminated with calcium sulphate is very difficult [12,23,24]. Furthermore, a comparison was made between the concentrations of sodium and chloride ions (Figure 11b), and a good correlation was observed for samples taken from Panel A (SG_6, SG_7, SG_8), Panel C (except for sample SG_29) and the samples taken from the top of the vault (SG_37A). This suggests that sodium chloride salts, such as halite, are present in these areas in addition to newly formed gypsum. Finally, the samples that showed correlations between potassium and nitrate ions (Figure 11c) were those taken Furthermore, a comparison was made between the concentrations of sodium and chloride ions (Figure 11b), and a good correlation was observed for samples taken from Panel A (SG_6, SG_7, SG_8), Panel C (except for sample SG_29) and the samples taken from the top of the vault (SG_37A). This suggests that sodium chloride salts, such as halite, are present in these areas in addition to newly formed gypsum. Finally, the samples that showed correlations between potassium and nitrate ions (Figure 11c) were those taken from Panel A (SG_6 and SG_7, with the exception of sample SG_8), the samples taken from Panel C (SG_5 and SG_29, with the exception of sample SG_4) and the samples taken from Panel E (SG_1 and SG_2, which show the highest concentrations of these species). For these samples, the presence of salts such as niter is assumed. Moreover, chloride and nitrate salts (particularly with sodium and magnesium) are more soluble and deliquescent than others and do not usually precipitate on the surface of the wall [25]. However, when the outdoor climate becomes exceptionally dry, efflorescence of this nature can be observed on surfaces. This means that these salts accumulate locally over prolonged periods of time, which can be as long as several decades, and tend to keep the masonry moist at all times, except in very dry conditions [12]. The types of decay that form on masonry usually result in swelling and detachment of the plaster, as was observed on the panels of the walls studied. The distribution of salts within a wall also depends on the actual mixture of salts present and their origin. As discussed by Steiger [26], based on the in-depth analysis of the north façade of a convent in northern Bavaria and other monuments [12], the presence of nitrate, potassium, and magnesium, as well as chloride and sodium, depends on the mixture of salts present and their origins, and is the result of rising damp.
Regarding the mechanism of the saline degradation [14], the results obtained in this study allow us to hypothesize that the degradation produced by the salts on the walls of this monument can be attributed mainly to capillary rise. Fractionation during ascending transport is revealed by the shift in the potassium/magnesium (K/Mg) ratio, which decreases with increasing height [12]. In fact, this ratio, calculated based on the data obtained from the different samples, showed a decrease in this parameter with increasing height. In the samples taken from Panel E (SG_1 and SG_2, K/Mg ratio = 1.03; 0.89, respectively), Panel C (SG_4 and SG_5, K/Mg ratio = 0.98; 0.24, respectively) and Panel A (SG_6 and SG_7, K/Mg ratio = 0.63; 0.38, respectively), the K/Mg ratio indicates an ascending transport of salts due to capillary rise from the ground. Finally, for the samples taken from the splays of the windows (SG_8, Panel A; SG_29, Panel C) or from the top of the vaults (SG_37A, SG_41B), it can be assumed that the detachment observed on the surfaces cannot be attributed to capillary rise from the ground, but is probably attributable to infiltration.

Conclusions
Thanks to the use of two analytical techniques (IC and ATR-FTIR), it was possible to characterize the degradation due to salt crystallization of the masonry of the fresco by Saturnino Gatti in the church of San Panfilo in Villagrande di Tornimparte (L'Aquila). The results highlighted the different types of efflorescence that probably caused the detachment of the painted surface and of the plaster.
Specifically, areas were identified where efflorescence was mainly caused by newly formed crystals of gypsum and calcium carbonate. These two types of salts were found in almost all the samples analyzed. Higher concentrations of gypsum were present, especially for those samples taken at a lower height, such as Panels A, C, D and E. Efflorescence composed of newly formed sodium chloride, halite, was present in samples SG_4, 5, 6, 7, 8 and 37A, with varying concentrations (higher concentrations for samples SG_6 and 8). This evidence was observed in the samples taken at slightly greater heights, with higher values detected in Panels A and C and in the sample taken from the upper vault (SG_37A). Efflorescence composed of potassium nitrate was present in samples SG_1 (higher concentrations) and SG_2, 6 and 7 (lower concentrations) (Panels E and A).
Finally, the results suggested that the degradation produced by efflorescence was due mainly to the capillary rise from the ground, with the exception of the samples taken close to the window splays and from the vault, in which infiltration could be responsible. The research carried out will allowed us to provide indications in order to perform targeted interventions regarding possible restoration work aimed at desalinizing the church walls.

Informed Consent Statement: Not applicable.
Data Availability Statement: Data sharing is not applicable. No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest:
The authors declare no conflict of interest.