Integrated Investigations to Study the Materials and Degradation Issues of the Urban Mural Painting Ama Il Tuo Sogno by Jorit Agoch

Abstract

This paper focuses on an integrated approach to study the materials and the degradation issues in the urban mural painting Ama Il Tuo Sogno, painted by the famous street artist Jorit Agoch in Matera (Italy). The study was conducted in the framework of a conservation project, aiming to contrast a progressive decay affecting the artifact that started a few months after its creation. Multi-analytical techniques were used to investigate the stratigraphy and chemical composition of the pictorial film within a low-impact analytical protocol for sustainable diagnostics. They included polarized light microscopy in UV and VIS reflected light, FTIR spectroscopy, Py-GC-HRAMS, and SEM-EDS. The mineralogical–petrographic composition of the mortar employed in the pictorial support was also studied with optical microscopy of thin sections and X-ray diffractometry. To know the mechanism underlying the degradation, IR thermography was performed in situ to establish the waterways and the distribution of the humidity in the mural painting. In addition, ion chromatography and X-ray diffractometry were used to identify and quantify the soluble salts and to understand their sources. The overall results allowed us to determine the chemical composition of the binder and pigments within the pictorial layers, the mineralogical–petrographic characteristics of the mortar of the support, and the execution technique of the painting. They also highlighted a correlation between the presence of humidity in the painted mural and the salt damage. The mineralogical phases were detected in the mural materials by XRD, and the results of ion chromatographic analyses suggested a supply of soluble salts mainly from the mortar of the support. Finally, the study provided basic knowledge for planning appropriate sustainable conservation measures.

1. Introduction

Urban art is related to a contemporary artistic phenomenon increasingly popular in the international and Italian scene [1]. This new artistic production, which is gradually becoming widespread in cities, is nowadays an essential part of the cultural heritage and related conservation activities. At the same time, in the last decades, scientific interest in graffiti materials [2,3,4,5,6,7,8,9] and graffiti murals of urban art [7,10,11,12,13,14], and consequently in their degradation processes has considerably increased [15]. Keith Haring’s mural paintings in Pisa and Paris have been widely investigated [11,12,13] through in situ non-invasive spectroscopic analyses [13] and in laboratory spectrometric investigations [11,12], where the binders, pigments, and additives were exhaustively characterized. More recently, murals painted by Alice Pasquini, Alexey Luchko, and by SEPE and Chazme in Lazio (Italy) were investigated using FTIR and micro-Raman spectroscopy to characterize not only the painting materials but also the degradation mechanisms [16]. The main binders detected were acrylic and alkyd resins, whereas the pigments identified were both organic (belonging to phthalocyanine, azo, and dioxazine classes) and inorganic (calcite, rutile, and gypsum), while the main degradation problems seem related to the presence of rutile pigments in the outer painting layers.

Deterioration of graffiti materials has been investigated both on mock-ups after natural and artificial aging [17,18,19,20,21,22] and on graffiti wall paintings [16,23,24]. Certainly, graffiti applied on outdoor walls, mostly within urban contexts, are highly exposed to common deterioration agents such as water, gases, saline solutions, and live organisms [23]. Knowledge about the materials used in these artifacts and how they interact with the different substrates under peculiar environments is essential for choosing measures to slow down the deterioration of the artworks. The chemical composition of the paint layers plays an important role as it determines different susceptibilities to decay in the form of fading, discoloration, and detachment [16,18,23,24,25]. Particular attention has been devoted to the identification of the binder, which is useful for assessing the technique adopted by the artist, and evaluating the possible effects of cleaning, as well as establishing the best conditions for preventive conservation. Alkyd resins are among the most used binders in street art mural paintings [11,16,18,26]. They have been revealed to be an unsuitable medium as they are affected by dramatic detachment phenomena [11,21,27].

In this paper, the mural painting Ama Il Tuo Sogno (Figure 1), painted by the contemporary street artist Jorit Agoch, was investigated.

Jorit Agoch, whose real name is Ciro Cerullo, is one of the most active Italian muralists in the urban art field. He was born in Naples, where he started his activity in the decade beginning in 2000. He became increasingly known internationally, and his artistic production can be found in many cities around the world. His subjects of choice are both famous and common local people who are assimilated figures of social martyrs associated with cultural and political commitment. The surfaces he paints on are usually large walls, where he focuses on the realistic depiction of the human face. He typically marks the faces of the characters with red stripes on the cheeks, which can be considered his signature style: they refer to the tribal ritual of scarification and symbolize belonging to a single human tribe. For this reasons, Jorit’s works are characterized by a message of great visual impact and an intense symbolic value.

The mural investigated was built for “Matera European Capital of Culture” in 2019 and portrays the eyes of Yvan Sagnet, a political activist for farm workers’ rights and against illegal hiring. The painting addresses an issue of a political and social nature, namely the exploitation of immigrants, which is particularly relevant to the historical culture of the Matera territory. The mural is part of the production of the Urban Muralism artistic movement, which is very distant from the Graffiti writing movement. Moreover, it was commissioned to the artist by the Matera Municipality with the use of public funds, and the local citizens now consider this artifact as a part of the heritage of the city. All these factors mark the importance of preserving this painting. It was painted in an outdoor context, namely on the surface of the masonry wall of “Carlo Levi” Art High School, which is made of curtain blocks of a local bio-calcarenite, set against a filler ground.

According to the statements of the artist and the workers involved, two superimposed preparatory layers obtained with premixed cement-based mortar were applied on the stone wall support. For the application of the paint, the artist combined the use of aerosol spray with the traditional painting technique of brushstrokes. The spray cans used are produced by Montana Colors^®. The artist used two specific products, Montana 94 and Montana Hardcore, both based on alkyd resin but with different “gloss” yields.

A few months after its creation, the artwork showed some decay on the surface. Nowadays, its state of preservation is deeply compromised as decay extends on both the painting layers and mortar. The pictorial film in the central area of the painting was initially affected by a lack of adhesion to the plaster (Figure 2a). Over time, the worsening of this phenomenon led to the detachment and loss of the painting layers, causing lacunae and micro-lacunae, which made visible the mortar layer underneath (Figure 2b). Here, the mortar shows a lack of cohesion, with cracking, powdering, and, in some periods of the year, efflorescence: some needle-shaped, some others in globular or powder forms (Figure 2c,d). Furthermore, white veils and efflorescence of soluble salts are evident on the mural painting in a regular texture, shaped like the joints of the wall ashlars underneath.

In view of conservation work, a study of the mural painting was undertaken. It aimed at characterizing the materials used in the painted panel and understanding the mechanism of the decay affecting the artwork. These are both important goals to be achieved to support the planning of an appropriate conservation project, to which the analytical identification of the constituent materials and decay products and their quantitative composition decisively contribute.

Moreover, before the analytical investigations, an in-depth conservation study of the mural and its executive techniques was carried out; thanks to the large quantity of historical photographic material from the Levi Foundation correlated to the direct observation of the surface, it was possible to recognize the peculiar pictorial differentiation in which the artist alternated the use of Montana’s gloss and matte spray cans, with different effects and thicknesses in the painting. Thanks to an interview with local workers, the original mortar of the preparatory layers was also traced to two specific commercial cement-based products from Fassa Bortolo. This basic information allowed the analytical investigations to focus, in a sustainable way, on only samples aimed at supporting and deepening previous knowledge on the executive technique, relating it to the individual morphologies of the decay process of the pictorial film.

Sustainability is one of the main pillars for conservation actions of cultural heritage, which aim at preserving it for future generations, and it should be ensured in the diagnostic process through minimal destructiveness. On-site, non-destructive, large-scale techniques [28] and portable analytical devices developed in recent years undoubtedly address this basic requirement as they have no impact on the artifact [29]. Nonetheless, the laboratory analytical approach remains necessary to gain certain diagnostic knowledge, such as the quantitative composition of constituent materials, decay products, and subsurface sequence layers. In this case, sustainability of the analytical approach is concerned in terms of microinvasive analytical protocols.

In the present study, a diagnostic strategy based on an integrated approach, including both laboratory material analyses and non-destructive investigations on site, was implemented in order to optimize the diagnostic knowledge in the framework of sustainable investigations.

Multi-analytical techniques were used to achieve an increased and deeper knowledge in terms of qualitative and quantitative composition of the materials used in the artifact and their decay products, with a limited sampling that reduced as much as possible the impact on the original artwork.

2. Materials and Methods

Fourier transform infrared (FTIR) spectroscopy and qualitative/quantitative pyrolysis–gas chromatography/high-resolution accurate mass spectrometry (Py-GC-HRAMS) microanalyses were performed to recognize the kind of the binder and evidence of degradation. The stratigraphy and the inorganic components were examined using polarized light microscopy (PLM) in ultraviolet (UV) and visible (VIS) light, as well by microanalysis with scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDS).

This multi-analytical approach relies on a low-impact analytical protocol that combines non-destructive or minimally destructive analyses—thus allowing the optimization of material knowledge by such an application. Material characterization was achieved through PLM [12,16], SEM-EDS [7,12], and FTIR [7,12,13,14,16] using the same cross-section samples, along with the advanced Py-GC-HRAMS [2,5,6,11], which typically has very low detection limits and needs very small sample amounts. In addition, to further minimize sampling on the artifact, analyses by Py-GC-MS at high resolution and FTIR were also applied to samples of the Montana Colors 94 spray paints, indicated by the artist as the materials he used, to gain more information about the possible variability in the chemical composition within the mural and to detect, by comparison, any degradation effects of the materials in the outdoor exposure conditions.

The processes and causes of deterioration were investigated by conservators and scientists with combined in situ infrared thermography (IRT) and laboratory analyses of the decay products by means of X-ray diffractometry (XRD) and ion chromatography (IC).

The main limitation of IRT comes from the fact that the data acquired are qualitative and not quantitative, unlike laboratory analyses. On the other hand, an undoubted advantage of this technique consists of a large-scale description of the state of the artifact in relation to the specific investigated parameter, while laboratory analyses provide data which refer to limited points in the artifact.

In particular, IRT was used to evaluate the painting mural’s thermal behavior to detect the humidity areas behind the wall. Infrared thermography is a non-contact and non-destructive testing technology that can be applied to determine the surface temperature of an object [30]. It can assess moisture-related phenomena as changes in moisture content are related to changes in surface temperature, and the latter can, therefore, be detected by this technique [31]. XRD and IC analyses were performed to detect salt mineralogical phases and for a qualitative/quantitative evaluation of soluble salt ions within the materials to identify their possible source. Also, in this case, the same powder samples were used.

2.1. Sampling

As the mural was created only a few years ago, extensive documentation on the materials used and the execution techniques was available from the artist; therefore, the sampling was limited and extremely focused on the verification and/or in-depth analysis of some material aspects and those relevant to the decay morphologies affecting the pictorial films. On the other hand, the use of multi-analytical techniques with low or no destructiveness contributed to containing the sampling.

In this context, samples including the pictorial layers and the mortar of the support were taken from the mural painting in order to characterize the constituent materials. A sample from the black color on the stone ashlar was also taken. In addition, five samples from the Montana Colors 94 spray paints used by the artist were analyzed to gain more information about the possible variability in the chemical composition of the spray paints, avoiding the sampling of more areas on the mural painting.

To study the composition of salt decay products, samples of efflorescence and white veils were taken from the surface of the mural painting. In addition, the raw mortar materials used for the plaster layers were analyzed to assess their eventual supply of soluble salts. Samples from the stone and joint mortar of the masonry support were also taken for comparison with decay products in the mural painting. The sampling points are shown in Figure 1, while the list of samples and the analyses performed are reported in

Table 1. List of samples and corresponding analyses.

Sample Name	Sample Type	Analytical Techniques
JA1	Fragment of mortar from the bottom layer of the painted panel	PLM (t.l.), FT-IR
JA2	Fragment of mortar from the top layer with red pictorial film from the painted panel	PLM (t.l.; r.l.)
JA3	Salt efflorescence powder on the pictorial layer of the painted panel	XRD, IC
JA4	Pictorial layer consisting of brushstrokes with red, pink and purple colors and affected by lifting from the painted panel	Py-GC-MS, FT-IR, PLM (r.l.)
A5	Powder from the top mortar layer just under the pictorial film of the painted panel	XRD, IC
JA6	Salt efflorescence from the mortar joint from the masonry just at the foot of the painted panel	XRD, IC
JA7	Mortar fragment from the joint from the masonry on the right of the painted panel	XRD, IC
JA8	Powder from the white veil on the pictorial layer of the painted panel	XRD, IC
JA9	Black pictorial layer from the masonry ashlar surface on the right of the painted panel	PLM (t.l.; r.l.), FT-IR, XRD, IC
JA10	Powder from the white veil from the masonry ashlar surface on the right of the painted panel	XRD, IC
JA11	Stone fragment from the masonry ashlar surface on the right of the painted panel	PLM (t.l.), XRD, IC
IG21:	Raw mortar powder used in the top layer of the paint support	XRD, IC
KD2	Raw mortar powder used in the bottom layer of the paint support	XRD, IC
RV-35	Montana Colors 94 spray paint—Chocolate Brown	Py-GC-HRAMS
RV-205	Montana Colors 94 spray paint—Warrion Brown	Py-GC-HRAMS
RV-136	Montana Colors 94 spray paint—Inca Brown	Py-GC-HRAMS
RV-100	Montana Colors 94 spray paint—Coffee Brown	Py-GC-HRAMS
RV-97	Montana Colors 94 spray paint—Chiapas Brown	Py-GC-HRAMS

Key: t.l.: transmitted light; r.l.: reflected light.

.

2.2. Characterization of the Mural Painting Materials

To characterize the materials of the mural painting, samples including the pictorial layers and the mortar of the support were analyzed with the following techniques.

The sampling was carefully carried out by detaching a micro-fragment (1–2 mm) of pictorial layers and about 50 mg of mortar from the support next to the edges of the mural painting.

• PLM in transmitted and reflected light.

Polished cross-sections of the pictorial layer samples were prepared with polyester resin (Colorchimica SRL, Reggello, Italy) and then carefully polished after curing the resin with progressively finer silicon carbide cards. Observations of cross-sections were performed using an BX51 optical polarizing microscope (Olympus Corporation, Tokyo, Japan), up to a magnification of 400×, in order to define the nature, the thickness, and the sequence of the layers. Observations were conducted under reflected visible light originating from a 50 W halogen lamp and in UV fluorescence by using a 100 W HBO HG vapor lamp and a BP 340–380 nm excitation filter. Thin sections of mortar samples were also prepared, and their petrographic features were examined in transmitted light with the same microscope. The degree of rounding and sphericity of the clasts, the grain size sorting, the porosity of the mortar, and the binder/aggregate ratio were estimated by visual comparison with reference charts normally used in optical microscopy [32,33,34].

All the photos were recorded with a Camedia C-4040-Zoom digital camera (Olympus Corporation, Tokyo, Japan).

• SEM-EDS

Cross-sections of the pictorial layers were analyzed by SEM-EDS in low-pressure mode (90 Pa, 25 Kv), without a conductive coating. An EVO 15 microscope (Zeiss, Jena, Germany) equipped with a tungsten filament operating at 30 keV with a resolution of 3.4 nm was used.

SEM images were collected in the backscattered electron (BSE) mode. EDX spectra (live time 300 s) and distribution maps of the elements (matrix 1024 × 800, area 700 × 560 μm, dwell timeb400 µs, 25 kV) were acquired by using the AZtec 6.0 SP1 software platform and an Ultim MAX 40 mm² SSD detector (Oxford Instruments, Abingdon-on-Thames, UK) with a resolution of 127 eV FWHM @MnKa and a detection limit of about 0.1 wt% percentage from 0.3 to 3 μm in depth. The software module uses a standardless ZAF quantification system.

• Micro-FTIR spectroscopy

Micro-FTIR spectroscopy was applied to polished cross-section samples and on five samples from the Montana Colors 94 spray paints used by the artist (Table 1). The spray paint samples were scraped from the glass on which they were applied, several days after their application, and analyzed to gain more information about the possible variability in the chemical composition of the spray paints, avoiding the sampling of more areas on the mural painting. An FTIR microscope (Hyperion 3000, Bruker Optics GmbH, Ettlingen, Germany) was used. It was equipped with two infrared detectors (standard single-element mid-band photoconductive MCT and focal-plane array photovoltaic MCT) and a 20× Cassegrain objective with a large numerical aperture (0.6). Data were acquired and managed with OPUS-IR™ 7.5 software (Bruker Optik GmbH, Ettlingen, Germany). The Hyperion microscope was connected to a vacuum FTIR spectrometer (Vertex70 v Bruker Optik GmbH, Ettlingen, Germany) with a standard KBr/Ge beamsplitter and an external source for high-performance experiments (24 V Glowbar, water cooled). During the measurement, the spectrometer was kept under vacuum (residual pressure 1 mbar). Spectra were acquired using a Germanium Attenuated Total Reflection (ATR) crystal (spectral range: 5000–600 cm⁻¹; penetration depth: 0.65 μm) in the 4000–650 cm⁻¹ region at a resolution of 4 cm⁻¹ and by summing 128 scans. The FTIR data were processed with OPUS software (Version 8.7).

• Py-GC-HRAMS

Analyses by pyrolysis–gas chromatography/high-resolution accurate mass spectrometry were performed both on the sample (JA4) from the mural painting (Figure 1) and on the five samples from the Montana Colors 94 spray paints (Table 1). The Montana spray paints were analyzed several days after their application on glass.

The pictorial layer from the wall painting was analyzed with and without TMAH, whereas all spray paints were analyzed only after derivatization to allow for hydrolysis and methylation and correct elution of more polar molecules such as polyols and fatty acids.

For Py-GC-HRAMS analyses, samples were derivatized with the thermally assisted hydrolysis and methylation method (THM) using tetramethylammonium hydroxide (TMAH) in aqueous solution at a concentration of 2.5% by weight (Sigma-Aldrich, St. Louis, MO, USA). A few mg of the paint film sample were derivatized with 5 µL of tetramethyl ammonium hydroxide (TMAH) for 1 h at 70 °C. One μL of the derivatized sample was placed into a stainless-steel cup and inserted into a micro-furnace. The Frontier Lab’s Multi-Shot Pyrolyzer™ (Frontier EGA/PY-3030D) with Auto-Shot Sampler™ (AS-1020E) was coupled to a Thermo Scientific™ TRACE™ 1310 Gas Chromatograph with a Thermo Scientific™ TraceGOLD™ TG-5SilMS 30 m × 0.25 mm I.D. × 0.25 µm film capillary column (P/N 26096-1420). The GC system was then coupled to a Thermo Scientific™ Exactive™ GC Orbitrap™ mass spectrometer. The pyrolysis was carried out in one shot, where the temperature was set at 550 °C for 20 s. The interface temperature of the pyrolyzer was 300 °C and the temperature of the GC injector was kept at 280 °C. The GC was equipped with a TG-5 SILMS (Thermo Fisher Scientific Inc., Waltham, MA, USA), similar to a 5% phenyl methylpolysiloxane (30 m, 0.25 mm i.d., 0.25 µm film thickness) capillary column. Helium (99.999%) was used as carrier gas at a flow of 1.2 mL/min. The following temperature program was used for the gas chromatographic separation: isotherm of 2 min at 40 °C, ramp of 10 °C/min up to 320 °C, isotherm at 320 °C for 5 min. The MS transfer and the ion source temperature were set, respectively, at 250 °C and 260 °C. Electronic ionization (EI) at 70 eV of energy was used with an emission current of 50 µA. Spectra of high-resolution EI fragments were acquired using a resolution of 60,000 (FWHM at m/z 200). Full-scan MS acquisition was performed in profile mode using an m/z range of 40–550. Nitrogen gas (99.999%) was used for the C-Trap supply. Data were processed with X-Calibur 4.0. and TraceFinder 4.1 (Thermo Fisher Scientific Inc., Waltham, MA, USA).

2.3. Investigation of Decay Issues

Decay issues were investigated in situ and by laboratory analyses.

In situ infrared thermography was used to identify the mechanism underlying the degradation by detecting the presence and the distribution of the humidity in the mural painting. In addition, an analytical study of the salts within the mural painting was performed by chemical–mineralogical analyses, including ion chromatography and X-ray diffractometry. These analyses were performed on samples of efflorescence and white veils taken from the surface of the mural painting, as well as on a sample from the preparatory mortar layers. The analyses were also performed on the raw mortar materials used for the plaster layers to assess their eventual supply of soluble salts. The raw materials are two premixed cement mortars (KD2 and IG21, by Fassa Bortolo). Technical sheets report that KD2, which was used for the plaster layer immediately under the painting, is a fiber-reinforced mortar based on hydrated lime, Portland cement, size-selected sands, and additives. IG21 is the mortar used in the layer of the plaster directly applied on the stone ashlars of the wall. The composition reported for this product is based on hydrated lime, white or gray Portland cement, size-selected sands, and additives. Samples from the masonry support (stone and joint mortar) were also analyzed.

• In situ IRT
• Infrared thermography was used to evaluate, through a non-invasive methodology [35], the thermal behavior of the mural painting over almost one year. A long-wave infrared camera (FLIR 396 produced by Teledyne Flir, Wilsonville, OR, USA) with a resolution of 640 × 480 pixels and an absorption band of 7.5–14 µm was employed. The calibration was performed through an NUC operation, which is an image correction performed by the thermal imaging camera software to compensate for any differences in the sensitivity of the elements of the detector and other optical and geometric disturbances (EN 16714-3). During this operation (offset update), a shutter is placed in the optical path and all detector elements are exposed to the same amount of radiation originating from the shutter.
• The painting was investigated through passive methodology using surface heating generated by solar radiation. Thermographs were acquired and images were processed in false color with a reference scale with shades from violet to black for the areas with low temperatures and to yellow/white for those with high ones. All acquisitions were performed when the painting was entirely in shadow.
• XRD
• XRD analyses were carried out on powdered samples to identify the mineralogical salt phases. The samples were ground in an agate mortar until an impalpable powder was obtained. A Philips (Philips, Amsterdam, the Netherlands) diffractometer (Mod. PW1729, APD—3.6j version) was used (CuKα radiation at 40 kV/20 mA, 0.02° step size, 1.25 s counting time, 2θ scan interval between 3° and 60°). The diffractometer was provided with manual divergence slit and graphite monochromator. Diffraction data were processed with X’Pert software 2.2e—Philips Analytical.
• IC

The ion chromatography analyses were performed to determine the qualitative/quantitative content of the soluble ions within the samples. Sample amounts of 50 mg were finely ground in an agate mortar, weighed, and dried in an oven at 60 °C for about 12 h. Once cooled in a desiccator, 100 mL of double-distilled water was added to each sample in glass flasks and stirred for 24 h. Then, the solution was filtered with Whatman grade 4 qualitative filter paper (20–25 μm) and measured immediately. Ion chromatography was performed using a Dionex ICS-1100 Ion Chromatography System (Thermo Fisher Scientific Inc., Waltham, MA, USA), with Na₂CO₃ [476 mg/L] and NaHCO₃ [117 mg/L] for the anion mobile phase and CH₄O₃S [1.2 mL/L] for the mobile phase of the cations. In the case of anions, a Dionex RFIC IonPac AS22 (4 × 250 mm) analytical column, together with a Dionex RFIC IonPac AG22 (4 × 50 mm) guard, was used for the separation, and 4.5 mM Na₂CO₃/1.4 mM NaHCO₃ was used as the mobile phase at a flow rate of 1.2 mL/min, with a Dionex Suppressor ASRS 300, 4 mm (AutoSuppression Recycle Mode), for chemical suppression. The quantification of cations was conducted by using a Dionex IonPac CS12A, RFIC (4 × 250 mm) analytical column, together with a Dionex RFIC IonPac CG12A (4 × 50 mm) guard. Methanesulfonic acid (20 mM) was used as the mobile phase at a flow rate of 1 mL/min. For each sample, three injections of 1 mL were performed using a Whatman Anotop 10 IC (0.2 μm—diameter 10 mm) syringe filter, and the chromatograms were acquired. The results in parts per million (ppm—mg/L) were converted to weight percentages relative to the dry sample weight. The mean values of three measurements and their standard deviation were determined.

3. Results and Discussion

3.1. Stratigraphy and Composition of the Pictorial Films

Stratigraphic sequences observed on the painting samples under PLM in reflected light appear quite simple, consisting of two painting layers (Sample JA1, Figure 3a), with an orange layer (layer b) which is above a whitish one (layer c) and directly applied on the mortar (layer a) and three painting layers (sample JA2, Figure 3b) with pinkish (layer b), purple (layer c), and pinkish (layer d) layers where the layer directly applied on the mortar appears thicker (35–40 µm) than the others (3–5 µm). A black paint layer (layer b), as shown in Figure 3c (JA9 sample), is relative to the writing external to the panel, and it is directly applied on the biocalcarenite of the wall (layer a).

The sample JA4 at the microscopic observation yields a stratigraphic sequence made of two layers (Figure 4): (b) a pink, irregular, and discontinuous pictorial layer (0–50 µm) containing some rounded particles visible under UV light and (a) a dark red/orange layer, which typically shows a significant thickness of 70–120 µm and is composed of a series of thin coats where white particles are mixed with very-fine-grained red/orange particles. The corresponding SEM-EDS results (Figure 5) for a portion of the sample stratigraphy allow us to identify titanium dioxide (PW6), iron oxides (PR101) or hydroxides (PY42), and a few silica particles in layer b; mainly iron oxides or hydroxides were detected in layer a. Iron-based pigments and titanium dioxide are very common pigments in modern paints [16,26].

The ATR-FTIR spectra of the JA1, JA4, and JA9 samples were collected to obtain preliminary information on the binder and inorganic components of the paints. The corresponding spectra (Figure 6) show the characteristic absorption bands of the styrene-modified alkyd resin: OH stretching (3600–3100 cm⁻¹), CH₂ and CH₃ stretching and bending (2957, 2855, 1465, and 1451 cm⁻¹), C=O stretching vibration (1730 cm⁻¹), COO asymmetric stretching of the ester group (ca. 1265 cm⁻¹), CCO asymmetric stretching (ca. 1070 cm⁻¹), and C C stretching of aromatic rings (1489, 1450, and 1580 cm⁻¹). The modification with styrene can be inferred from the detection of the bands due to the out-of-plane bending of the aromatic CH (between 750 and 700 cm⁻¹) [27,36]. Silicates were detected in all paints (at approximately 1000–1015 cm⁻¹). In all spectra, the shoulder at 1176 cm⁻¹ and the bands at 983, 633, and 610 cm⁻¹ could be ascribed to barium sulfate (PW 21) as an additive. Only in the spectrum of the brown JA4 sample could the two low signals at 795 and 895 cm⁻¹ be related to PY42 (synthetic hydrous ferric oxide).

In the FTIR spectrum of the black JA9 sample, the bands at 1650 and 1299 cm⁻¹ (N-O stretching) and the shoulder at 842 cm⁻¹ (C-N stretching) could be due to the addition of nitrocellulose [7].

The FTIR-ATR spectra collected for the mural painting samples were compared with those collected with fresh Montana spray paints in order to find some degradation features due to aging. In Figure 7, the FTIR-ATR spectra acquired for the fresh Montana spray paints (RV205) and for the JA4 paint sampled, which was exposed for four years to natural and anthropic agents, are compared. The spectra reveal that the oxidation reactions related to aging and producing alcohols and carbonyl species lead to an increase in the bands in the region 1730–1680 cm⁻¹ due to carbonyl stretching, and the broadening at 1635 cm⁻¹ could be due to the stretching vibration of the C=O groups in diketones and their enol form (Figure 7) [36]. A decrease in the band at 1265 cm⁻¹ related to the asymmetric stretching of the ester group was also observed.

To compare the chemical degradation between the fresh spray paints and the mural painting samples, a semi-quantitative approach was used by normalizing two significant integration areas after the same baseline correction. The bands concerned are ascribable to the carbonyl group in the range between 1775 and 1680 cm⁻¹, which is more susceptible to degradation, and the band between 1520 and 1430 cm⁻¹ due to CH₂ stretching vibration, which is considered stable and constant [20]. The obtained average values were 8.05 ± 0.98 for fresh samples and 5.01 ± 0.10 for the mural painting samples, and this result indicates an effective reduction of the carbonyl group.

The pyrogram of the JA4 sample without TMAH shows the presence of phthalic anhydride and benzoic acid, together with palmitic acid, oleic acid, and stearic acid. Furthermore, the presence of dibutyl phthalate was found, added as a plasticizer. In order to reveal all components of the alkyd resin, the same paint sample was also analyzed with Py-THM-GC-HRAMS, i.e., after derivatization with TMAH, to allow a suitable elution of more polar molecules such as polyols and fatty acids.

Alkyd resins are prepared by condensation polymerization of three types of monomers: polyalcohols (i.e., glycerol, pentaeryhritol, etc.), phthalic anhydride or terephthalic acid, and fatty acids or triglyceride oils [5]. In the pyrogram of the JA4 sample, the main components of the alkyd resin are pentaerythritol (tetramethyl ether of pentaerythritol), phthalic anhydride, and phthalic acid dimethyl ester as polybasic acids and fatty acids (

Table 2. List of compounds identified in the spray paint samples (RV100, RV136, RV205, RV35, and RV97) and in the paint sampled from mural painting (JA4).

RT	Compound	RV100	RV136	RV205	RV35	RV97	JA4
7.35	Benzaldehyde	X	X	X	X	X	X
7.61	Phenol	X	X	X	X	X	X
7.67	α-methyl styrene		X	X		X	X
7.74	Benzonitrile	X	X	X	X	X	X
8.49	Butanedioic acid, ME	X	X	X	X	X	X
8.63	Octanoic acid, ME	X	X	X	X	X	X
9.56	Benzoic acid, ME		X	X	X	X	X
9.75	1-propanol, 3-methoxy-2,2-bis-(methoxymethyl)	X	X	X	X	X	X
9.96	Nonanoic acid, ME		X	X		X	X
10.63	Benzoic acid, ME	X			X	X
10.88	Pentaerithrol, tetramethyl ether	X	X	X	X	X	X
11.46	Decanoic acid, ME	X	X		X	X
11.57	Styrene		X	X		X	X
12.76	Phthalic anhydride	X	X	X	X	X	X
12.87	Undecanoic acid, ME	X	X	X	X	X	X
13.48	Trimethoxy benzene		X		X	X
13.54	Dodecanoic acid, ME		X			X
13.75	Biphenyl	X	X	X	X	X	X
13.87	Heptanedioic acid, 2ME	X	X	X		X
14.27	N-methyl.phthalamide	X	X	X	X	X	X
14.45	Octanedioic acid, 2ME	X	X	X	X	X	X
14.59	Dimethyl phthalate	X	X	X	X	X	X
14.87	N-propyl benzamide	X	X	X	X	X	X
15.70	Nonanedioic acid, ME	X	X	X	X	X	X
16.25	2,3-dimethoxy benzoic acid, ME				X	X
16.38	Decenedioic acid, diethyl ester	X		X	X	X	X
16.87	Decanedioic acid, DME	X	X	X	X	X	X
17.28	N-4-(2-aminoethyl) -N4 ethyl-2-methyl-1,4 benzenediamine			X		X
17.76	Tetradecanoic acid, ME	X	X	X	X	X	X
18.10	1,4-benzene dicarboxylic acid, ME	X	X	X	X	X	X
18.83	Pentadecanoic acid, ME	X	X	X	X	X	X
19.41	Dimethyl phthalate	X		X		X	X
19.86	Hexadecanoic acid, ME	X	X	X	X	X	X
19.94	Isopropyl phthalate	X	X	X	X	X	X
20.83	Heptadecanoic acid, ME	X	X	X	X	X	X
20.95	Dibutyl pththalate	X	X	X	X	X	X
21.54	(Z)-octadec-9-enoic acid, ME	X	X	X	X	X	X
21.73	Phthalic acid, cyclobutyl hexyl ester	X	X	X	X	X	X
21.77	Octadecanoic acid, ME	X	X	X	X	X	X
21.90	Hexyl methyl phthalate	X	X	X	X	X	X
22.02	Phthalate	X	X	X	X	X	X
22.14	Docosanoic acid, ME	X	X	X	X	X	X
22.33	(9Z,12Z)-octadeca-9,12-dienoic acid, ME	X	X	X	X	X	X
22.62	Methyl propyl phthalate	X	X	X	X	X	X
23.13	oxiraneoctanoic acid, 3-octyl, ME	X	X	X	X	X	X
23.50	Tetracosanoic acid, ME	X	X	X	X	X	X
23.52	Eicosanoic acid, ME	X	X	X	X	X	X
23.70	Isobutyl methyl phthalate	X	X	X	X	X	X
28.42	Phthalic acid, methyl phenyl ester		X	X	X	X	X
30.17	Phthalic acid, methyl phenyl ester		X

).

Moreover, the presence of styrene suggests the use of an alkyd resin modified with this compound. Styrene is usually added to alkyd resin to improve brilliance and mechanical resistance [25]. The methylated fatty-acid peaks are relative to saturated fatty acids ranging from C8:0 to C24:0 and dicarboxylic acids, such as heptanedioic, octanedioic, nonanedioic, and decenedioic acids, together with unsaturated acids, such as (9Z)-Octadec-9-enoic acid (oleic acid, C18:1) and (9Z,12Z)-octadeca-9,12-dienoic acid (linoleic acid, C18:2). The highest amounts of fatty acids are related to palmitic acid and stearic acid, whereas the low quantities of unsaturated fatty acids (C18:1 and C18:2) versus the higher quantities of dicarboxylic acids are probably due to the aging of the paint film (Table 2, Figure 8), which follows the same drying mechanism as lipidic binders. Indeed, aging of lipidic binders involves oxidation and cross-linking of unsaturated fatty acids, producing dicarboxylic acids [5]. The oxidation and cross-linking of alkyd resins make the paint film more rigid and brittle, and this effect may be responsible for the lack of adhesion and lifting in some areas of the painting.

The compounds related to the alkyd contribution were also detected in the Montana spray paints although at different percentages (Figure 8,

Table 3. Data related to the peak area percentage in all pyrograms (% BA, % PBA, % PE, and % FA) and the ratios of the components detected in the alkyd samples (A/P, P/S, and PhA/A).

ID	% BA	% PBA	% PE	% FA	% PhT	A/P	P/S	PhA/A
			Montana 94 spray paints
RV100	16.31	81.74	0.14	0.92	0.89	0.21	7.70	3.50
RV136	17.10	76.22	0.10	2.98	3.61	0.38	4.92	44.85
RV205	9.60	83.34	0.08	1.83	5.15	0.46	3.57	56.11
RV35	7.21	88.78	0.43	2.66	0.91	0.32	4.16	0.27
RV97	12.78	82.61	0.11	2.13	2.37	0.48	3.94	0.67
			Paint sample
JA4	7.05	85.53	0.13	2.95	4.35	0.08	3.29	132.58

Key: BA = benzoic acid; PBA = polybasic acid (phthalic anhydride and phthalic acid, dimethyl ester); PE = pentaerithrol; FA = fatty acid; PhT = phthalates; PhA = phthalic anhydride; A = azelaic acid; P = palmitc acid; S = stearic acid.

). The modification with styrene was absent in the RV35 and RV100 spray paints. The results related to the peak area percentages in all pyrograms (%BA, %PBA, %PE, and %FA) and the ratios of the components detected in the alkyd samples (A/P, P/S, and PhA/A) are listed in Table 3.

Data show that the spray paints can be divided into two groups based on the proportion of the benzoic acid in comparison to the other components. Benzoic acid ranges from 12 to 17% for RV100, RV136, and RV 97 and from 6 to 10% for RV205 and RV35. However, because benzoic acid, an aromatic monobasic acid, is more likely present as a stopping agent in the commercial formulation rather than due to a pyrolysis product of the polybasic acid [37], the comparison of the percentages of the major components of the Montana spray paints revealed a similar composition between them.

Furthermore, for these samples, the palmitic acid/stearic acid ratio (P/S) and the phthalic anhydride/azelaic acid (PhA/A) ratio of the fresh spray paints are considered, respectively, to determine the kind of oil and to estimate the oil length. The P/S ratio of the main oils used for the alkyd resin ranged from 1.4 to 2.9 [5], whereas the P/S values obtained for all samples analyzed were too high (3.29 to 7.70, in Table 3) and are not useful for establishing the kind of oil used in manufacturing; therefore, probably, fatty acids rather than oils have been used for the manufacturing of these spray paints.

The low values of the PhA/A ratio calculated for the RV100, RV35, and RV97 samples indicate the longer lengths of the oils in the alkyd resins, whereas RV136 and RV 205 are characterized by a short oil due to the higher values of the PhA/A ratio (Table 2). The length of the oil affects the drying speed of the paints: in general, the shorter the oil, the faster the drying.

The alteration of the alkyd resin may also be due to photo-oxidative processes affecting the oil component [11,12,25], leading to a very stiff, brittle material and/or fading and yellow discoloration [4,5,21], as well as to this process affecting some pigments in the formulation and the type of substrate under the mural painting [17]. Inorganic pigments make paint more resistant to fading [38], but the simultaneous presence of titanium oxide can cause a loss of color [17]. Therefore, the iron oxides and hydroxides, along with the titanium oxide, which were found in the JA4 sample analyzed under SEM-EDS, could make the paint prone to such a discoloring effect.

3.2. Microscopic Characteristics and Mineralogical Composition of the Mortar of the Preparation Layers

The following characteristics were observed for the two mortar preparatory layers under optical microscopy in transmitted light.

The top mortar layer (Figure 9a) is made of carbonated air-hardening lime or weakly hydraulic lime as binders, mixed with a silica/calcareous aggregate. The binding matrix is mainly composed of calcium carbonate and has a micritic texture and a homogeneous structure free of lumps. The aggregate consists of calcareous clasts with micritic or microsparitic texture and fragments of metamorphic rocks made up of polycrystalline quartz sometimes feldspars and/or micas. The clasts have rounded or sub-rounded edges, medium or high sphericity, and poor particle-size selection. The dimensions of the aggregate vary from 0.04 to 0.45 mm. The porosity is about 15% by volume and is due to longitudinal micro-cracks. The binder/aggregate ratio is 1:2 by volume.

The bottom mortar layer (Figure 9b) is made with a cement binder mixed with a calcareous aggregate. The binder matrix has a micritic texture and contains non-hydrated cement clinker relicts. Calcareous clasts were obtained by crushing limestone with a micritic or microsparitic texture. Limestone has a sub-angular to rounded shape, medium or low sphericity, and a moderate granulometric selection. The dimensions are between 0.04 and 0.75 mm. The porosity of the mortar is between 25 and 30% in volume and is due to circular pores, which are often interconnected, and microcavities. The binder/aggregate ratio is 1:2.5 by volume.

XRD analyses performed on the raw materials show that the IG21 mortar, used in the top layer of the painted panel, is made of calcite magnesian ((Ca, Mg)CO₃), portlandite (Ca(OH)₂), belite (Ca₂SiO₄), alite (Ca₃SiO₅), silicate phases such as quartz and albite attributable to the aggregate, and traces of a phosphatic phase such as brushite (CaHPO₄·2H₂O) (Figure 10a). The KD2 mortar used in the bottom layer consists of calcite magnesian, belite (Ca₂SiO₄), and celite (Ca₃Al₂O₆) (Figure 10b).

3.3. Decay Features

3.3.1. Humidity Presence and Distribution

Thermal images acquired by in situ IR thermography are reported in Figure 11a,b.

The images show different thermal behavior on the area of the wall external to the panel. Cold areas (blue areas) were evident and widespread at different heights from the ground level. Their pattern suggests the presence of rising humidity from the bottom of the wall, as well as due to water seepage across the filler ground behind the wall. Thermal contrasts are also evident on the panel. Most cold areas fall in a rounded portion at the center of the artifact and extend horizontally on its left. Over the yearly monitored period, cold areas were well evident on both the panel and the surrounding areas of the wall during the spring and denoted an increase in humidity in this season (Figure 11a). The thermal contrasts decreased during the autumn after drying of both the panel and the wall likely took place in the summer (Figure 11b). Nonetheless, they persisted in the upper part of the wall, probably due to a high humidity content within the filler ground caused by the infiltration of water. The presence and distribution of humidity detected by IR thermography suggest that water seepage moving by gravity across the filler ground on the back of the wall, rather than rising capillary humidity, affects the painted panel. The thermal image acquired in the spring also shows cold areas shaped on the texture of the wall and following the joints between the ashlars, which likely are the preferential paths for the migration of water towards the surface.

Thermal contrasts in the panel also seem to partially relate to the different responses of the elements of the decoration pattern: during the spring, it can be observed that obliquus hot areas follow the straight brown lines which typically mark the face portrayed in the painting.

The IR thermography results show correlations with the map of the decay visually recorded. Thermographs highlight a correspondence between the coldest areas relating to a high presence of water and the most degraded areas of the painted panel, which helps to better understand the decay scenario in which the artifact is placed and the primary role of the water seepage in the salt decay phenomena. The area with a higher presence of humidity corresponds to that where decay in the form of salt depositions and mortar decohesion strongly affect the painting, and white veils also follow the cold net of the mortar joints (Figure 11c). Therefore, the results of the thermographic survey let us identify the specific correlation between the water seepage across the filler ground and the decay observed on the surface of the artifact. These suggest that water from the ground plays a central role in the painting’s deterioration and, after the evaporation phenomenon, promotes the consequent salt damage. Water mobilizes soluble salts, which afterward are deposited over and underneath the pictorial film as efflorescence and sub-efflorescence [39]. The crystallization–dissolution cycles of the salts take place thanks to the wetting–drying cycles due to seasonal variations in humidity within the panel.

3.3.2. Salt Identification and Quantification

Weight percentages of ions detected by ion chromatography in all the analyzed samples are reported in Table 4 and Figure 12.

Table 4. Weight percentages of each ion, along with total amounts in the samples from the mural paintings, in the raw mortar materials of the preparatory layers and in the samples from the masonry wall.

	IG21	KD2	JA5	JA3	JA8	JA6	JA10	JA7	JA11
Chloride	0.60	0.37	0.08	0.04	-	0.16	0.56	0.09	0.61
Bromide	-	-	-	1.10	0.03	-	0.06	-	-
Nitrate	0.11	0.06	0.03	0.07	0.05	0.09	0.06	0.05	0.12
Phosphate	0.03	0.93	0.01	5.20	8.66	0.02	9.74	0.02	-
Sulfate	0.74	0.08	0.65	-	0.31	17.45	6.46	2.53	0.88
Total anions	1.48	1.44	0.78	6.41	9.05	17.71	16.87	2.69	1.61
Sodium	0.52	0.24	0.23	4.80	0.22	2.40	0.30	0.27	0.33
Potassium	-	-	-	0.49	-	0.27	-	-	-
Magnesium	-	0.07		1.86	0.20	0.06	0.16	0.06	0.16
Calcium	6.36	2.75	0.83	5.11	2.65	1.05	1.79	1.12	0.57
Total cations	6.88	3.06	1.06	12.26	3.07	3.78	2.25	1.44	1.06
Total ions	8.35	4.50	1.83	18.67	12.11	21.49	15.49	4.12	2.67

Table 4. Weight percentages of each ion, along with total amounts in the samples from the mural paintings, in the raw mortar materials of the preparatory layers and in the samples from the masonry wall.

	IG21	KD2	JA5	JA3	JA8	JA6	JA10	JA7	JA11
Chloride	0.60	0.37	0.08	0.04	-	0.16	0.56	0.09	0.61
Bromide	-	-	-	1.10	0.03	-	0.06	-	-
Nitrate	0.11	0.06	0.03	0.07	0.05	0.09	0.06	0.05	0.12
Phosphate	0.03	0.93	0.01	5.20	8.66	0.02	9.74	0.02	-
Sulfate	0.74	0.08	0.65	-	0.31	17.45	6.46	2.53	0.88
Total anions	1.48	1.44	0.78	6.41	9.05	17.71	16.87	2.69	1.61
Sodium	0.52	0.24	0.23	4.80	0.22	2.40	0.30	0.27	0.33
Potassium	-	-	-	0.49	-	0.27	-	-	-
Magnesium	-	0.07		1.86	0.20	0.06	0.16	0.06	0.16
Calcium	6.36	2.75	0.83	5.11	2.65	1.05	1.79	1.12	0.57
Total cations	6.88	3.06	1.06	12.26	3.07	3.78	2.25	1.44	1.06
Total ions	8.35	4.50	1.83	18.67	12.11	21.49	15.49	4.12	2.67

Salt anions in the efflorescence (JA3) and white veil (JA8) samples from the painting surface are mainly phosphates. Very small amounts of sulfates are also present in the white veil.

XRD analyses of the samples from the painted panel indicate the presence of magnesium phosphate in the efflorescence (Figure 13a) and gypsum in the white veil (Figure 13b).

As regards the JA5 sample taken from the upper layer of the painting, the quantities of ions, except calcium and chlorides, are comparable with those present in the commercial product IG2, i.e., the mortar used to make this layer within the panel. Calcium ions are present in a considerably higher amount in the IG21 mortar, and it is to be ascribed to hydrated lime. It decreases in the JA5 sample as a result of setting when Ca ions are incorporated into the insoluble calcium carbonate. The comparable salt contents in the IG21 and the JA5 mortar samples suggest that the presence of salt ions in the top layer of the painted panel is not the result of an external supply but intrinsic to the nature of the mortar itself and that also the provenance of the salt ions detected in the efflorescence and white veil samples from the painting could be related to the nature of the raw mortar materials.

The ion chromatography analyses show that KD2 mortar, used in the bottom layer of the mural panel, contains a non-negligible ion phosphate amount (Figure 12). Phosphates are probably related to the cement’s additives. Phosphorous additives are used in cements as setting retardants. It has been reported that the presence of phosphorus in the range of 1–2% in Portland cement clinkers slows the rate of hardening of the cement [40], and it could be the origin of the predominant phosphate component found in the white veils and in the efflorescence on the painting.

Phosphates may have other sources as they may come from fertilizers in the soil [41], as well as from pigeon droppings [42]. In the case of such supplies, they would also be present in the samples taken from the wall areas external to the panel. On the contrary, phosphates were absent in these samples, except in the white deposit sample (JA10) taken from the stone surface of the wall at the foot of the panel. This sample was rich in phosphates, which were detected in quantities comparable to those found in the white veil on the painting. These findings, and the location of the sample at the foot of the panel, suggest that the source of the phosphates in this case are also the mortars of the mural painting, from which they are washed and then deposited on the wall surface.

In a similar vein, the presence in the white veil (JA8) of sulfate ions detected by IC and gypsum found by XRD may be related to the sulfates contained in the IG21 mortar.

Bromine ions found in the JA3, JA8, and JA10 samples, in a higher amount in the former, could be related to the composition of the spray paints and are probably due to the pigments of the perinone series containing bromine [43]. The efflorescence sample from the panel also shows especially high amounts of cations such as Ca and Na, followed by Mg and K. Ca, Na, and Mg were also found in the white veil in notably lower amounts than those detected in the efflorescence.

The raw mortars (IG21 and KD2) have high sodium contents that could be the source of the high accumulations of this ion in the JA3 efflorescence. Mg is present in extremely small quantities in the KD2 mortar; it could derive from the magnesium lime detected diffractometrically in this mortar, while it is absent in IG21. Potassium ions were not detected in the two commercial products.

It should also be noted that, in the light of the results obtained from the analyses of the samples from the wall external to the panel, it cannot be excluded that additional sources for the sulfate ions, as well as for some of these cations, could be the constituent materials of the masonry under the mural painting.

As regards the samples from the wall external to the painted panel, with the exception of the JA10 sample, mainly sulfates were detected among the anions (Figure 12).

The highest amounts of sulfates, along with notably lower quantities of chlorides, were measured in the efflorescence from the mortar joint between the stone ashlars (JA6), where they are the most exclusive component, as well as within the white veil on the stone ashlar (JA10 sample). In the latter, phosphates were also present; moreover, they were in higher amounts than the sulfates. In the joint mortar powder (JA7), too, sulfates, although at lower entities compared to the previous samples, were present in a significant amount, along with low quantities of chlorides. The lowest contents of these ions were detected in the sample from the surface of the stone ashlar (JA11) in the wall on the right of the panel.

When we consider cations, it should be noted that the JA6 efflorescence sample was especially rich in Na and contained K, as in the case of the efflorescence on the painted panel. Ca and notably lower contents of Mg ions were also detected. Mainly Ca, followed by lower Na and negligible Mg ion contents, were measured in the other samples (JA7, JA10, and JA11). XRD analyses identified gypsum (CaSO₄·2H₂O) and epsomite (MgSO₄·7H₂O) in both the efflorescence on the mortar joint and the white veil on the stone surface, as well as within the mortar of the joint (JA6, JA10, and JA7 samples, respectively). In addition, thenardite (Na₂SO₄) was found in the JA6 and JA7 samples (Figure 13c). These salts, especially sodium sulfate and magnesium sulfate, have been found to be particularly efficient in salt weathering as they can cyclically change their hydration states, and when they are in mixtures, they may exhibit a greater damage potential [44,45].

XRD analysis of the JA7 sample also evidenced a hydraulic nature in the mortar of the joints in the masonry support, where the presence of the mineralogical phases of the tobermorite group was detected [46,47]. It is well known that cement-based mortars may be a source of soluble ions [48], such as sulfate, Ca, Mg, and alkali metals (Na and K), coming from cement leaching and also be responsible for a strong effect on the long-term performance and durability of the cement itself [49]. These ions are the components of the salt phases detected by XRD in the investigated samples from the wall support. Therefore, in this case, the soluble salts also seem to be related to the nature of the materials used in the masonry structure. It is not excluded that the mortars of the joints in the wall support also contribute to the salt ions within the panel, especially for sulfate and sodium.

Nonetheless, other factors could contribute to the supply of soluble salt ions on the surface of the wall and painted mural in the outdoor exposure conditions. In particular, due to the exposure to vehicular traffic, sulfates could also originate from air pollution and the consequent sulfation processes of the calcareous stone of the wall [50], which could account for the high presence of sulfates found in the area of the wall rather than on the mural. The ubiquitous presence and comparable amounts of sodium and chloride ions found in the analyzed samples from the panel and the wall do not exclude a contribution of sodium chloride from the atmosphere in the form of marine aerosols [51], which may penetrate the porous materials of the artifacts by rainwater infiltration and surface condensation. This supply has already been recognized in the city of Matera [52].

4. Conclusions

Integrated investigations were conducted on a representative example of contemporary urban art by Jorit Agoch to characterize the materials used for its creation, as well as to investigate decay issues. Sustainable investigations, able to achieve a comprehensive knowledge of these aspects, were supported by a multi-analytical approach based on microinvasive analytical protocols combined with non-destructive in situ IR thermography.

In particular, the results of FTIR and Py-GC-HRAMS analyses allowed us to identify spray paint based on an alkyd resin. Alkyd resin is one of most used binders in street art and urban art, which is susceptible to photo-oxidative processes. Indeed, chemical degradation of the alkyd resin in the painting was highlighted by the FTIR and Py-GC-HRAMS data.

According to the results of microscopic observation under VIS-UV reflected light and SEM-EDS analyses, the spray paints were applied by the artist in a single layer or in multiple paint layers. Pigments and extenders consisted of titania, silica, iron oxides, and hydroxides and calcite.

Investigations at the macroscale by in situ IR thermography highlighted thermal contrasts related to water seepage phenomena through the filler ground on the back of the wall where the mural was built and their correlation with salt damage. The latter was the main cause of decay observed on the mural painting, responsible for the lifting and subsequent loss of the pictorial film, as well as for the decohesion of the mortars of the paint support. The analytical study by ion chromatography and XRD allowed us to identify soluble salts—primarily phosphates and secondarily sulfates—affecting the mural and to recognize their source mainly in the cement-based nature of the mortars of the support, which was recognized by PLM and XRD. In the area of the wall around the painting, a significant presence of gypsum, epsomite, and thenardite was detected, too, which defines a risky situation for the painting and also suggests in this case a provenance from the constituent materials of the wall, likely across the mortar joints.

Finally, the study contributes to increasing the scientific background on the materials used in contemporary urban mural art, which is an important part of the cultural heritage at risk that deserves attention for conservation. In this regard, the study also provides basic diagnostic knowledge to support suitable and sustainable conservation strategies. Different measures may be suggested in the specific case study and they include structural works aiming to isolate the wall under the paintings from the ground, where water accumulates and then penetrates the mural along with soluble salts; application of conservation treatments addressing both a consolidation function and the ability to inhibit soluble salt action, such as those based on inorganic barium hydroxide products successfully used in the conservation of murals [53,54]; and removal of the painting for musealization indoors. The first two options seem to be the strategies preferred as the most suitable ones to allow the open fruition of this artistic production in the urban art field. On the other hand, the local community was involved by the artist in the creation of the artifact and then in the sharing of the preservation needs. This dialogue increases the public engagement through the awareness of the value of cultural heritage and of the importance of contributing to its safeguarding. In this perspective, the choice of possible preservation measures is also in the direction of a public engagement with the local community.

Nonetheless, for the preservation strategies to be effective, they must be supported by a preventive conservation approach based on monitoring over time and periodic maintenance work, which are the basic rules to ensure the preservation of cultural heritage in outdoor exposure conditions, as is the case for urban art.