2. Materials and Methods
- Baveno granite (hereinafter G, Figure 1 and Figure 2a,e,f), which is a medium-grained and pink granite, composed of white plagioclase, gray quartz, pink K-feldspar, and biotite partially replaced by chlorite [35,36]. It takes its name from the main quarry, located in the small town of Baveno on the west bank of Maggiore Lake (in the Verbano-Cusio-Ossola quarry district, northern sector of the Piedmont region) . In Turin, it has been employed with both a polished and unpolished surface, depending on the application. Within this work, a polished surface finish was considered, as it occurs, for example, for several columns of the typical arcades in the city center. It is known that during the polishing process, an epoxy resin filler is used to fill any micro fissures . The EDX spectra reported in Figure 2 highlight the presence of this resin on the granite (Figure 2e,f), showing C, Al, and Si as major elements and, to a lesser extent, Mg, Na, S, Cl, K, Ca, and Fe;
- Vico diorite (hereinafter D, Figure 1 and Figure 2b,g,h) is a quartzdiorite with a granular texture and characterized by a light to very dark gray colour, depending on the grain size and the percentage of femic minerals, represented by amphibole, biotite, and rare pyroxene. With regards to sialic minerals, plagioclase mainly occurs, in addition to K-feldspar and rare quartz . Vico diorite comes from the quarry district of the Chiusella Valley, which is about 70 km from Turin . As for the granite, a polished surface was selected for the Vico diorite, as this surface type was observed for several columns and paving of the arcades of blocks in the city center. Si-rich fillers were detected in the superficial voids and fissures of this stone (as highlighted in Figure 2g with an arrow), as a result of the silicone-based chemical applied to enhance the absorption qualities, increase the resistance to staining, and/or alter the stone´s appearance ;
- Luserna Stone (Pietra di Luserna, hereinafter Gn, Figure 1 and Figure 2c,i,j) is an orthogneiss. It is named after the municipality of Luserna San Giovanni, which is about 65 km from Turin (NW Italy), where the main quarry district was historically located . It is characterized by a micro-augen texture, medium-fine grained size, and good fissibility along the schistosity planes, well-defined by muscovite crystallized under high-pressure conditions . It displays a light gray colour, occasionally turning slight darker or greenish; magmatic porphyroclasts of K-feldspar, in addition to quartz and albite, are clearly visible. As for the forming minerals, it is composed of quartz, plagioclase, K-feldspar, muscovite, and biotite; epidote, chlorite, and sphene are also present as minor minerals . Luserna Stone has been frequently employed in Turin, such as in the case of the slabs of the dome of the Mole Antonelliana (a major landmark building in the city) and the façade of the Automobile Museum. As for numerous real applications, a flamed surface finish was considered for this stone within the present work;
- Travertine (hereinafter T, Figure 1 and Figure 2d,k,l) is a very porous orthochemical sedimentary rock, with fine grains. It is completely composed of a calcite matrix and often contains plant prints. Macroscopically, it is characterized by a more or less intense beige color, with thin bands highlighted by different contents of impurities . Among the four lithotypes selected, it is the only one not from the Piedmont region, with the main mining area being located in the region of Rome . Nevertheless, it was included in the present work since it has been frequently employed in Turin, such as in the case of the façade and arcades of the Santissima Annunziata church, as well as several façades of buildings and shops in the city center. As is mostly seen in Turin, a smooth disc-cutting finish was selected for the travertine.
2.2. Anti-Graffiti Products
2.3. Graffiti Paints and Application
2.5. Analytical Techniques
- The coated surfaces were observed with an OLYMPUS SZ ×10 stereomicroscope, with an OLYMPUS Color View I digital camera, in order to observe any visible effect of the anti-graffiti product applied on the different stones. Moreover, all surfaces, after having been treated with the different cleaning methods, were evaluated by stereomicroscopy to identify the graffiti remains. Moreover, in order to determine the thickness of the anti-graffiti coatings and the graffiti paints, fragments of 1 cm × 1 cm × 1 cm were embedded in resin to be visualized by means of stereomicroscopy;
- The water absorption of both the coated surfaces and the surfaces, after having been treated with the different removers, was evaluated by the contact sponge method. A CTS contact sponge kit was used, following . It is a non-destructive water absorption test, which may be performed in both the laboratory and in situ and has been proven to be directly comparable with other water uptake laboratory measurements (e.g., the capillary rise method) . A Spontex1 Calypso-type sponge, in natural fibers, having pre-determined characteristics and dimensions, was charged with an adequate amount of water (2 mL within the present work), after preliminary tests to verify that the water did not leak when the sponge was put in contact with the surface. The sponge was applied on the stone sample surface for 180 s, by using a Falcon 1034 Rodac® circular plastic plate: by placing the borders of the plate in contact with the examined surface, the maximum applicable manual pressure was determined. The amount of water absorbed by the surface was calculated by the difference, by weighing the sponge before and after the application. The difference in weight corresponds to the amount of water which has been absorbed by the material through the surface. The results (water absorption, Wa) were then expressed by the mass difference as a function of area and time. For each surface, three measurements were performed, by drying the sample between measurements;
- The hydrophobicity of the surfaces after cleaning was evaluated by means of a SEO Phoenix-300 Touch goniometer (Surface Electro Optics Co., Suwon, Korea) following BS  by applying the sessile drop method. Three drops of 6 µL of deionized water per sample were applied;
- The variation of roughness of the surfaces after having been treated with the different cleaning methods were evaluated using a PLu 2300 Sensofar® optical imaging profiler (Sensofar, Barcelona, Spain). The images with the optical imaging profiler were collected with an EPI 10X-N objective, an overlap of 25%, a depth range of 2 mm, and a lateral resolution of 1 nm. The system allowed us to obtain 3D images of the cleaned surfaces and therefore, the roughness parameter (), R3z (third maximum peak-to-valley height), was obtained using the Gwyddion 2.47 software. For each sample, three images of 4 mm × 4 mm areas were taken and used to obtain the roughness parameter;
- A microscopic evaluation of the cleaned surfaces was performed using a scanning electron microscope (FEI Quanta 200) in BSE mode to find the location of the graffiti and anti-graffiti remains and the damage induced on the surfaces. The optimal conditions of observation were the same as used to characterize the paint layers on the stone.
- The removal of the green acrylic paint was satisfactory from all four stones. Nevertheless, as for the samples with sacrificial anti-graffiti product, paint remains filling the voids were found on travertine surfaces. Therefore, the commercial graffiti remover employed (a blend of alcohol and terpene solvents) proved to be effective in the dissolution of this paint;
- The cleaning of the violet alkyd paint was totally unsatisfactory, leaving the graffiti layer apparently unaffected. However, an observation by SEM revealed a few fissures and cracks in the paint layer, possibly showing the initial step of the detaching process from the protective coating.
Conflicts of Interest
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|Violet||Solvent mixture A||GVSa|
|Green||Solvent mixture A||GGSa|
|Violet||Solvent mixture B||DVSb|
|Green||Solvent mixture B||DGSb|
|Violet||Solvent mixture A||GnVSa|
|Green||Solvent mixture A||GnGSa|
|Violet||Solvent mixture B||TVSb|
|Green||Solvent mixture B||TGSb|
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