## 3 Results and Discussion  
### Artistic Patinas on Bronze Specimens  
In Figure 2, stereomicroscopy images of the artistic patinas are reported. From a morphological point of view, it can be observed that all the studied patinas present quite homogeneous surfaces. In addition, from the aesthetical point of view, the microscopic observations suggest a good uniformity in terms of colour and appearance of the surfaces. In few cases (Figure 2b,e), the traces of brush strokes are still visible due to the application of chemicals by brush, for their realisation.  
Figure 2: Images recorded by stereomicroscopy on surfaces of bronze specimens with artistic patinas. (**a**) Red 103—Rosso fiorentino; (**b**) Green 201—Verde Messina; (**c**) Green 206—Enamel green; (**d**) Blue 502—Blu note; (**e**) Black 605—Medium black; (**f**) Grey 802—Silver grey.  
In Figure 3 the FTIR spectra of the red patina 103, green patinas 201 and 206 and of the blue patina 502 are reported. These spectra are quite similar; this is probably due to the employment of copper nitrate for the production of these patinas (Table 1). In particular, the main characteristic peaks of copper nitrate hydrated (Cu\({}_{2}\)(NO\({}_{3}\))(OH)\({}_{3}\)) can be observed at 3541 cm\({}^{-1}\), 3420 cm\({}^{-1}\), 1420 cm\({}^{-1}\), 1340 cm\({}^{-1}\), 1046 cm\({}^{-1}\), 880 cm\({}^{-1}\), 665 cm\({}^{-1}\), and 507 cm\({}^{-1}\). Such peaks could be attributed both to gerhardtite [59] and to rouaite [107], two polymorphs of basic copper nitrate. Moreover, the peaks at 1300 cm\({}^{-1}\) and 760 cm\({}^{-1}\) could be attributed, thanks to the comparison with a reference spectrum, to the presence of unreacted copper nitrate hydrated. As concerns the blue patina 502, in addition, the peaks at 3040 cm\({}^{-1}\), 1597 cm\({}^{-1}\), 1495 cm\({}^{-1}\), 1188 cm\({}^{-1}\) and 1156 cm\({}^{-1}\) can be ascribable to the presence of methylene blue on the surface [108], employed according to the foundry recipe to obtain such an intense blue hue. XRD analysis confirmed the presence of basic copper nitrates as main compounds, identified specifically as rouaite in case of green 201 and blue 502 (Figure 4). In addition, on patina red 103 hematite, employed in the patination procedure to obtain a bright red colour, have been clearly identified.  
FTIR spectra of black 605 and grey 802 patinas, instead, did not show the presence of any characteristic peak. For such patinas, XRD analysis (Table 3 and Figure 4) revealed the  
Figure 4: XRD spectra of four representative artistic artificial patinas produced by _Fonderia artistic Battaglia_ (N = copper nitrate hydroxide; C = cuprite; H = hematite; R = rouaite; G = gerhardtite; T = tin dioxide; CH = chalcotic).  
Figure 3: FTIR spectra of artistic artificial patinas produced by _Fonderia Artistic Battaglia_ (G = gerhardtite; R = rouaite; MB = methylene blue; N = copper nitrate hydrate).  
presence of tin dioxide (Sn2O) and chalcocite (Cu2S). The former is probably due to the oxidation of tin present in the bronze alloy, whilst chalcocite is probably the result of the reaction of copper with the ammonium sulphide employed for the patination. For grey 802 patina, instead, cuprite (Cu2O), copper nitride (Cu3N) and copper nitrate hydroxide (Cu\({}_{2}\)(NO\({}_{3}\))(OH)\({}_{3}\)) were detected by XRD. Cuprite may be the result of the oxidation of the surface caused by its heating and by the presence of corrosive substances, whilst copper nitride and copper nitrate hydrated are probably the result of the reaction of copper with silver nitrate.  
The artistic patinas were also investigated by means of VIMP during a previous work [64], results were in quite good agreement with FTIR and XRD results.