## 3 Results and Discussion
### Electrochemical Characterisation  
The Rp values obtained by LPR and EIS measurements are reported in Figure 5. It can be observed that all the artificial patinas showed a significantly lower corrosion resistance with respect to the not patinated bronze. Moreover, such Rp values are lower also with respect to those measured on natural corrosion layers formed upon atmospheric corrosion in non-marine outdoor environment. In such cases, in fact, Rp values that normally range between 2 and 20 \(\Omega\)-m\({}^{2}\)[75; 109; 110; 111; 112; 113; 114]. In particular, patina black 605, constituted by Cu2S and SnO2 resulted the most stable from the corrosion point of view, showing the highest Rp values while all the nitrate-based patinas showed Rp lower than 1 \(\Omega\)-m\({}^{2}\). Concerning the not patinated specimens, XRD analysis did not provide relevant information regarding the composition of the very thin corrosion layer present over the surface. Based on the literature [1; 10; 11; 89], it can be assumed that such tiny corrosion layer was mainly constituted by cuprite (Cu\({}_{2}\)O), that is reported as the first corrosion product that is formed on bronzes [1; 10; 11; 89].  
Figure 5: Polarization resistance values measured by LPR and EIS on artistic artificial patinas on bronze.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Patina** & **Mineralogical Composition of Patinas** \\ \hline Red 103—_Rosso forentino_ & Copper nitrate hydroxide—Cu\({}_{2}\)(NO\({}_{3}\))(OH)\({}_{3}\) Hematite—Fe\({}_{2}\)O\({}_{3}\) \\ \hline Green 201—_Verde Messina_ & Rouaite—Cu\({}_{2}\)(NO\({}_{3}\))(OH)\({}_{3}\) \\ \hline Green 206—Enamel green & Copper nitrate hydroxide—Cu\({}_{2}\)(NO\({}_{3}\))(OH)\({}_{3}\) \\ \hline Blue 502—_Blu notte_ & Rouaite—Cu\({}_{2}\)(NO\({}_{3}\))(OH)\({}_{3}\) \\ \hline Black 605—Medium black & Tin dioxide (SnO\({}_{2}\)) \\  & Chalcocite (Cu\({}_{2}\)S) \\ \hline Grey 802—Silver grey & Copper nitrate hydroxide—Cu\({}_{2}\)(NO\({}_{3}\))(OH)\({}_{3}\) Copper Nitride (Cu\({}_{3}\)N)Cuprite (Cu\({}_{2}\)O) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Mineralogical composition of artistic artificial patinas assessed by means of XRD.  
Such data suggest thus that these artistic patinas do not seem to provide any protection to the substrate leading instead to an increase in the reactivity of the metallic surfaces and a consequent decrease in the corrosion resistance of the patinated surfaces. This, in turn, confirms the fundamental need of developing and setting up optimal procedures for the preservation of this kind of surface finishing. Their presence appears in fact as potentially critical for the conservation of the metallic substrate as they apparently could promote an increase in the corrosion rate of the surfaces. This is in agreement with the literature data indicating that freshly patinated bronze surfaces exhibit low corrosion resistance [115], that gradually improves over time, when exposed to humid conditions, since it slowly converts to a \"natural\" patina [55]. Nevertheless, artistic patinas have an important aesthetical and artistic value, and it would be important to find procedures for their preservation and stabilisation.