## 3 Results and Discussion
### Laboratory Artificial Patinas on Copper Specimens
#### 3.2.4 Electrochemical Characterisation  
The corrosion behaviour of the specimens with the laboratory patinas so far described was evaluated by means of LPR and EIS measurement, from which Rp values were obtained (Figure 17). Bode and Nyquist plots of selectedcuprite patinas are reported in Figure 18. Rp values calculated for all kind of laboratory patinas are summarised in Figure 17. As it can be observed, when cuprite patinas are considered, the one obtained by \"boiling solution\" procedure shows a significantly lower Rp value with respect to the one of not patinated copper and to those typically measured on natural patinas that normally range between 2 and 20 \(\Omega\cdot\)m\({}^{2}\)[75; 109; 110; 111; 112; 113; 114]. Cuprite patinas realised by means of wet and dry cycles in deionised water or artificial seawater allowed instead the development of less reactive surface layers. In particular, the patina formed in deionised water plays a quite protective role with respect to the metallic substrate, while the cuprite formed in artificial seawater showed Rp values more similar to those measured on non-protected natural patinas. Such results clearly suggest that the composition of corrosion layer is not the unique or main factor that influences the corrosion behaviour of the specimens. In terms of composition, the cuprite formed in artificial seawater could have showed the lowest Rp value due to the presence of a little amount of chloride on its surface, compared to the boiling one that is constituted by pure cuprite. In addition, the two \"pure cuprite\" patinas showed a significantly different value of Rp.  
Figure 16: FTIR spectrum of nitrates-based patina. G = gerhardtite, N = Copper nitrate hydrate, R = rouaite.  
Figure 15: Nitrate-based patina (**a**) picture of a specimen and (**b**) stereomicroscopy observation.  
As regards atacamite-based patinas, different corrosion behaviours have been observed. Both the \"applied paste\" patinas showed quite low Rp values, suggesting a high reactivity of the patinated surfaces. In particular, the \"applied paste\" containing only chloride-based compounds, was characterised by a Rp value 2 order of magnitude lower with respect to the not patinated copper. Such decrease in the surface corrosion resistance is even more pronounced for the sulphate containing \"applied paste\" patina, which Rp is four orders of magnitude lower with respect to bare copper. A higher stability was instead shown by the atacamite patina realised by wet and dry cycles in CuCl2 solution, which Rp value is comparable to the one measured on natural patinas developed in presence of chlorides [75]. Concerning brochantite laboratory patina, an Rp value perfectly comparable to those measured on copper-based surfaces with natural patinas formed in urban environment was measured, confirming the suitability of such procedure for the reliable reproduction of natural brochantite patina on laboratory specimens. Eventually, gerhardtite showed a quite high stability, with a measured Rp of 22 \(\Omega\cdot\)m2, that is even higher than the Rp typically measured on naturally aged surfaces.