## 3 Results and Discussion
### Laboratory Artificial Patinas on Copper Specimens
#### 3.2.1 Chloride Rich Patinas  
Atacamite-based patinas were also realised following three different recipes. In one case the copper specimens underwent wet and dry cycles in a 0.25 M solution of CuCl2 [100, 117]. In the other cases, instead, the \"applied paste\" method was adopted [51, 98]: two mixture of salts, one rich in copper chlorides and the other rich in copper chlorides and sulphates, were properly diluted in water to obtain a sort of paste, that was applied on the copper surfaces by brush. For these latter patinas, preliminary tests were performed to identify the optimal number of layers of paste to be applied to obtain a uniform and adherent corrosion layer. In both cases, 2, 4, 6 and 8 layers were considered for the realisation of both \"applied paste\" patinas. The artificial corrosion layers produced with both the \"applied paste\" recipes showed all a quite uniform aspect, both from the macroscopic (Figures 8a and 9a) and microscopic (Figures 8b and 9b) point of view, regardless the number of layers applied. Nevertheless, as expected, the homogeneity and the thickness of the patinas apparently was improved by applying more layers. However, in all cases, the patinas appeared powdery regardless the number of layers, and they showed a more pronounced tendency to detach from the substrate as the number of applied layers increased. The optimal number of applied layers was identified in eight layers for the patina with only chlorides, and in six layers for the patina containing both chlorides and sulphates.  
Figure 8: Patina obtained by “applied paste” method, with chloride rich paste applied in 6 layers: (**a**) picture of a specimen and (**b**) stereomicroscopy observation.  
FTIR analysis allowed detecting the main peaks of atacamite at high wavenumbers (3445 cm\({}^{-1}\), 3340 cm\({}^{-1}\)) on all the specimens patinated by \"applied paste\" method (Figures 10 and 11). In the case of the \"applied paste\", patina containing both chlorides and sulphates, the peaks ascribable to the presence of CuSO4.5H2O, one of the chemicals employed for the patination, are still clearly visible (Figure 11). As it can be observed in Figure 11, the intensity of the atacamite peaks increases as the number of paste layers increases, while the signals of copper sulphate pentahydrate decrease.  
Also, the atacamite-based patina produced by wet and dry cycles in a CuCl2 solution was characterised by a uniform appearance. On a microscopic scale, the presence of large crystals, probably constituted by atacamite, is clearly visible. Underneath them, a light green and apparently more compact layer is visible. Such observation basically confirm that this patination procedure allows obtaining a quite uniform distribution of the patina on the metallic substrate. This patination, however, resulted very aggressive, as it caused severe thinning of the sample. From a compositional point of view, in this case, the presence of atacamite is confirmed, and it is the only compound detected by FTIR (Figure 12) with a very good correspondence between spectrum of the realised patina and the atacamite reference one.  
Figure 10: FTIR spectra of atacamite patina produced by the “applied paste” method, with chloride rich paste (Cl) applied 8 times, and with a reference spectrum of atacamite.  
Figure 9: Atacamite patina obtained by “applied paste” method, with chloride and sulphates rich paste: (**a**) picture of a specimen and (**b**) stereomicroscopy observation.