## 1 Introduction
### Natural Patinas
However, due the high concentration of sulphide ions and the reducing atmosphere needed for the formation of such minerals, they form normally on objects buried in soil (where sulphate-reducing bacteria can also promote the formation of sulphides) or in seawater sediments. In cultural heritage field, therefore, they are normally found on archaeological objects [2; 35]. When chloride-based salts are present in the atmosphere, they can dissolve in the electrolyte film and dissociate producing Cl-. Then, they react with positive copper ions produced by the dissolution of cuprite and the highly reactive nantokite (CuCl) or eriochalcite (CuCl\({}_{2}\)) are formed. Their reaction with atmosphere is quite quick and the formation of a more stable trihydroxy copper chloride, in the crystalline form of atacamite (Cu\({}_{2}\)Cl(OH)\({}_{3}\)), or in its polymorphs paratacamite or clinoatacamite is observed. The corrosive action of the chlorides is of particular importance when bronze surfaces are considered. In fact,the presence of nantokite in specific conditions [36] can cause the onset of a cyclic and autocatalytic severe corrosion phenomenon, the so-called bronze disease [37; 38; 39]. Basic copper carbonates have been detected in natural patinas of objects exposed to atmospheric corrosion. They are thanks to the reaction of the surface with the atmospheric CO\({}_{2}\) or with HCO\({}_{3}\)\({}^{-}\) dissolved in the condensed water film [2; 10]. However, since a high concentration of CO\({}_{2}\) or HCO\({}_{3}\)\({}^{-}\) is necessary, they are quite rare as products of atmospheric corrosion, while they are quite common on objects buried in soil [2]. In this case, after prolonged permanence of the object in soil, a uniform patina of malachite (CuCO\({}_{3}\)-Cu(OH)\({}_{2}\)) often covers the surfaces. In addition, discrete crystals of azurite (2 CuCO\({}_{3}\)-Cu(OH)\({}_{2}\)) can be found in association with malachite. Azurite is in fact a less stable phase and tends to transform in malachite in presence of moisture through loss of carbon dioxide [2]. Therefore, basic copper carbonates are usually present on archaeological objects [2; 40; 41]. Also basic copper nitrates can be found among the corrosion products of copper-based surfaces. In particular, the most common compound in this class is gerhardite (Cu\({}_{2}\)(NO\({}_{3}\))(OH)\({}_{3}\)) [2; 15]. However, it has rarely been detected in patinas of outdoor exposed surfaces [42]. In fact, gerhardite is highly soluble in water, therefore it is very soon eliminated through rain runoff from the patinas [2]. Copper formate, acetate and oxalate can also be found as corrosion products [15; 43] since formic, acetic and oxalic acids can be present as atmospheric pollutants [10; 15; 43]. Besides the natural corrosion products, artificial patinas intended as deliberately produced corrosion layers also play a key role in the field of the conservation of the metallic artefacts of the cultural heritage. In particular, in the preservation of cultural heritage field two main kinds of patinas should be considered, on one side the artistic patinas, and on the other, those produced on specimens and coupons by laboratory procedures. The formers are produced since ancient times on artistic surfaces in order to mimic their natural ageing and to confer the desired colour and finishing. The latter are instead realised as a model system to perform specific diagnostics and experiments. Therefore, laboratory patinas are aimed to simulate the chemical composition, morphology, and micro-structure of the natural ones.