## 1 Introduction
### Artistic Patina
Moreover, artificial patinas evolve with time, taking part in the natural corrosion process and changing their chemical compositions [55; 56]. Therefore, few works in recent decades have reported on the compositional characterisation of corrosion layers on copper-based artefacts of supposed artificial production. A black patina has been identified on a series of Roman, Mycanean and Egyptian bronzes, known as \"black bronzes\". On such artefacts, the presence of an intentional patina obtained by means of metallic Au and/or Ag (probably nanoparticles) in a cuprite matrix was demonstrated [48; 49; 50; 57; 58]. A similar realisation technique was hypothesised for the so called _shakudo_ patina, observed on ancient Japanese artefacts [48]. In the same works, the investigation of the corpus of Egyptian archaeological artefacts have moreover led to the identification of reddish cuprite-based artificial patina, while a further black patina on a roman balustrade was demonstrated to be constituted by tenorite. Another kind of black patina with compact and lustrous aspect and constituted by copper sulphur, was detected on ancient Greek and Roman artworks. Its presence was hypothesised by [PERSON] [49; 42] on two Roman daggers and by [PERSON] and [PERSON] [42] on the renowned [PERSON]'s bronzes. On contemporary artworks, instead, artistic patinas have been more easily characterised. In these cases, in fact, the identification of artificial patinas is normally easier as their colour may differ significantly from those of natural corrosion layers, and because often the design of the artist is well known. [PERSON] et al. [59], as an example, identified ebfardtite (a basic copper nitrate), copper phosphates and copper sulphide (probably malcocite) on a contemporary bronze sculpture realised in 1988 and exposed to outdoor environment. [PERSON] [56] characterised instead the composition of the artificial patinas (mainly dark-brown) on two contemporary sculptures of the [PERSON] contemporary art museum in Rome. On the first sculpture, the dark patina resulted constituted by cuprite and tenorite, while on the other cuprite was associated with copper sulphur, employed probably to obtain a darker colour shade. Moreover, traces of copper nitrate (gheardtite) and iron hydroxides were detected. Copper nitrates and FeCl3 are in fact often used in the artificial patination process [51]. In the work of both [PERSON] et al. [59] and [PERSON] et al. [56], relevant amounts of natural corrosion products (e.g., brochantite and atacamite) were detected, confirming that the artificial corrosion layers tend to transform upon exposure to the atmosphere. In addition, some authors reproduced the artificial patinas according to traditional recipes and then characterised the obtained surface layers. As an example, [PERSON] et al. [47] have performed a Raman characterisation of a series of artificial patinas with different compositions (namely, nitrate- carbonate- sulphate- and chloride-based artificial patinas). Raman spectroscopy was included also in a multi-analytical investigation of a series of patinas produced by torch-techniques by [PERSON] et al. [53]. In this work, in particular, an artificial ageing in spray chamber of the artistic patinas was performed, and the changing in their composition upon such treatment was assessed. In addition, in several works, potassium sulphide associated with the heating of the metallic substrate, have been employed to realise black patinas, according to traditional recipes used also in modern artistic foundries [49; 52; 56; 60; 61; 62; 63]. Even if more rarely reported, the electrochemical characterisation of the artistic artificial patinas and of their corrosion behaviour has also been carried out [54; 64]. [PERSON] et al. [65] performed electrochemical impedance spectroscopy (EIS) on two nitrate-based artistic patinas and suggested a dependence of their corrosion behaviour on their microstructure.