## 1 Introduction
### Laboratory Patination  
The laboratory artificial patination and ageing of laboratory specimens may be performed also with a scientific aim. In fact, artificially patinated and pre-corroded laboratory coupons can be adopted as model systems to carry out specific investigations. The artificial patinas realised on laboratory specimens, thus, are intended to mimic as much accurately as possible the composition and microstructure of natural corrosion layers, in order to obtain a reliable reproduction of the real conditions of metallic surfaces that have been exposed to different environments. The aim is to realise laboratory mock-ups, intended as material replicas of the historic surfaces, to be used to investigate, e.g., mechanisms of decay or new conservation treatments. Such studies typically include invasive and destructive measurements and may lead to severe deterioration of the surfaces and therefore cannot be performed directly on the artefacts. In the case of new protective coatings and/or corrosion inhibitors, the possibility of testing and optimising their performances and of verifying their influence on the appearance of the surfaces before their application on real artworks is of crucial importance. In some cases, specimens with natural patinas developed upon outdoor exposition have been employed for this kind of studies, especially for the investigation of the patina formation processes and for the investigation of the corrosion rate of different metallic substrates to different environmental conditions [4; 5; 69; 70; 71; 72; 73; 74; 75]. However, such procedures of natural ageing are time demanding and, therefore, the production of laboratory artificial patinas is then carried out in order to more rapidly obtain corrosion layers with the desired features (composition, morphology, etc.). Many patination procedures have been adopted in the literature to this aim. They can be distinguished in three main categories: artificial ageing of the specimens, electrochemical methodologies and chemical procedures. The electrochemical patination is normally carried out by immersing the specimens in saline solution with specific composition according to the desired corrosion layer, and by enhancing corrosion process by means of imposed currents or potentials [67; 76; 77; 78; 79; 80; 81; 82; 83]. It has been employed to reproduce the most common natural corrosion products, with the great advantage of a significant rapidity of the patination procedures. The artificial ageing is based, instead, on the exposition of the specimens to specific environmental conditions reproducing the natural environments which corrosivity must be studied. Several artificial ageing procedures have been proposed, depending on the exposure conditions that needed to be reproduced. In some cases, the artificial ageing of copper-based coupons has been achieved by exposing specimens to specific temperatures and relative humidity levels in controlled environments (e.g., climatic chambers) [55; 84; 85]. In many cases, however, the specimens to be aged have been exposed to artificial rain, namely a salt solution reproducing the rain composition of a specific geographic area [62; 81; 86; 87; 88; 89; 90; 91; 92; 93; 94]. Depending on the conservation condition to be reproduced on the specimens, slightly different exposure methodology have been proposed. Wet and dry cycles typically suffered by surfaces exposed to outdoor sheltered conditions have been simulated by alternatively immersing metallic specimens in artificial rain, for a series of cycles [86; 91; 92; 95; 96]. In other cases, to simulate the leaching action of the rain on unsheltered surfaces, artificial ageing of the specimens has been carried out by dropping artificial rain on the specimens surfaces [52; 81; 82; 87; 92; 97]. To achieve an optimal adjustment of the precipitation rate [81; 97] and a uniform distribution of the artificial rain drops on the ageing specimens [81], some dropping devices have been also proposed.