## 2 Materials and Methods
### Artificial Ageing Protocol (AAP)  
Each material responds differently to any type of environmental stress, such as climatic variations, extreme temperature exposures, and crystallization of salts dissolved in water, producing characteristic effects of a degradation process [24]. Nowadays, through experimental laboratory treatments and an artificial ageing test on samples, it is possible to observe and study the response of this material to the processes of degradation in progress. The different artificial ageing tests, coupled with physico-mechanical tests, could be a valuable tool for the study of the causes, processes, and effects of degradation on ornamental materials, like plasterwork, used in historic and modern buildings. Artificial ageing also allows the evaluation of the effectiveness of conservative treatments by reducing the risks of unsuccessful choices of products that are not appropriate to the real needs of historical works. Therefore, an appropriate ageing diagnostic program is a valuable tool for the quality and sustainability of conservative intervention. At the present time, however, bibliographic research has revealed the fragmentary nature of such ageing tests and the lack of a unified protocol aimed at identifying and characterizing the degradation of ornamental stones. Moreover, although there are a limited number of norms and protocols, the literature offers some interesting tips and  
Figure 3: (**a**) Simulated Hispano-Muslim plaster samples up to the original polychrome layer. In this case, P1 refers to the sample simulating azurite pigment mixed with Arabic gum (P1 aGa) and animal glue (P1 aC); P2 refers to the sample simulating vermilion pigment mixed with Arabic gum (P2 vGa) and animal glue (P2 vCa). (**b**) Simulated Hispano-Muslim plaster samples up to the repainting layer. (**c**) The scheme shows the final distribution layout of the test samples after the application of the repainting layers. Band A corresponds to the reference of the simulated original polychromy (P1 aGa(A), P1 aCa(A), P2 vGa(A), and P2 vCa(A)). Band B refers to the repainting layer composed of linseed oil and artificial ultramarine blue with zinc white (P1 aGa(B), P1 aCa(B), P2 vGa(B), and P2 vCa(B)), while Band C corresponds to the repainting layer made of linseed oil and ochre pigment (P1 aGa(C), P1 aCa(C), P2 vGa(C), and P2 vCa(C)). suggestions about the test models to be adopted and invites continuous testing to develop unified procedures. Considering these previous studies [25; 26; 27], a specific ageing protocol is proposed. Furthermore, this protocol is based on the study of the climatic conditions of temperature and relative humidity of those regions where the Hispano-Muslim plaster is found, in southern Spain. The ageing protocol is carried out after the application of the altered oil layers on the polychromed samples, which have previously undergone a 1-month natural curing process. The reason for performing the artificial ageing at this final stage of sample preparation, rather than before and after the application of this layer, lies in the nature of the materials used to reproduce the original stratigraphy. Both the gypsum and the binders used are highly sensitive to humidity above all, but also to temperature, which directly affects the preservation of this specific type of works [28]. Previous studies have also been taken into consideration, as they highlight the aggressive nature of applying an ageing protocol to these materials, which in this case must be artificial and accelerated, potentially causing destructive effects on the samples [25; 26; 27]. Furthermore, there is the added challenge of fully reproducing, both materially and technically, the typology of Hispano-Muslim gypsum artworks and subjecting them to ageing without any existing precedent. For this reason, the artificial ageing process is applied only after completing all the layers of the samples, including the alteration layer.