## 4 Discussion  
The analytical results obtained from the study of the wall paintings in the Church of Saint Nicolas de Myre at Semsales summarized in Table 2, provide insights into the painting materials and the techniques used by [PERSON] for his first monumental wall decoration in Switzerland. [PERSON] started depicting the _Trinity_ in the apse by painting _a tempera_ on an existing \"rabitz\" plaster, as he reported in his notes [10]. The term \"rabitz\" is generally used to indicate a substructure made of woven needs or wire mesh, which supports a gypsum or cement plaster. The analytical investigations of the _Trinity_ have confirmed the presence of a gypsum-based plaster mixed with sand aggregates. However, the \"colla\" (i.e. the animal glue) described by [PERSON] [10] as part of the \"rabitz\" mixture was not identified. This layer does not contain any organic material. As clearly revealed by the cross-section analyses, over the gypsum plaster there is a protein-based primer (thickness of ca. 2-3 \(\upmu\)m) followed by a lime-based preparatory layer of variable thickness (between 250 and 40 \(\upmu\)m) not containing any organic binder. It is possible that [PERSON] started by applying the protein-primer on the gypsum and then the lime-based preparatory layer to level the surface. In his notes, contemporary to the work in Semsales, [PERSON] describes several ways to prepare a \"rabitz\" wall for painting [9]. These recipes could correspond to textural differences revealed by visual examination. However, the stratigraphy and the materials identified in the cross-sections do not correspond exactly to any of these preparation recipes and an extensive invasive sampling campaign would be necessary to characterize them. Another question relates to the presence of an additional yellow preparatory layer visible in the stratigraphy of the two angels over the _Trinity_ in the upper most portions of the apse, but missing in the samples from the other areas. This layer contains pigments used by the artist elsewhere, but its role and its presence in the lower portion of the _mandorla  
Figure 11: **(a)** Image of the sampling area (image size ca. 35 \(\times\) 20 cm) along a fracture in the tree trunk behind the figure of [PERSON]; **(b)** optical microscope and **(c)** SEM-BSE images of the cross-section of the micro-sample collected in **(a)**; **(d)** area analyzed by FTIR-FPA imaging (the location is shown in **(b)** with a yellow rectangle); **(e)** FTIR-FPA imaging results with integration of the following spectral range: 1453–1350 cm\({}^{-1}\) (calcium carbonate); 1061–947 cm\({}^{-1}\) (silicates); 1669–1603 cm\({}^{-1}\) and 1331–1309 cm\({}^{-1}\) (oxalates). remains unclear. Regarding the paint layer, the analyses confirm that the _Trinity_ mural was painted _a tempera_ with a protein-based binder composed of animal glue and possibly casein. Although non-invasive FT-IR investigations identified a lipid component (indicated by the IR ester band at ca. 1740 cm\({}^{-1}\)), the exact nature of this material remains unknown and the results of chromatographic analyses are inconclusive as only a pine resin was identified. A further sampling campaign would be necessary to clarify the nature of this lipid-based material.