The plaster samples from the nave and the chancel have been analysed using the XRD technique. Both results are very similar and reveal a plaster dominated by quartz, while the calcite content is lower. The plaster also contains some feldspars and dolomite; the lattercould be part of both the binder and the aggregate. Therefore, the plaster mainly consists of aggregate, while the binder, in this case lime, is less abundant. Nevertheless, the plaster in the chancel exhibits slightly higher calcite content and somewhat less quartz, which could indicate that the two works originate from different periods or workshops. The sample cross-sections also reveal a plaster rich in aggregate, especially in the case of the plaster from the nave (Fig. 1). The high content of varicoloured sand grains, ranging from translucent to ochre, red, and brown, is clearly evident. Many of the grains are slightly rounded, so river sand may have been used. Underneath the plaster, a white layer is visible on the sample. This could be the top layer of the arriccio, or a layer of limewash may have been applied before the plaster to ensure its better adherence to the wall. On the other hand, the sample taken from the chancel (Fig. 2) shows more lime and translucent aggregate (crushed limestone, marble, or quartz), which, however, could also result from this sample being smaller. On one of the samples taken in the nave, limewash is also clearly visible (Fig. 1). It seems that the red colour of the flag was applied on top of it – meaning that the plaster had dried before the mural was completed, or that the low amount of lime in the plaster did not have enough binding power, so a layer of limewash was added.