# The Characterization of a Fragment of a Medieval Fresco from Corbii de Piatra Cave Church
## 3 Experimental Results  
Microstructural characterization by optical microscopy is a frequently used method for the characterization of heritage materials and assets [2, 7, 8, 9, 10, 11, 12]. The aim was to identify the morphology, size, and type of aggregates, binders, additives, apparent porosity, cracking, and degradation processes, and the characterization of the interfaces between the constituents. To enable its characterization, the fresco sample was embedded in resin and sanded with abrasive paper until it achieved a roughness of 2000. The optical microscope examination was performed on the entire surface of the sample, which allowed an overall characterization. The analyzed sample shows a continuous red pictorial layer, which in certain areas penetrated through the existing cracks in the mortar before its application (Figure 2a). The matrix has inclusions, cracks, pores, and even voids (Figure 2b). The cracks observed are short in length, with some of them appearing some are detached from the tip of the straw insert, as shown in Figure 2c,d. The quality of the inlay-mortar interface is good, as no cracks have developed along its length that extends from the interface to the interior of the mortar. An area of carbonic residue can be observed following the degradation of the straw inserts used in the preparation of the mortar (Figure 2a,c). The microstructure shows open, yellow-orange, large, rounded phases inside the mortar (Figure 2c,d), while others are small and globular and located near the compaction defects. The observed light separations may be attributed to the presence of precipitates or the use of organic additives, such as sheep tallow or mineral oil, which were commonly utilized during that time period to enhance the quality of the mortar. The pictorial layer in Figure 3 has a thickness between 71.7 and 81.5 \(\upmu\)m (Figure 3a). Above it, a remnant of the carbonation layer formed during the drying of the fresco can be identified (Figure 3b, top right). The carbonation layer on the surface of the pictorial layer has a protective role and determines the transparency of the color and a decrease in its intensity [11]. Under the red pictorial layer, there is a thick area of pink color from the diffusion of the red pigment in the wet mortar (Figure 3b) [7; 11]. The microscopic analysis of the investigated fresco fragment using the cathodoluminescence technique [13; 14; 15] revealed the presence of two distinct areas: an outer, unaltered layer, and a strongly altered internal layer. The outer layer (Figure 4a,b) is unaltered and has orange luminescence, specific to calcite-type carbonate minerals. The thickness of this layer ranges from 0.3 to 0.5 mm, and it is composed of a mixture of sedimentary particles, ranging in size from very fine arenite (63-125 \(\upmu\)m) to silty particles (3.9-63 \(\upmu\)m), that are poorly sorted and have subrounded contours. These particles are trapped in an extremely fine matrix, possibly composed of calciti  
Figure 3: The pictorial layer in section through the fresco (OMPL). Figure 2: Fresco microstructure by polarized light microscopy (OMPL). Beneath the unaltered layer, the mortar shows a faint red luminescence that is characteristic of carbonate minerals such as dolomite (CaMg(CO\({}_{3}\))\({}_{2}\)). This suggests that lime of dolomitic origin (CaMgO) was used in the preparation of the fresco. At this level, the component sedimentary particles are found in low percentages. Carbonate particles are not observed in the internal layer, these being completely dissolved. However, in contrast to the fine sedimentary particles observed in the altered layer, large bioclasts of carbonate, possibly gastropods, are present and are millimeters in size. These bioclasts exhibit partially dissolved tests and a dull orange luminescence (Figure 4b1). Quartz grains appear in low concentrations, showing purple luminescence colors, specific to quartz of volcanic origin (Figure 4b1).