## 3 Results and Discussion
### Glazes Composition of Gilded Tiles  
The thickness of the glazes ranges between 100 and 200 \(\upmu\)m. The glassy matrix is lead-based as reported in the EDS spectrum in Figure 8.  
The morphology, the chemical composition and the distribution of grains dispersed in the glassy matrix, is almost comparable from sample to sample. Few bubbles up to 40 \(\upmu\)m in diameter and few metal-oxide complexes (such as cassiterite, SnO\({}_{2}\)) responsible for the glaze opacity and colour, are dispersed in the matrix. In some cross-sections, tin (Sn) appears more concentrated, while in a few areas, the vitreous phase is more abundant.  
At the body-glaze interface the formation of wollastonite (CaSiO\({}_{3}\)) is clearly present in almost all samples. The interface thickness ranges between 50 up to 100 \(\upmu\)m and its extent is related to the temperature gradient that the glaze experiences during the firing [36; 37].  
To the naked eye, the aspect and the colour of the sampled tiles are comparable. All the micro-fragments are sampled from gilded tiles with the peculiar, corrugated surface as previously shown (Figure 6). The analysed glazes can be grouped into three distinct types based on their structure as revealed by the SEM technique. Morphologically, the first group shows a double layer of the glaze highlighted by the images in SEM backscattered electrons (BSE) of sample F01 (Figure 9). The colour of the glaze is uniform. It is transparent over the entire surface. In the lower area in contact with the terracotta, the presence of bubbles is observed and this phenomenon, probably not intentional, confers to the glaze a greater opacity.  
As highlighted in Figure 9, in the upper part of the glaze of sample, the characteristic needle-shaped calcium silicate crystals are totally absent. Few spare non-homogeneous bubbles are visible, suggesting the intention to obtain a transparent glass.  
Figure 8: SEM-EDS spectrum of an area corresponding to the glaze of the sample F02.  
Figure 9: Visible reflected light and SEM-BSE images (**a,b**); detail of an area on the cross section of sample F01; (**c**) EDS spectrum of one of the crystals highlighted in the white box on the sample F01.  
In the second group of samples, the glaze exhibits calcium silicate crystals, homogeneously distributed throughout the thickness, as shown in Figure 10. The glaze looks darker with respect to the other samples. In fact, the sampled tile shows a darker hue and a more opaque and less brilliant gilding than the other tiles.  
The third group, on the other hand, is characterised by translucent glazes without any inclusions, except for the presence of some calcium silicate crystals present only in a thin layer in contact with the terracotta (Figure 11). The vitreous phase is transparent and characterised by the absence of bubbles.  
SEM coupled with EDS has allowed a preliminary chemical analysis of various parts of the polished glaze sections. The interfaces, in general, as compared to the average glaze composition, are richer in Al, K, Ca and Si. The glaze composition for the three different groups obtained from SEM-EDS analyses is shown in Table 3.  
Figure 11: Visible reflected light and SEM-BSE images of sample F02.  
Figure 10: Visible reflected light images (**a**,**b**) and SEM-BSE image of details of sample F05 fr2 (**c**,**d**), highlighting the presence of calcium silicate crystals.  
Observations carried out at the glaze-body interface by SEM did not allow us to determine with any doubt if the glazing mixture was applied on fired or unfired bodies. Nevertheless, as stated by [PERSON] et al. [36], the interaction is often increased in unfired bodies. Thus, the presence of an important interface zone may indicate the application of the glazing mixture on an unfired body. It was also demonstrated that a slower cooling or modification of the glazing mixture can generate a pronounced interface zone. In order to solve this critical issue, an evaluation on the terracotta composition has been performed.