## 4 Results  
### X-ray Diffraction  
The XRD pattern of the deposited ceramic layers is shown in Figure 1. In accordance with the evolution of XRD patterns for P1, after the first firing (2_P1_C), of the opaque layer, all diffraction peaks were indexed as KAlSi2O6 (JCPDS no. 01-085-1626) and ZrO2 (JCPDS no. 01-078-0047) at percentages of 58% and 42%, respectively (Figure 1a). The second firing determined the complete formation of a KAlSi2O6 phase with a tetragonal symmetry, and no formation of impurity phases was observed in the 3_P1_C sample. The third and fourth firings (4_P1_C and 5_P1_C) only decreased the crystalline quality of the KAlSi2O6 phase (Figure 1a). \begin{table}
\begin{tabular}{c c c c} \hline
**Component** & **Dentin, Enamel (wt \%)** & **Glaze (wt \%)** & **Opaquer (wt \%)** \\ \hline SiO2 & 59.5–65.5 & 61.0–68.0 & 35.0–55.0 \\ Al2O3 & 13.0–18.0 & 5.0–8.0 & 8.0–14.0 \\ K2O & 10.0–14.0 & 5.0–8.0 & 7.5–13.0 \\ Na2O & 4.0–8.0 & 5.0–8.0 & 3.5–5.5 \\ ZrO2 & - & - & 14.0–39.0 \\ ZnO & - & 2.0–4.0 & - \\ Other oxides & 0.0–4.0 & 3.5–17.0 & 0.0–3.0 \\ Pigments & 0.0–2.0 & 0.0–1.0 & 4.0–20.0 \\ Glycols & - & 30.0–40.0 & 26.0 \\ \hline \end{tabular}
\end{table} 
Table 4: Chemical composition of IPS InLine (Ivoclar, Schaan, Liechtenstein) ceramic layers (as given by the manufacturer). P2 showed a different behavior during the first firing, following the depositing of the opaqueer, additionally highlighting the Si crystalline phase. The KAlSi2O6:ZrO2:Si crystalline phase distribution is 55%:13%:32% (Figure 1b). After the second firing, KAlSi2O6 became the unique crystalline phase. Subsequent deposition decreased the crystalline structure. In the case of the 5_P2_C sample, when subjected to the fourth firing, the ordered structure of the ceramic layer was destroyed, becoming amorphous. Thus, the Si content has a certain impact on the ceramic's behavior. As shown in Figures 2 and 3, the structure of the Co-Cr-W dental alloy is strongly affected by the repeated firings of the ceramic layers, especially by the first firing of the opaqueer, which takes place at 930 \({}^{\circ}\)C for P1, and at 940 \({}^{\circ}\)C for P2 (samples 2_P1_M and 2_P2_M, respectively). The modification of the crystalline quality is clearly reflected in the intensity of the diffraction peaks. The intact 3-D-printed samples of 1_Co-Cr-W_alloy display a low, crystalline, ordered structure based on the mixture of two polymorphs of cobalt metal with hexagonal symmetry (JCPDS no. 00-001-1278, space group: P63/mmc) and cubic symmetry (JCPDS no. 00-015-0806, space group: Fm3m). The structure was transformed into a high crystalline cubic structure after the first firing, regardless of the ceramic used for veneering. Figure 1: (**a**) X-ray diffraction (XRD) patterns of P1_C samples at room temperature; (**b**) XRD patterns of P2_C samples at room temperature. Figure 2: (**a**) XRD patterns of the P1_M samples at room temperature; (**b**) enlargement of the indexed XRD peaks of the Cr2O3 crystalline phase. The subsequent firings caused a different evolution of the M samples, depending on the ceramic used and variations in the firing temperature. In the case of P1, the structural changes of the dental alloy were moderate, almost as if the high crystalline cubic structure is a stabilizing influence. Conversely, in the case of P2, the crystalline structure of the dental alloy continued to change abruptly (Figure 4a-c). All P1_M and P2_M samples also revealed the presence of the Si crystalline phase (cubic, JCPDS no. 01-080-0018, space group: F-43m). Figure 4: The evolution of the diffraction peak intensity in the function of the number of firings: (**a**) (111) crystal plane of Co; (**b**) (200) crystal plane of Co, and (**c**) (110) crystal plane of Cr\({}_{2}\)O\({}_{3}\). Figure 3: (**a**) XRD patterns of the P2_M samples at room temperature; (**b**) enlargement of the indexed XRD peaks for the Cr\({}_{2}\)O\({}_{3}\) crystalline phase. Furthermore, a new crystalline phase of Cr\({}_{2}\)O\({}_{3}\) with rhombohedral symmetry (JCPDS no.