## 3 Results
### X-ray Diffraction  
The XRD patterns of the zinc silicate glass and glass-ceramic heat treated at different temperature ranging from 700 to 950 \({}^{\circ}\)C for 4 h are discretely presented in Figure 2. At room temperature and  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sintering Temperature (\({}^{\circ}\)C)** & **27** & **700** & **750** & **800** & **850** & **900** & **950** \\ \hline Archimedes density (g/cm\({}^{3}\)) & 2.9118 & 3.2186 & 3.2413 & 3.2581 & 3.3001 & 3.3225 & 3.4138 \\ True density (g/cm\({}^{3}\)) & 2.3745 & 2.5482 & 3.4036 & 3.4164 & 3.4240 & 3.4329 & 3.4472 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of the values measured from the Archimedes density and true density.  
Figure 1: Density accompanied by the linear shrinkage of the zinc silicate glass and glass-ceramics that were heat treated at various sintering temperatures.  
700 \({}^{\circ}\)C, the XRD patterns showed a broad amorphous characteristic, indicating that the samples are still glass in nature. However, as the sintering temperature increased to 750 \({}^{\circ}\)C, a sharp peak started to appear and the \(\beta\) phase of the zinc silicate (\(\beta\)-Zn\({}_{2}\)SiO\({}_{4}\)) with a JCPDS no. of 14-0653 was detected to be formed. \(\beta\)-Zn\({}_{2}\)SiO\({}_{4}\) is known to be the thermodynamically metastable phases of zinc silicate (willemite) that has an orthorhombic crystal system (from a recent study) [11; 12]. In Figure 2, thirteen major diffraction peaks of \(\beta\)-Zn\({}_{2}\)SiO\({}_{4}\) at 20 = 22.04\({}^{\circ}\), 25.23\({}^{\circ}\), 27.10\({}^{\circ}\), 31.20\({}^{\circ}\), 36.23\({}^{\circ}\), 37.84\({}^{\circ}\), 42.44\({}^{\circ}\), 44.90\({}^{\circ}\), 47.84\({}^{\circ}\), 56.19\({}^{\circ}\), 61.22\({}^{\circ}\), and 65.38\({}^{\circ}\), corresponding to planes (210), (020), (12-1), (112), (221), (022), (231), (420), (40-2), (322), (511), (332), and (33-3), respectively, were detected and indexed as shown in Figure 2. This \(\beta\)-Zn\({}_{2}\)SiO\({}_{4}\) continued to exist until the sintering temperature reached 850 \({}^{\circ}\)C.  
Meanwhile, when the sintering temperature of the sample increased to 900 \({}^{\circ}\)C, the \(\beta\)-Zn\({}_{2}\)SiO\({}_{4}\) phase transformed into \(\alpha\)-Zn\({}_{2}\)SiO\({}_{4}\) phases (JCPDS no. 37-1485). The \(\alpha\)-Zn\({}_{2}\)SiO\({}_{4}\) phase is a neosilicate, or previously known as an orthosilicate, consisting of isolated SiO\({}_{4}\)\({}^{+}\) tetrahedrons and ZnO\({}_{4}\)\({}^{-}\)[13; 14]. It is the most common practical crystalline phase of zinc silicate. At 900 \({}^{\circ}\)C, there were still a few of the \(\beta\)-Zn\({}_{2}\)SiO\({}_{4}\) phases being traced. However, as the temperature increased to 950 \({}^{\circ}\)C, only a single phase of \(\alpha\)-Zn\({}_{2}\)SiO\({}_{4}\) was seen. There were twelve major diffraction peaks found, as indicated in Figure 1, where the diffraction peaks positioned at 20 = 22.12\({}^{\circ}\), 25.59\({}^{\circ}\), 31.53\({}^{\circ}\), 34.07\({}^{\circ}\), 38.88\({}^{\circ}\), 45.10\({}^{\circ}\), 46.84\({}^{\circ}\), 49.01\({}^{\circ}\), 57.85\({}^{\circ}\), 59.77\({}^{\circ}\), 65.73\({}^{\circ}\), 68.77\({}^{\circ}\), and 70.45\({}^{\circ}\) corresponded to planes (330), (220), (113), (410), (223), (060), (431), (333), (8-70), (006), (8-73), (636), and (416), respectively. Therefore, it can be concluded that, as the sintering temperature increases, the glass gradually changes into glass-ceramics, where the crystallization of the \(\beta\)-Zn\({}_{2}\)SiO\({}_{4}\) occurred first at the lower temperature, and then gradually was converted into the thermodynamically stable state of \(\alpha\)-Zn\({}_{2}\)SiO\({}_{4}\).  
Figure 2: X-ray diffraction patterns of the zinc silicate glass and glass-ceramics that were heat treated at various sintering temperatures.