Besides, as known the sintering of glass may be impeded by crystallization. However by enhancing the heating rate, it is still possible to sinter the glass to full density before the onset of crystallization [28]. While crystallization depends on nucleation and growth, glass sintering is merely related to viscous flows. Because for nucleation not only rheological properties but also the amount of undercooling is critical, a faster heating rate gives an advantage to sintering process [28,29].