Heat treatment is a key process for producing a predetermined crystalline phase and a glassy phase of the glass-ceramics. After the composition is determined, the structure and properties of the glass-ceramics mainly depend on the heat treatment system (heat treatment temperature and holding time). During the heat treatment, phase separation, crystal nucleus formation, crystal growth, and secondary crystal formation may occur in the glass. For different types of glass-ceramics, the above processes are performed in different ways. The heat treatment process can generally be divided into two stages: the first stage is the fine tuning of the glass structure and the nucleation, and the second stage is the crystal growth.
The nucleation and crystal growth of the glass-ceramics are generally performed at a transition temperature Tg or higher and a melting point of the main crystalline phase. The nucleation treatment is generally performed at a temperature equivalent to 10 to 10 Pa·s for a certain period of time to form a certain number of uniformly distributed nuclei in the mother glass. For some highly eutectic glasses (such as systems with low melt viscosities and high levels of alkali metal oxides), it is also possible to eliminate the nucleation stage and heat them directly to the crystal growth temperature because these glasses are heating up. The nucleation can be completed and a large number of nuclei can be produced. In general, the crystal growth temperature is about 150-200°C higher than the nucleation temperature.