The process of the glassy substance from the molten state to the solid state is gradual, and the changes in its physical and chemical properties are continuous and gradual. This is obviously different from the crystallization process of the melt. A new phase will inevitably appear in the crystallization process, and many properties will suddenly change around the crystallization temperature point. The glassy substance is completed in a wide temperature range from the molten state to the solid state. As the temperature gradually decreases, the viscosity of the glass melt gradually increases, and finally the solid glass is formed, but no new phase is formed in the process. On the contrary, the process of heating the glass into a melt is also gradual.