Tempered glass is made from float glass. After heated to its softening point, float glass is cooled rapidly and homogeneously so that the glass surface gets compressive stress. In the cooling process, the external part of tempered glass is rapidly cooled and solidified, while the internal part is cooled slowly. So, when the internal part keeps on cooling and shrinking, compressive stress forms in the glass surface and tensile stress inside, so as to improve the strength and the thermal stability of glass.