Tempered glass is made of float glass which is firstly heated to softening point nearby then rapidly cooled down by air, to enable even compression stress on the glass surface. During the Instant cooling procession, external glass is solidified because of the rapid freezing. And as internal glass cools down slowly and would make surface of glass produce compression stress, meanwhile, internal glass produces tensile stress which further makes mechanical strength of glass up to double increment and to be with good thermal stability.