Heat strengthened glass is produced by a similar process as tempered glass but with a different cooling pattern. The glass is thermally brought to the softening point and then cooled in steps and not rapidly as in case of tempered glass. This gives the glass higher mechanical and thermal strength than annealed glass but less than fully tempered glass. However, it is less vulnerable to spontaneous breakage as in case of Tempered glass. However it breaks almost like an annealed glass and can not be used as a safety glass.