Question: How is tempered glass made? What is the process by which it is produced for use in wood stoves or fire place doors? Answer: Tempered glass is glass which has been heated to a certain temperature and then cooled. This changes it's properties. Glass used in woodstoves and fireplaces is rarely tempered...most is a ceramic glass which is good to 1000 degrees plus...tempered starts going at about 500 degrees. The exception are some fireplace glass doors, which use tempered if they have a lot of cooling air flowing around them.