It was invented in 1874 by Francois Bastie (1830–1901) of Paris, France by quenching almost molten glass in a heated bath of oil. A process he later patented in England. Tempered glass is also called Bastie glass after its creator. It is indistinguishable from annealed or plate glass in appearance.
Today rather than oil baths like Bastie used, tempered glass is made by heating glass past about 1100 °F and then quickly cooling the surface of glass down with forced air while leaving the center of the glass free to cool slowly.
The result is essentially a multi-layer piece of glass with tightly compressed outer layers and an inner layer under tension. This creates a much stronger piece of glass that is upwards of four times as strong as annealed glass.
Though it has greatly increased strength on the surface of the glass the edges of the glass are not nearly as strong. These edges are its Achilles heel and cause breaking very easily, so do be careful when handling it.
Tempered glass is also referred to as safety glass due to its strength and ability, when broken, to break into thousands of dime-sized pieces that, though they may scratch you, are far less dangerous than the irregular shards that come from annealed glass.
This makes tempered glass the go-to choice, and in some cases required choice, in areas that demand extra safety such as those below.
- Car windows (not windshields)
- Shower doors
- Doors & sidelights
- Table tops
- Glass stair railings
Can Tempered Glass Be Cut?
Unlike annealed glass which can be cut rather easily by scoring it, once a piece of glass has been tempered there is no way to cut it without the whole piece turning into a big pile of broken glass. The tempering process creates a glass that is under stress and when breaks that starts a chain reaction that undoes the entire piece of glass.
If you must cut through a pane of tempered glass, there is only one way to do it without destroying the glass. You’ll need to heat it to nearly 1,100 °F and then slowly cool it. This process is called annealing, and it will undo the tempering process, weakening the glass so that it can once again be cut by scoring.
The tempering process is made after cutting glass to the size needed and cutting any holes.