What is Tempered Glass?

Tempered glass is a type of safety glass that is used when there is a fear of breakage if standard glass had been used. Tempered glass is four to five times stronger than standard glass. When it breaks, it breaks into thousands of tiny (about 1/8 inch)pieces versus large sharp pointed pieces. This is why it is used in applications like shower doors, skylights, windows, and refrigerator shelves. However, you do still need to wear gloves when handling broken tempered glass since it can be a little sharp.

This type of glass is manufactured through a process of extreme heating and rapid cooling, making it harder than normal glass. Recycled glass artists can use this type of glass in casting (after it is broken and therefore, no longer tempered). I often obtain large sheets of donated tempered glass and then need to break it up in order to stack it into a plaster/silica mold.

First, I place a sheet of plastic on the floor that is a little larger than the glass sheet. I then lay the glass sheet flat on top of it. I next take a pointed metal hole punch (see photo above) or a screwdriver with a pointed tip and hammer in the same exact spot. It can take several minutes of hammering and you must keep the point of your tool in the exact same position. Eventually you will see a small chip in the glass and you continue to hammer the metal tip into this glass chip. You will soon hear this loud pop sound and crackling noise as thousands of little cracks expand throughout the glass piece. You can also use a drill with a bit for drilling a hole.

Taking ordinary glass into a tempered state involves heating the glass in a special furnace to approximately 1260degrees Fahrenheit then setting a permanent tension between the glass “core” and outer surfaces by rapidly cooling the glass in a high pressure quench. So when fully tempered glass is broken, the release of tension between the surfaces initiates a cascade of much smaller glass fragments than ordinary annealed glass. It is very difficult if not impossible to create large sheets of tempered glass in non-factory type glass studios.