Tempered glass is up to 10 times stronger than standard annealed glass. What tempering does is put stress into the glass. When glass is made, the batch or raw materials are melted at around 2300 degrees, where it is can be poured like honey. The glass is then brought down in temperature to its annealing point, around 1000 degrees. This is where the glass stops moving. The glass is then slowly cooled to room temperature, allowing the glass to cool evenly. If it doesn’t cool evenly the glass will have stress and break along the different temperatures.

So tempering is controlling the stress to make it stronger. First an annealed piece of glass is cut to size. All holes, cutouts and polished edges must be done before tempering. The glass is placed in a long shallow oven, heated to over the annealing point. The glass is then quickly rolled into a chamber where a large blast of air, from top and bottom, quickly cools the surfaces of the glass. Now the surface shrinks just a small fraction, while the core of the glass is still hot.

Now there is stress between the surface and the core, but because the surfaces shrank first, it squeezes and creates a balance of force between the surface and core. If the glass does break, the glass will shatter into a million tiny pieces, spreading out the weight so a large piece of sharp glass cannot fall onto anyone or damage walls and floors.