The tempering process of glass explained

Tempered glass was invented in 1874 – and since then, it's become a critical material in a huge range of applications. But how exactly does glass become tempered?

Like so many aspects of glass manufacturing, the answer centres on heat. The glass is heated and then cooled. This sudden shift in temperature gives tempered glass its famous strength and breakage pattern.

It's this breakage pattern that makes it suitable for critical locations where extra safety precautions are a must. It can then be laminated and, once laminated, bear loads.

How is glass tempered?

The glass tempering process consists of three stages. First, a sheet of standard annealed glass is cut to size. This is important as glass can't be cut once it's tempered.

Next, the glass is passed through a furnace on a roller table. It's heated at a high temperature close to its melting point – around 650°C (1,200°F).

It's then taken out of the furnace and rapidly cooled or "quenched" with high-pressure jets of cold air from above and below. This cools the outer surfaces and causes them to quickly harden.

This process gives tempered glass its strength and its distinctive breakage pattern. This is because the heating and quenching creates stress inside the glass.

The inner core of the glass cools more slowly than the surfaces. As it cools, it contracts. This pulls in the outer layers, which have already solidified. The outer layers are now in a state of high compression, and the inner core in a state of tension.