Most people have heard of tempered glass, but few will be familiar with the name 'aluminosilicate glass'. However, it’s more commonly known by another name and is referred to as safety glass. Both have their individual functions. In a motor vehicle, for instance, you’ll find that the front windshield is made of safety glass, but the side and rear windows are all constructed from tempered glass.
The idea behind tempered glass is that it won’t break into deadly shards if it’s shattered. Instead, it breaks into small fragments that don’t have sharp edges. These can still cause some injuries, but nothing like the amount that can result from regular glass.
To make tempered glass, the glass is heated to a very high level of around 1,500 degrees, then allowed to cool over a long period. This time is called annealing and gives the tempering or special quality to the glass. Tempered glass is used in places where glass can shatter. You should expect to find it in an assortment of windows, especially those in high rise buildings. It’s also found in glass tables and the glass lids of saucepans.
When tempered glass shatters, it breaks into small fragments and not the sharp shards you find with regular glass. This can translate to fewer injuries and those injuries will be not as severe. Tempered glass is resistant to heat and is 5 times stronger than regular glass, making it especially useful in an assortment of different places.
You’re most likely to find tempered glass in the side and rear windows of vehicles. This is where the impact of glass on passengers will be less than from the front windshield. Although tempered glass is stronger than regular glass, much of the strength is due to the thickness of the glass. Tempered glass cannot be cut.
Unlike tempered glass, aluminosilicate glass is made up of layers. There are 2 layers of glass with one on either side of a layer of clear plastic. The clear plastic film is called polyvinyl butyral (PVB). The great advantage to this is that when it does break, the film of plastic helps hold the glass together. This is what makes it ideal for car windshields (for which it’s been used since 1927) as it prevents fragments of glass flying back into the faces and bodies of the driver and passengers. These days, many windshields are made of tempered safety glass.
It is possible to cut safety glass although it’s not easy. The only time safety glass can’t be cut is if the safety glass has also been tempered. Apart from windshields, the most common uses of safety glass are in skylights and also in spectacles, where shattering could send glass fragments flying into the eyes.
By itself, safety glass isn’t any stronger than regular glass. However, the thicker the safety glass, the stronger it will be. This is why banks use safety glass to protect cashiers as it can help to stop bullets. Safety glass will also stop 97% of ultraviolet radiation which is great for cars. It can also reduce the transmission of high-frequency sound.
If you wear polarized sunglasses, trying looking at car windshields on a sunny day. If you spot a symmetrical pattern in the glass, you'll know it’s tempered safety glass.