Spontaneous glass breakage in tempered glass

The reports sound scary: “It just exploded!” “It sounded like a gun going off!” People report a small number of cases each year of tempered glass breaking in their homes. Tempered glass can be found in a number of places. Most commonly, it’s found in oven doors, shower doors and glass tabletops. What cases spontaneous glass breakage in tempered glass and what can you do about it?

Spontaneous glass breakage is uncommon

Spontaneous glass breakage is an uncommon phenomenon, but it does happen. When ordinary glass breaks, it reduces into random shards. These shards are for the most part jagged and sharp. Because the potential for injury is very high, tempered glass is used in certain applications to reduce the risk of injury. Tempered glass is a safety glass. When it breaks, it reduces into tiny cubed pieces, and often stays in one piece or several large broken pieces.

But what causes tempered glass to break spontaneously? The seeds of spontaneous destruction of a sheet of tempered glass are actually sown when the glass is manufactured – even before it has cooled. The tempering process heats glass up to a temperature of about 1,200 degrees, then cools the glass quickly. (Ordinary glass is cooled gradually.) The quick cooling changes the surface tension of the inner and outer surfaces of the glass, and strengthens it. Tempered glass can resist impacts and other insults that would cause ordinary glass to break.

When the glass is heated to its liquid or near-liquid state, nickel alloys in the glass are able to move more freely inside the glass. If they pool together (particularly toward the center) the tempered glass becomes vulnerable to spontaneous glass breakage.

Breakage can occur at any time – or not at all. The glass can break as the result of a fairly innocuous impact or other environmental factor, or it can occur for no apparent reason. Spontaneous glass breakage in tempered glass can be “explosive.” That is, it can create a tremendous, loud noise as it shatters. It can also happen very quietly.

The Consumer Product Safety Commission receives about 100 reports annually regarding spontaneous glass breakage in tempered glass. Most reports involve tempered glass shower doors, but a few involve other products like tabletops, oven glass and automotive side and rear window glass. (Windshields are made from laminated glass – a different kind of safety glass that also resists splintering when broken.)

There is currently no way to identify tempered glass that is vulnerable to spontaneous glass breakage. The good news is that when tempered glass breaks – though it may be scary – it does what it’s supposed to do: prevent injuries. Spontaneous glass breakage may be covered by manufacturers’ warranties or by homeowners insurance policies, and the number of reports of breakage is small compared to the amount of tempered glass that is used in homes around the country.

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Photo Credit: Evelyn Köster, via FreeImages.com