Chicago Window Expert

Cracks in tempered glass

Tempered Glass Breakage
When tempered glass breaks, the energy retained in the glass due to internal tension/compression releases explosively and produces a breakage pattern sometimes called “cubes’.

Seeing a cubist breakage pattern does not tell you why the glass broke, it only tells you that the glass was tempered. Generally, there are three reasons tempered glass will break: impact, edge damage or nickel sulfide inclusions.

Normally, when tempered glass breaks, it falls down into a pile of little cubes. Only the most patient window consultants with the most generous client would ever consider piecing the cubes together to determine the cause of breakage. However, occasionally the pieces of broken tempered glass will stay in the opening, locked to each other like blocks in a masonry arch. And just like in a masonry arch, if you remove the keystone, the arch – or glass in this case – comes tumbling down.

If you look closely, you can see the point of impact on this tempered glass. A laminated interlayer holds the cubes in place.

Spontaneous Breakage in Tempered Glass
Glass, and especially tempered glass, sometimes breaks all by itself. This can be quite disconcerting when, as has happened in a public place which will go unnamed here in our great city, large, thick panes of tempered glass basically blew up fairly frequently. The unusual cause in this rare instance: the glazing contractor attempted to grind the edges of the glass after it was tempered, creating a series of time bombs. It is a very bad idea to modify glass after it is tempered!

A more well-known, but also quite rare cause of spontaneous breakage is nickel sulfide inclusions. If you read the previous post, you will already be down with the fact that glass is made from melted powders. A nickel sulfide inclusion is a tiny rock of unmelted material that remains in the glass. Below see an artists graphic representation of a nickel sulfide inclusion.

Artists representation of nickel sulfide inclusion.

Stress Cracks
A “stress crack” will usually only happen in annealed or heat strengthened glass. Stress cracks emanate from the edge of the glass and meander about apparently without purpose. But there is a purpose: to relieve stress. However, the term “stress crack” can be misleading.

If annealed glass is subjected to thermal fluctuations beyond its capabilities, it will break in a way that will relieve the stresses induced by thermal changes. This type of failure is a design issue. Heat strengthened glass should have been specified for the application.

However, there can be a near-identical breakage pattern which emanates from damage in the glass edge that fails as stresses, such as thermal, are applied.

Classic meandering pattern of heat-related stress crack, accompanied by evidence of edge damage. In this case, the building had both: edge damage and underspecified glass. Reflective blinds and a South exposure combined to create a high frequency of this type of breakage.

To tell the difference between a true stress crack and a crack due to edge damage, look at the edge of the glass for a chip, which we window linguists call an “oyster”. You might have to look hard because the oyster could be buried in the primary seal on the #2 or #3 surface.

Another clue would be the distribution of glass breakage in the building. It would be normal to find stress-like cracks on elevations with greater temperature swings. But does the breakage also coincide with the use of reflective interior blinds, especially in a partially opened position? That would be indicative of a true stress crack, rather than a crack induced by edge damage.

Great additional resource:
Viracon Technical Information: Thermal Stress Breakage

Mark Meshulam observing that glass is not broken