What is Spontaneous Glass Breakage?

Spontaneous glass breakage is a phenomenon with toughened glazing whereby installed glass suddenly breaks, seemingly without cause. This can be a huge issue, especially if this occurs in overhead glazing or balustrades where people could be standing underneath the panes that shatter.

Of course there is always a cause, even if not immediately evident. With glass becoming increasingly popular in new build, the number of instances of spontaneous glass breakage is increasing too. It has become an essential consideration for ALL new glazing installations.

Fortunately, practices exist that can reduce the risk of spontaneous glass breakage occurring in the first place, and solutions exist that reduce the risk of injury it when it does happen.


What causes Spontaneous Glass Breakage?

There are many external influences that could cause glass to break unexpectedly, such as impact with the glass pane, or excessive loads such as wind or snow.

However, the single largest cause of spontaneous glass breakage, is Thermal Stress.

What is Thermal Stress?

As solar energy passes through glazing, some of the energy is absorbed by the glass, heating up the glass pane. The heat absorbed causes the glass to expand, exerting pressure on the edges of the pane. Thermal stress is a natural occurrence in all glazing.




What can cause Thermal Stress Breakage?

All glass has a natural limit of thermal stress that it can withstand before it breaks. In toughened glazing this limit is relatively high. Therefore, in good, undamaged, and correctly installed glazing, the risk of breakage due to thermal stress remains low. However, if the pane were to heat unevenly, or if there were flaws in the glazing system, thermal stress breakage could occur prematurely.

Breakage due to Temperature Differences in the glass pane

Thermal Stress breakage can occur as a result of temperature differences across a glass pane, as two areas expand at different rates.

Solar energy warms all reachable areas of a pane of glass, which rises the temperature and makes it expand. The edges of a pane, perhaps hidden behind a frame, remain cool and become under stress to expand. If the stress imposed by the expansion exceeds the natural breaking strength of the glass, thermal stress fracture of the glass occurs. Partial shadows across a glass pane can also have the same effect, as the light and dark areas of a pane heat and expand at different rates.


The greater the difference in temperature, the greater the risk of breakage.

Breakage due to Flaws in the glazing system

Flaws in the production or installation of a glazing system can create weak points that are less resilient against thermal stresses. Examples include:

  • Poor quality glass or installation – e.g. edge chips, surface scratches
  • Framing defects – e.g. too tightly installed
  • Microscopic internal defects within the glass – e.g. particles of refractory brick, or undissolved silica
  • NiS Inclusion – see below.

Any of these defects could impede the natural movement of the glass as it expands, resulting in glass breakage originating at the flaw.


Breakage due to Nickel Sulphide inclusion

Nickel Sulphide (NiS) is a tiny impurity that can remain in glass following the tempering process. Such inclusions are relatively rare, at 1 per 300m2 of glass.

An NiS inclusion can be a problem as it will continue to expand over time, post installation, trying to return to its natural (larger) state. This puts increasing pressure on the surrounding glass, until it breaches the tolerable limit and breakage occurs.


The speed of expansion can be accelerated by heat, but the point at which breakage will occur is unpredictable. It could be within weeks or years of manufacture, if at all. However when tempered glass spontaneously breaks as a result of NiS inclusion, it’s typically in the first 2 to 7 years after installation. The probability of breakage decreases with time.1

NiS Inclusions cannot be detected post manufacture, but additional steps can be taken during the manufacturing process to expose such flaws in the factory (effectively forced breakage), so that such panes are never installed. (More information below.)

Nickel Sulphide inclusion remains the most common cause of thermal stress breakage.


Identifying thermal stress breakage

The breakage pattern can be used to identify the type of breakage that has occurred. For example, in thermal stress breakage the breakage typically runs from, and perpendicular to, the edge of the glass. However, breakage caused by an inclusion usually follows a butterfly break pattern, although the exact type of inclusion (NiS or otherwise) cannot be identified without sophisticated investigative equipment.


Our glazing surveyors will be able to assist with any investigation into glass breakage, and help to identify breakage type, and suggest potential causes of the breakage.


Reducing the likelihood of breakage

Use Heat Soaked Glass

The heat soaking of glass is designed to expose panes affected by NiS inclusion at the point of manufacture, forcing them to break before they leave the factory.

The process reduces the risk of breakage due to NiS inclusion to 1 in 7000 sealed units, or 1 in 14, 000 panes of glass.2. It can be specifically requested and is indeed seen as best practice in high traffic areas where glass breakage could lead to considerable injury.


Ensure good quality installation

As identified above any flaws with the glass or installation can create a weak point that can be exposed by thermal stresses, leading to glass breakage. Ensure the glazing is installed correctly, in line with industry standards and manufacturers guidelines, by a reputable glazier.


Window Film and Thermal Stress

The installation of solar and thermal window films can increase (or decrease) the thermal stress levels borne by glazing, but when correctly specified and installed these remains very low and well within tolerable limits.

In this way window film cannot directly cause glass breakage, but could expose existing flaws within the glazing system.


Ensure correct specification

The aim of correctly specifying window film is not only to ensure the film performs as specified, it is also to check that the film is compatible with the glazing system itself.

All good Window Film professionals use sophisticated software to model solar energy absorption and thermal stress levels to ensure that they remain low and well within tolerable limits. The software is extensive, and considers a wide range of factors which include (but are not limited to) frame type, building orientation, existing shading, glass construction, and of course the proposed window film product.

A positive compatibility assessment means that the risk of breakage remains low, not that there is no risk. There is always a risk of thermal stress breakage in every pane of glass, regardless of whether film is installed or not.


Options for Glazing Containment in the event of failure

Buildings with multiple instances of spontaneous glass breakage often have huge expanses of glass, which automatically increases the probability of breakage.1 It can be prohibitively expensive to remove and replace all the remaining glass to reduce risk, so alternatives must be sought.

The installation of a glazing containment system (comprised of a window film and edge retention system) is a fast, non-disruptive, and inexpensive route to reducing risk from further instances of glass breakage. The retrofit system will help to hold glazing in place should further breakage occur, reducing the risk of shattered glazing falling onto anyone below the affected areas.

Once the risk has been identified it is always best to be proactive to reduce the risk of injury from further breakages.


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