FAQ – What is Heat Soaked Glass and when is it recommended

Heat soaking is a process that exposes NiS inclusions in toughened glass, reducing the risk of spontaneous glass breakages.

What is heat soaked glass?

Heat soaked glass is glass which has gone through a heat soak testing process. During the testing the toughened glass is heated to a very high temperature (290°C), then left for several hours. This allows any pane of glass which would have likely fractured in service, to fracture during this process.

Due to the nature of glass, spontaneous glass breakages can never be completely eliminated but this process greatly reduces the risk.

When is heat soaked glass recommended?

Heat soaked glass is recommended in areas where a breakage could cause serious injury. Or for pieces of glass which would be very difficult to replace at a later stage, such as large panes of glass above the ground floor level or where a crane would be required for installation.

Specifying heat soaked glass minimises the risk of spontaneous glass breakages and therefore having to replace the glass in the future, the future cost to do so and the disruption caused.

What causes toughened glass to break?

Tempered glass can break for many reasons, some are associated with an event e.g. accidental impact, or vandalism. Other times there is be no obvious reason. These are often referred to as spontaneous.

There will always be a cause when glass breaks. The cause of the break can very often be identified by the break pattern of the glass.

Causes include:

Impact damage

A sudden hit which exceeds the elastic strength of the glass. With this cause, the damage is visible instantly.

Face damage

The face of the glass has received an impact which did not exceed the elastic strength of the glass.

This type of damage will not always cause a break to happen immediately. The break can occur at a much later date. Often after heat stress caused by weather (hot days & cold nights), where the glass expands and contracts.

Installation edge damage

The edge/side of the glass has been damaged on or before installation. If the glass comes into contact with the fittings and fixings, with no allowance for the glass to expand and contract, it will likely result in an edge break.


When the glass is spec’d the wind / structural load, clearances, design flaws, excessive heat stress etc, all need to be taken into account. If the glass has not been correctly spec’d for the design/location this can cause breakages.


Once the glass has been cut to size, it will go through a toughening process. If the glass has not been toughened correctly, lower stress levels can cause the glass to break. Compared to what a correctly toughened glass unit could handle.

Nickle Sulphide Inclusions

Nickel Sulphide Inclusion (NiS) occurs during the process of manufacturing toughened glass. As a result of nickel contaminants present in the glass melt, reacting with sulphur.
Visually you are not able to see these tiny particles, but they may still be contained within the glass. The particles remain benign until they are aggravated.

If NiS is present in the glass, there is a risk that the glass will spontaneously break, if the particle changes size or shape. Find our FAQ on NiS here, to learn more.

It is this spontaneous breakage that heat soaking can help reduce.

How does heat soaking reduce spontaneous breakage for Nickle Sulphide Inclusions?

The heat soak testing process is designed to return the nickle sulphide to it’s natural larger state. Which as a result causes the glass to break, if present during the testing process, rather than months, or even years later once the glass has been installed.

When toughened glass is manufactured it goes through a process of extreme heating and rapid cooling. During this process NiS inclusions change size from a low-temperature structure to a high-temperature crystalline structure. When the glass is cooled quickly, the NiS particle is unable to change completely back to its original low-temperature form. Over a period, the particle slowly converts back to its original form but with an increase in volume of about 2-4%. This increase in size can cause a seemly spontaneous breakage.

As annealed non-toughened glass doesn’t go through the rapid heating/cooling process, spontaneous breakages of this nature does not occur.

How much does heat soaking reduce the chances of glass breaking from Nickle Sulphide Inclusions?

The heat soaking process doesn’t eliminate the risk of spontaneous breakage from NiS completely, however it is greatly reduced. The Glass and Glazing Federation estimate the risk is reduced from 1m2 in every 10,000m2 of glass, to 1m2 in every 1 million m2 of glass.

Does heat soaking have an implications on the cost / lead time of the glass?

Heat soaking does incur an additional cost, however it is a modest cost for the peace of mind the process can offer.

Due to the additional process the glass goes through, the lead time is slightly increased. Our sales team will be able to advise you on what this would be.

Is spontaneous breakages of heat soaked glass covered by warranty? 

Due to the inability to eliminate spontaneous breakages via NiS completely, however very rare, our glass suppliers are unable to offer a warranty against breakages from this phenomena or any other failure mechanism, therefore Aspect Windows are unable to pass on any warranty to our customers.