What Is Heat Soaking and Where Should I Specify It?

Heat soaking is a testing process carried out in the manufacturing stage of glass, used to reduce the risk of spontaneous breakage in architectural glazing. It is often advised that heat soaked glass be specified on high level, inaccessible or difficult to fit installations due to these installations of architectural glazing being harder to access and replace if required in the future. Minimising the risk of having to replace glass in the future reduces future cost and disruption to a client or building. It is always recommended that a glass replacement plan is put in place for extensive glass installations.

a nickel sulphide breakage in structural glass

What is a Nickel Sulphide Inclusion?

A nickel sulphide inclusion is a naturally occurring phenomenon within toughened glass in which small particles of nickel sulphide may be present in glass after the manufacturing and toughening process.

Although the inclusions are not visible, these particles can cause spontaneous glass breakages if the glass is aggravated by sudden or extreme temperature changes.

You can read more about the science behind these inclusions in our technical article: Nickel Sulphide Inclusions.

What is a Nickel Sulphide Inclusion?

What is Heat Soaking?

Heat soaking is a destructive test treatment that takes place after any toughening and laminating process’. The finished glass is placed into a large heat soaking oven and heated to extreme temperatures.

After being heated to around 290 +- 10 degrees Celsius and held at that temperature for approximately 2 hours, it is removed from the oven facility. During this thermal treatment, any glass containing nickel sulphide is forced to shatter, leaving behind only the panes with a reduced risk.

The risk of spontaneous breakage for heat soaked glass drops dramatically from 1m2 in every 10,000m2 to just 1m2 in every 1 million m2.


structural glass heat soaked glass
slim sliding sash

Where Should I Specify Heat Soaked Glass?

Heat Soaked Glass can be specified for any installation of architectural glazing. However, it has more value in glass installations that may be difficult to access or replace once construction is completed.

Some areas where Heat Soaked Glass can add value are:

  • large glass installations above the ground floor level
  • where a crane is required for the glass installation
  • complicated or intricate glass designs
  • for glass units glass that are supporting other elements (to ensure the installation wouldn’t have to be taken apart if a breakage occurred)
  • glass installations using oversized or very specialised glass units

Another note for specification would be that heat soaking is only really necessary for glass panes 6mm thick and over.

The general consensus within the glass industry is that 4mm toughened glass isn’t thick enough to support a Nickel Sulphide inclusion and that it would automatically break during the toughening process. So heat soaking glass of that thickness would not provide any further reduction in the likelihood of a NSI.


What Next?

Our experienced team of glass experts are on hand to answer any of your questions, including questions about whether heat soaked glass would be suitable for your project. Just get in touch with us by calling 01494 722 880 or visit our contact page for other ways to get in touch. We look forward to speaking with you soon.