What’s the difference between toughened and heat soaked glass?

When it comes to glazing, safety glass should always be used to ensure no injuries are sustained in the event of a breakage.

Toughened glass is an excellent type of safety glass as when broken the glass shatters into small harmless pieces, posing less of a risk than large, sharp shards of glass. This used in conjunction with an interlayer greatly reduces the risk of harm.

The process of toughening glass involves heating the glass and then cooling it rapidly. The sudden change in temperature causes stresses within the glass which causes the glass to shatter into tiny pieces when broken.

Heat soaking is an extra step from toughening glass. This eliminates any panes that would be susceptible to nickel sulfide breakages, a naturally occurring phenomenon in glass.

It can be difficult to know which types of projects need toughened and which should use toughened and heat soaked glass. This article will help you decide which type is right for your project.

testing toughened and laminated glass

Toughened glass

The process of creating toughened glass involves heating the glass to temperatures of around 650°C and the rapidly cooling the pane until it is cool to the touch.

Sudden temperature changes can cause glass to shatter due to the differences in temperatures across the surface creating stresses. This is also known as thermal shock, and the toughening process causes any weak panes to shatter which significantly reduces the risk of thermal shock or fracturing breakage.

By cooling the glass at speed, the outer areas become cool to the touch whilst the middle remains at a high temperature. This difference in temperatures across the glass causes stresses which results in a tougher pane that is significantly more difficult to break than float glass.

This results in glass that is up to five times stronger than standard float glass. It also means that due to the tension in the glass, in the event of breakage it will shatter into small, rounded pieces which are less likely to injure someone.

Sieger uses toughened glass as standard for all glazing applications as our customers safety is of the upmost importance to us. Depending on the glazing solution, we also use toughened laminated glass for an added level of safety. This is always used on the external pane of a floorlight, or for internal pane of a rooflight, for example.

Laminating toughed glass increases the safety by holding the shattered pieces in place when broken.

shattered glass pane that is toughened and laminated
invisio thermally broken frameless rooflight with toughened glass and laminated inner pane

Heat Soaked glass

Toughened glass panes have a very low risk of spontaneous breakage. This is due to the nickel contaminants in the glass reacting with sulphur that comes from the heating part of the toughening process.

When the nickel contaminant reacts with the sulphur it can create nickel sulphide inclusions which grow over time and then seemingly at random will grow to the point where the glass breaks.

Although extremely rare, all toughened glass could suffer from a nickel sulphide inclusion. This can happen days, weeks and even years after the toughening process.

These breakages can be avoided by heat soaking the glass. This extra step in the glass manufacturing process involves putting the toughened glass panes involves into an oven where it is heated to around 290°C for a prolonged amount of time.

This controlled heating cycle accelerates and nickel sulphide expansion that would occur over time resulting in any weak panes that would have been at risk of a nickel sulphide shattering.

Although heat soaking can identify more than 95% of problem panes, this process does not eliminate 100% of nickel sulphide inclusions.

It is important to remember that nickel sulphide breakages are extremely rare, this extra step is worth the cost in certain glazing applications.

There is no need to specify toughened glass at Sieger as all our glazing is toughened as standard. In certain installations, we may recommend specifying heat soaked glass.

Glass balustrades, especially frameless ones, should undergo the heat soaking process as the possibility of missing a handrail, whether it’s internally or externally poses a severe safety risk for people.

Heat soaked glass may be necessary for installations located in hard to reach places, as replacing the glass would be difficult and possibly costly.

For large glass installations, or glass units that support another element of the building this may be a good idea as similar to above, the replacement of a glass pane could cause issues and incur a large cost if the structure becomes damaged.

Glazing with unique or intricate glass designs such as kiln-formed glass could be heat soaked in order to avoid having to replicate the intricate design. This is especially important if the glazing was manufactured years ago as it may be difficult to make an exact replication.


If you have any other questions about the glass used at Sieger or any of our glazing solutions don’t hesitate together in touch with our team.

Call 01494 722 882 or email [email protected]

flat rooflight with toughened and laminated inner pane for safety

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