Last update: 14.08.2019
Safety & security

Heat-soaked tempered glass

Each basic glass contains extremely low quantities of nickel sulphide (NiS) crystals, which are inevitably introduced into the glass via the raw materials. In normal annealed float or patterned glass, these crystals do not have any relevance.

Nickel sulphide


The extremely rapid cooling-off period during the tempering process “freezes” the NiS particles in a high temperature crystal modification. When heat is later applied, for example, through solar energy absorption, this crystal structure may change because the volume of the crystals change, i.e. increases, and this may cause the glass to suddenly burst apart as soon as the particles exceed a critical size.

Any safety-relevant glazing and panes, such as façade glass, which are exposed to high temperature fluctuations, should therefore be subjected to an additional heat soak test.


Heat-soak oven

This test is carried out in accordance with EN 14179 and should be documented. This test forces the NiS crystals that may be present to react quickly. Those panes of glass that do have these invisible crystals are intentionally destroyed during this test.

For this purpose, the tempered glass panes are heated to a defined 260 °C ± 10 % for at least 2 hours.

Tempering glass

This process is typically monitored by internal and external controllers and should be permanently documented for each pane supplied (mandatory in Germany). Moreover, these panes should be visibly marked with the tempered-heat soaked glass label.


Identification of tempered heat-soaked glass