Heat Soak Test - HST

With tempered glass, spontaneous breakage can occur occasionally resulting from inclusion of materials
such as nickel sulfide. These inclusions are inherent to in the production process. When the window breaks,
the tempered glass splinters into numerous pieces, which can fall out of the opening either wedged together
or as individual pieces. Although the risk of spontaneous breakage is usually minimal, planners should take
alternatives into consideration, where applicable. We recommend heat-strengthened glass, for which such
cases of spontaneous breakage are unknown. In areas, for which tempered glass is mandatory, we can offer
additional heat soak tests on the tempered glass. When nickel sulfide inclusions are heated, their volume can
increase. If this happens, small transverse forces can then lead to spontaneousbreakage. Heat soaking accelerates
this heat up and expansion process.

The heat-soak test consists in heating the tempered glass at 290°C for 8 hours. 

If inclusions are present, breakage is far more apt to occur during heat soaking than later at the installation site.

Nevertheless, heat soaking is not an absolute guarantee that tempered glass cannot break after it is installed.

However, this process does significantly decrease the probability of spontaneous breakage resulting from 

nickel sulfide inclusions