One of the specific characteristics of glass during the manufacturing process is that during fusion of the raw materials, small quantities of nickel sulfide (NiS) crystals may be formed. This impurity has no effect on regular glass, nor can they be detected.

Heat strengthened safety glazing, however, is a different story. During the rapid and violent cooling process which glass intended for space protection undergoes, as part of the heat strengthening process, the nickel sulfide (NiS) impurities retains high-temperature crystal structure. When these crystals are exposed to high temperatures at a future point in time, for whatever reason, they expand and in the event that they expand beyond a critical point, then the stored tension is released in the heat strengthened glass and as a result it breaks, without warning or any indication. Even though this phenomenon is not very common, it is evident that when human life depends on the integrity of the safety glazing pane, it is of the utmost importance to eliminate the possibility of failure of heat strengthened glass to 0%.

This is accomplished by an additional process, the Heat Soak Test, which ensures the performance of security windows. This method of accelerated aging, as it is known, comprises of a cyclical heating-cooling process, with temperatures reaching 290°C. During the test, any glass pane containing nickel sulfide impurities breaks. As a result, all heat strengthened glass panes that retain their integrity throughout the test are in no danger of spontaneously breaking in the future.

In VASGLASS, the Heat Soak Test thermal processing of heat strengthened safety glazing is offered under certification (CE marked) according to the European standard EN 14179 (Heat Soak Test Glass), under the brand-name VAS-HST.

Προσθήκες θειϊκού νικελίου (Nis) που έχουν βρεθεί μέσα σε υαλοπίνακα, σε φωτογραφική μεγέθυνση.