Heak soak test
After tempering the glass can break spontanuously. This break is due to very small impurities (very often nickel sulfite) inside the tempered glass. Depending where there are (on the surface or deeply inside) the glass can break or not a long time after tempering.
During the tempering (between 610° and 660°) the nickel sulfite NiS takes a not steady crystallin structure at normal glass using temperature. It is the changement to a steady crystalline structure (can be a long time after tempering) for the NiS which creates tension inside the tempered glass and the break.
This break can be very damaging (the glass has to be replace) and dangerous for people.
To prevent this, a destructive test has be created: the Heat soak test.
The idea is to create inside a furnace the conditions which can bring the critical glass to break very fast (inside the furnace or just after)
The glass to test are send inside a special furnace where there are heated up to around 290° Celcius and slowly cooled;
The total time for the test depends on the glass thickness
These conditions cause the impurities volume growing and the break of the glass in case the nickel sulfite impurities are present
This test is not 100% reliable. But he decreases drastically the number of spontaneous breaks.
The cost for glass supplied with Heat soak test is around 25% more important than the normal tempered glass.
