There have been reported cases of spontaneous breakage of fully tempered glass.  Although this is a rare phenomenon, many architects and contractors have become increasingly wary of this.  Experts have determined that microscopic traces of Nickel Sulfide (NiS) inclusions within the glass is the reason for this spontaneous breakage.

 

The heat soak test was derived to help find these Nickel Sulfide stones.  While it is not 100% successful, this destructive testing generally accelerates the aging process so that the glass with these stones will fail during the test rather than after the installation in the building.

We follow our heat soak test strictly to the German Standards, DIN 18516, Part 4.  The tempered glass is heat soaked at 300ºC for a full 8 hours.  During this time, glass which have Nickel Sulfide in them will break, and all the surviving glass is considered acceptable for shipment.  As a note, 7-8% of the tempering strength will be lost in using this process.