In our article, Spontaneous Breakage and Its Causes, we understand that the nickel sulfide (NiS) content present in tempered glass might cause it break spontaneously. One way to minimize spontaneous breakage on-site is to let the tempered glass product undergo a Heat Soak Test (HST). Heat Soak Testing is a destructive process wherein we subject the glass to a temperature of 290°C in a heat-soak oven. While heat soak testing, the reversion of the NiS inclusions its original larger beta form will be accelerated.

Why We Allow the Nickel Sulfide to Expand

Heat soak testing works on the principle of “break now, than later”. When the nickel sulfide expands, there is a probability that the tempered glass would break in the heat soak oven. It is generally a safer and a more socially responsible practice to let the tempered glass break in a controlled condition in the factory rather than allowing it to break on-site. Injuries may occur if spontaneous breakage happens on-site which can lead to damages and litigation.

For glass that have been subjected to heat-soak test, we can assure with 95% confidence that the batch of glass that have been heat-soak tested are free from NiS inclusions. In other words, there is no 100% guarantee that all heat-soaked test glass will not exhibit spontaneous breakage.


Heat-soaked tempered glass is recommended where the risk of spontaneous breakage would cause difficulty, especially if the location does not permit glass replacement easily. Furthermore, it is also unsuitable in areas where the glass fragments may fall and injure humans such as
  • Roof
  • Canopy
  • Glass Railings
  • High level glazing

Chain Glass Enterprises Inc. being a pioneer of the tempering glass industry in the Philippines offers heat-soak testing to our customers. Our heat soak testing conforms with European Standards BS-EN 14179-1:2005.