During float glass manufacture, impurities in the glass batch can result in inclusions in the finished product. These inclusions are so small that they are normally invisible to the naked eye and yet they can cause spontaneous breakage in Toughened Safety Glass.
The most notorious of these inclusions is nickel sulphide (NiS) crystals (stones), which can be contained in the raw material during the production of the glass.
The glass toughening process requires the glass to be heated to just below its softening point and then rapidly cooled. Because this heating and rapid cooling process induces substantial tension in the glass, nickel sulphide stone inclusions in the tension core can cause spontaneous breakage, as they are known to change phase (expand) some time after the toughening process.
Heat soaking is a secondary process to the Toughening process. Heat soaking reduces the incidence of Toughened Safety Glass breaking spontaneously after installation. Therefore reducing the associated replacement, maintenance and disruption costs and the risk of the building being classified as unsafe.
This secondary process after toughening, reduces the chance of Nickel Sulphide spontaneous breakage from 1 in 10,000m2 to 1 in 1,000,000 (Glass And Glazing Federation UK).