Tempered glass is made by heating the glass in tempering furnace until the softening point. From 620°C till 670°C, with many different recipes, following very rapid cooling it down to ambient air temperature. After such thermal treatment it becomes resistant against thermal stress and about 4 - 5 times stronger than ordinary float glass. The weak point of such glass is vulnerability against tiny NiS (Nickel Sulphide) inclusions which are getting entrapped within glass during the float glass production process. Such inclusions are completely harmless for float and heat strengthened glass, but may cause tempered glass to break spontaneously, and that is the reason why additional treatment - heat soak test - is often requested for tempered glass. It reduces the probability of spontaneous breakage to less than one case per 400 tons, although does not eliminate it completely. Tempered glass is one of safety glass types, because it is very hard to brake, and if broken, it shatters into lots of small blunt pieces (called dice), which cannot cause serious injuries.
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