The heat soak test is used to minimise the risk of spontaneous breakages of heat-treated glass caused by Nickel sulfide inclusions by subjecting the glass to an additional thermal process named “Heat-Soak Test” (HST).
Being destructive, this treatment disposes most of the glass that represents risk.
 
This treatment is proper in all situations in which the safety of users and working environment may be threatened by the braking of a heated glass.