Heat soaking is a quality control check carried out, when specified, to detect and reject any toughened glass that may have inclusions (Nickel Sulphide ) or edge defects that may cause breakage after installation.
Gold plus heat soaked glass can be used in critical application areas to minimize (if not totally eliminated) the risk of spontaneous breakage.
Nickel Sulphide (NIS) has been known to the cause of spontaneous burst in tempered glass. During tempering the nickel sulphide is transformed to the high temperature a- phase (t>380C) and has no time to return to the B- phase on quenching. With the passage of time NIS slowly inverts to the low temperature B – phase with an increase in volume of about 4%. This volume expansion may cause glass breakage.
The heat soak is a destructive treatment which heats the glass for several hours at about 290 C to speed up the a – NIS to B- NIS transformation and consequently forces the contaminated glasses broken within oven.