During the glass manufacturing process, a nickel sulphide inclusion may embed in the
glass, often undetectable by the naked eye. This inclusion expands and contracts at a
different rate than the surrounding glass, creating internal stress that can lead to

spontaneous glass breakage. Heat soak testing is an additional safety procedure that
can be done to detect these nickel sulphide inclusions. The glass is heated to 700°C
and then rapidly cooled under high pressure. If a nickel sulphide inclusion is present,
the glass will break during testing. Glass that passes this rigorous test ensures a
significantly reduced risk of spontaneous breakage, offering added safety and reliability
for tempered glass applications.