The heat soak test or heat soak process is used to minimize the risk of spontaneous breakages of heat-treated glass caused by nickel sulfide inclusions. In this process, glass channels are placed inside a chamber and subjected to an oven temperature of 550°F (287°C) to accelerate nickel sulfide expansion. This causes glasses containing nickel sulfide inclusions to break in the heat soak chamber, thus reducing the risk of potential field breakages. The full heat soak process involves testing all glass in the order, virtually eliminating any chance of nickel sulfide breakage.

Heat soak testing is recommended for all tempered glasses destined for exterior applications subject to changing temperatures. It is normally not required for interior applications.