The Heat-Soak Test is a destructive test for the presence of nickel sulphide inclusions that might cause glass failure (spontaneous breakage) in service. It involves placing the toughened glass in an oven and heating it to a temperature of 290 decrees centigrade. If NiS inclusions are present in the glass, it is hoped that they will cause breakage within the oven. Heat soaking will not detect other inclusions such as silica that can also cause breakage.
Heat soaking is of particular value in safety-critical work such as balustrades and overhead glazing as well as in highly trafficked areas such as airports and shopping malls where breakage would be very imotive.