When glass is manufactured there can be a nickel sulphide inclusion that embeds itself in the glass and is hard to detect with the naked eye. This nickel sulphide inclusion expands and contracts at a different rate than the glass which creates stress that can cause the glass to break spontaneously. Heat soak testing adds an additional element of safety testing for nickel sulfide inclusions. The glass is heated to 700° C and cooled rapidly under immense pressure. If a nickel sulfide inclusion is present in the tested tempered glass the glass will break. All glass that successfully passes through this test will minimize the occurrence of spontaneous breakage.