After the manufacturing of conventional safety glass spontaneous breakages can occur without any recognizable outside influences. These breakages are caused by corrosive nickel sulfide inclusions. With the aid of the so-called heat-soak-test a spontaneous breakages are tried to be caused deliberately by heating the glass. The safety glass panes are stored at a temperature between 280 to 300 degree for 24 hours. Predistinated panes usually break during this time. In this way the probability of breakage regarding the remaining safety glass panes is minimized.