If some tiny particles, such as unmelted sand, refractory stone, nickel sulfide inclusion, etc. remain in the glass, glass may stay well when it is coming out of the tempering furnace. But it will soon be fractured for destroyed stress balance when it encounters mechanical shock, temperature fluctuation or changing of nickel sulfide crystal phase. The function of our heat soak oven is to use heat-treating method to create an environment with a destroyed stress balance so as to lead the possible self-fracturing pieces breaking in advance. The heating source can be heated by either electric or gas fuel.