Depending on their properties, for glasses with same composition but different thickness, the tempering stress relates primarily to the cooling rate. Based on Thermal Stress formula, if we want to get the same result in surface stress, the cooling rate is inversely proportional to the square of the glass thickness. As to a tempering furnace, the size of the outlet of the quenching section is constant. While the air temperature and the blowing distance are predetermined, the cooling rate relates only to the air blowing pressure.

To receive the best result on surface finishing, the temperature of the glass coming out the furnace should be as low as possible, provided all tempering requirements are met. Therefore, you need to take the glass out of the furnace fast to prevent the excessive heat loss during the process and have it cooled as soon as you can. When the temperatures of both surface and inner layers drop below the glass transition temperature, the tempering procedures are completed. You may now cool it down further.