Resistance of Glass to Thermal Shock--

The resistance of glass to thermal shock may be increased considerably by tempering which is the controlled introduction of strain. Tempering and annealing represent opposite extremes in heat treatment. Annealing removes strain by slow cooling while tempering introduces strain by rapid cooling. Class fractures originate in regions of tension. When hot glass is subjected to a cold medium, a thermal gradient is introduced and the resulting strain distribution places the surface in a state of tension. If the tension exceeds the tensile strength of the glass a fracture will occur. Condenser lenses of projection machines are often subjected to thermal shock of this type. Tempering the glass places the surfaces under compression. A much greater thermal shock may then be applied without causing fracture, because sufficient stress must be introduced to completely neutralize the compression before the surface can go into tension and fail.