Temperature is not the only factor that determines when glass breaks, as the speed in which the glass is heated, as well as the distribution of heat, play a large role in determining expansion and contraction of glass which can ultimately cause shattering and cracks. Known as a thermal break, applying heat to glass can cause it to crack or shatter.
Temperature is not the only factor in determining why glass breaks. Thermal stress occurs when one area of a panel of glass becomes hotter than the other, such as the center becoming hotter than an edge. This temperature change causes the center of the glass to expand, placing stress on the glass and causing breaks or cracks. If the stress becomes greater than the strength of the glass, the glass will break.
Thermal breaks and stress can be alleviated or prevented by heating glass dishware or glasses very slowly to allow the entire item time to expand evenly. With windows or glass panels, such as patio doors, some glass companies will apply a special coating or color to help slow or even out the distribution of heat. Some companies may also suggest additional outdoor shading for windows and doors to help maintain a cooler temperature.