Temperature differentials across the surface of ordinary, annealed float glass with good clean cut edges greater than 40°C are likely to cause the glass to crack. The frame retaining the glass can shield the edge from heat sources and may not conduct heat at the same rate as the glass creating uneven heating. Partial shading of glass and localised heat sources can also lead to stress caused by uneven temperatures.
In a fire situation the glass is likely to break within the first few minutes and the ability of the glass to hold back smoke and flame is lost. The cracking pattern is usually extensive and results in the total loss of the pane. The glass, depending on its fragmentation pattern is likely to fall from the frame. Pane loss can happen immediately the glass breaks or some time afterwards. For this reason, monolithic annealed glass is not suitable for use as a fire-resistant glass.
For information on glass for fire resistance, please click here.