the fire - resistant window of fire resistance class e according to din en 357 has a window frame system and at least one thermally or chemically pre - stressed monolithic fire - resistant glass pane mounted in the window frame system . the at least one thermally or chemically pre - stressed monolithic fire - resistant glass pane is made of a high - temperature - melting aluminosilicate glass with a softening point above 875 ° c ., wherein η is the viscosity , and with a bending strength of over 100 n / mm 2 . the glass pane is also substantially impermeable to ultraviolet radiation .