Plate glass , , initially at 850 K, is cooled by blowing air past both surfaces with an effective surface coefficient of 5 W/m 2 K: It is necessary, in order that the glass does not crack, to limit...

Plate glass,

, initially at 850 K, is cooled by blowing air past both surfaces with an effective surface coefficient of 5 W/m²
K: It is necessary, in order that the glass does not crack, to limit the maximum temperature gradient in the glass to 15 K/mm during the cooling process. At the start of the cooling process, what is the lowest temperature of the cooling air that can be used?