On Thursday, May 22, 2014 11:54:48 AM UTC+1, Martin Brown wrote:
I'm a bit surprised it is always the inside. At a guess the outside
cools faster and the frame is slightly too rigid so that the inner
surface is in tension.

That can't be right Martin. If the outer was cool and shrank faster,
it would put the outer in tension and the inner in compression.

(You may be thinking of a single sheet of toughened glass which is
made by cooling the surface rapidly. The surface layer shrinks and
deforms the inside plastically. Eventually the inside cools and
shrinks, but the outside is solid - so the inside ends up in tension
and the outside in compression.)