THC
Daft Vapour
Sounds like you hit the nail on the head m8 the raid contraction caused by the cold water will defiantly not be good to stress the glass with rapid temp changes even if has boron added to the silica it still will move and if there is not enough room for it to do so as it is a super viscous liquid (glass) then moves quite allots and if it does not have the room to do so and it has sufficient forces involved the liquid will part or 'crack'.