Quote:
Originally Posted by cheng076
It always amazes me that people whine so much about 'shocky' glass and then I watch as they take a cold rod and wave it rapidly up and down through the flame..shock, shock, shock, shock!! How could it not shatter? It's just been heated and cooled over a many hundred degree temp change repeatedly in a couple seconds.
shocking.
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Actually the waving is done for a different reason and works quite well at preventing shock. When you remove the rod (rapidly) from the flame the heat does not stop. The heat on the surface of the glass now has some time to travel inward while the hot outer surface cools some. Basically what you are trying to avoid is having a very hot molten outside and a very cool inner core. The difference in expansion will tear apart the rod resulting in the "shocky" glass.
Shock can also be caused by other reasons (mixed colors such as pink, scratches, non amorphous structure, chemical composition etc) which is why some glass is more prone to shock than others.
Yes, there are other ways to avoid shock, preheat, heat slower, heat indirectly, heat with rotation, or heat with waving. All are valid methods that try to accomplish similar tasks... bring up the core temperature before the outer gets too hot.