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Why does thick glass cracks when hot water is poured into it but thin glass does not?

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I'm answering this question from the perspective of a glass artist...

Thin glass warms up more quickly because it IS thin and not subject to differences in temperature within the glass itself that cause internal stress. With thick glass, the surface may be hot, but the center may still be cold, and the difference in temperature will cause it to shatter or crack.

However, to answer your question, it is most likely related to the TYPE of glass that was used to create the product and not the thickness.

For those of us who work with hot glass (cast/torch/fused) an important concept is one of thermal shock. Glass for fusing is assigned a number called a "Coefficient of Expansion" that relates to how much the glass expands and contracts when you heat it to its melting temperature and cool it back down.
Coefficients of Expansion Beadmakers (lampworkers) who melt rods of glass use "softer" glass with COEs of approximately 104 (Moretti). This type of glass melts at a lower temperature and is fairly easy to work with. Glass fusers usually choose between one of two standards...System 96 (96 COE) or Bullseye (90 COE). Pyrex, another familiar glass (Corning 7740 Borosilicate), has a COE of 32.5. It is worked at a much hotter temperature. The higher the COE, the more that the glass expands and contracts. The lower the COE, the stronger the glass, and therefore less likely to break.
If you've ever watched a glassblower...the kind you used to see at the mall or the county fair, they most likely used Pyrex for its durability and resistance to thermoshock. Also, you cannot mix these different types of glass because as the glass cools it contracts at different rates and causes stress within the glass. This leads to immediate or even future breakage.
More Info About Pyrex... So a little more about Pyrex, which is very resistant to breaking...its predecessor was a low-expansion glass originally invented by Corning Glass Works for the railroad industry. The railroads needed shatterproof lantern globes that wouldn't break when hit with rain or snow. Corning found that the demand for replacement globes went down significantly after the introduction of this new product.

So how did Pyrex come into existance? The wife of a Corning scientist, Bessie Littleton, was having problems with a casserole dish breaking, and made a request to her husband to bring home samples of this new shatterproof glass. He brought home two samples of sawed-off battery jars, and she cooked a sponge cake in them. She found that the cooking time was shorter, the cake was uniform, and most importantly, they didn't break. In 1915, Corning introduced Pyrex bakeware to the world. Check out the article posted at the end of this answer.


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