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Old 09-29-2009, 07:51 PM   #11
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I have Visions too and have abused them horribly. I just wondered what kind of glass they were.

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Old 09-29-2009, 10:58 PM   #12
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Originally Posted by Dillbert View Post
>>Pyrex used to be made from a special formulation of glass that transfered heat throughout the whole of the container much more quickly, and thus prevented the thermal shock problem. But now, they just use tempered glass as it's cheaper.

this is incorrect. "Pyrex" glass is a formulation of glass that has a low coefficient of thermal expansion.

"tempered glass" is altogether a different concept.

the "coefficient of thermal expansion" is the amount of expansion a material experiences per degree of thermal difference.

"thermal difference" - that's about about taking it directly from the freezer to a hot oven.

"amount of expansion" if you stretch (any) material past its limit, it breaks. so a glass with a high expansion per degree of heat change will more rapidly expand/stretch to the point where the elastic limit of the material cannot withstand the strain - and then it breaks.

yeah, I used to work for a glass company.....
Pyrex used to be made from borosilicate glass, if I recall. And you are correct in that it expands more evenly. But the bimetalic strip analogy is still correct. I'm not sure if the glass transfered heat more quickly or not. But the result is still the same. I was at least close.

What would be too cool would be to find bakeware made from fused quarts glass. It can be taken to 1100 degrees and imersed in cold water without damage due to thermal shock. That would be amazing.

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Old 09-30-2009, 07:10 AM   #13
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>>from borosilicate glass

ayup! still is I imagine [g] the coefficient of expansion has units of how many inches per inch the material expands per degree of heat change - borosilicate formulations are on the low end of the scale.

the "exploding trick" is most associated with some manner of damage - a small nick or chip that becomes a stress riser - and when the right amount of thermal shock/stress comes along - ka-blewie. it can happen either 'through' the thickness of the glass (the bi metal is an apt description) or in a 'lineal' dimension.

this is easily 'demonstrated' - but hard to 'prove' after the fact because. . . well, it's just that there's a lot of little pieces.....

not that 'manufacturing defects' don't exist - frosted areas, dimples, orange-peel, mold marks, folds, inclusions.... lots of stuff can become a stress riser.

>>cast iron can be made to shatter
if you've played around with liquid nitrogen, it's good for making all sorts of things shatter.
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Old 11-16-2009, 10:09 PM   #14
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I have an 11x15 Pyrex casserole I've had over 20 years. I haven't babied it at all. I just always used a trivet to place it on. The troubles I've heard about often came from taking a hot Pyrex dish from the oven and set on the stove burners, either hot or cool. My trivets are Kitchenaid plastic ones, I make sure they are not wet.
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Old 11-17-2009, 11:32 AM   #15
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I have a large butcher block cutting board that I put on the counter next to the stove when I'm baking or roasting with any of my Pyrex or Corningware dishes. It protects the solid surface countertop and the dishes from any liquid spillage. The only Pyrex stuff I have is a couple of 9 x 13 pans.

I used to have an oval Visions Dutch oven style casserole. One day it came out of the oven with a crack all the way around the bottom. I don't even know what was holding it together. It did not go into the oven cold (just room temperature), and the oven was just at 325°. Since then I've been a big fan of Corningware casseroles, which I've never had a problem with.

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