Originally Posted by Dillbert
>>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.
Seeeeeeya; Goodweed fo the North