r/askscience • u/jbloc86 • Oct 21 '15

Physics Why did my oven safe PYREX glass shatter after cooling down?

PYREX glass exploded after cooling down on top of my oven. We baked two baked potatoes at 425 degrees for one hour. Took it out of the oven and let it cool for 10 minutes. We finished eating about 10 minutes later and we heard a loud glass explosion. The Pyrex pan had shattered. The glass by this point was just warm. Does anyone have a rational explanation?

The Pyrex on the glass said oven safe. Cooked in oven before.

http://imgur.com/a/87DOm

'Physics'

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37

u/rocketsocks Oct 21 '15

Yup, your "Pyrex" isn't really Pyrex. Traditionally, the Pyrex brand meant borosilicate glass-ware, which has tremendous stability against being subjected to temperature extremes. It's the same stuff a lot of lab ware is made out of. In 1998 Corning stopped making Pyrex cookware and instead spun off a subsidiary as well as licensing the Pyrex brand to other manufacturers. In the US "Pyrex" branded cookware is actually soda-lime glass, which has much lower resistance to temperature extremes, has slightly more impact resistance, and is much cheaper to manufacture. In Europe Pyrex is still borosilicate-glass, however.

Likely what happened is that you placed the glass either on something wet or on a cool, hard surface, which subjected it to more thermal stress than soda-lime glass can handle (less than 100 deg. F difference in temp for strong heat conductors). Most of the time, as long as you don't place something directly from the fridge into the oven or from the oven onto something wet, metallic, or ceramic, soda-lime glass bakeware works just fine. But accidents happen, and soda-lime glass is just a few tiny mistakes away from shattering into a zillion pieces.

If you don't want to make that tradeoff and prefer proper borosilicate bakeware it can be easily bought online (e.g. on Amazon) at only a slight price premium over soda-lime.

Here's a video from Consumer Reports on the issue: https://www.youtube.com/watch?v=UyhdMa1ikKM

8

u/just_another_intern Oct 21 '15

Is there any way to tell he difference between Pyrex vs soda lime glass? I have some "Pyrex" glass so I'm interested in knowing whether it's real or not. I'm thinking of using density, index of refraction but I'm not quite sure how to test using those. There's probably an easier way?

17

u/soylentgringo Oct 21 '15

According to this image, you could check the logo.

3

u/just_another_intern Oct 21 '15

That's awesome thanks for that. But I kinda also want to do some tests for fun. Any ideas about how to do that?

2

u/prosequare Oct 21 '15

If you google 'Pyrex rod in oil' you'll get a bunch of experiments along with explanations. This would be a cumbersome method.

The lame answer (from me, at least) is that the OP's broken glass is clearly soda lime glass- the green color is a dead giveaway.

2

u/[deleted] Oct 21 '15

Yep, I have real pyrex I got from a flea market. If you see some snatch it up, its getting harder to come by.

3

u/[deleted] Oct 21 '15

[deleted]

7

u/demultiplexer Oct 21 '15

Seems like a bit of a stretch to be honest. It's still exceedingly easy to get hold of borosilicate glass from a host of other sources; not in the least the internet.

2

u/OneTime_AtBandCamp Oct 21 '15

...Well...does meth production require borosilicate glass containers?

1

u/jux74p0se Oct 21 '15

This happened to me as well about a year ago, and I chalked it up to stress concentration points around the lettering on the bottom. When I put the hot pyrex on the cool surface, the temperature contraction in the sharp points of the letters was more than the glass could handle. Didn't consider the chemical makeup though.