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Be Careful! Newer Glass Pyrex Seems To Be More Subject To Breakage, Follow Directions!
on December 13, 2010
Glass kitchenware is absolutely terrific, I have used Pyrex for many years and have a lot of it. Original "pyrex" is a very hard and durable glass that is tough and can withstand fluctuations in temperature without getting too plussed about it. With my love affair of the stuff, I purchased a few new clear round pyrex pieces recently, and was surprised to see that the clear yellowish color had changed, it is now clear greenish. I am a glass artist and melt different kinds of glass regularly, and the color of the new pyrex dishes in the store reminded me of what is called float (window) or soda-lime glass. I thought maybe it was the store lighting but when the new pyrex dishes arrived home they were still greenish.
Then I got the current issue of Consumers Reports last week...they had an article on the safety of pyrex. Apparently it has been breaking and exploding unexpectedly in home kitchens, in some cases causing traumatic injury. It turns out that the "pyrex" name, first of all, was sold by Corningware to two other companies...Corningware left the home glassware business in the late 1990's. I was not aware of this. Then I read about the complaints that have been filed about shattering, cracking, and worst of all, exploding dishes. Why this is happening is fuzzy - some claim that the "pyrex" base glass formula has changed. To me it looks like it - my new pyrex looks like greenish soda-lime glass, not thermal shock resistant yellowish borosilicate glass. But the company making the new pyrex denies this, and of course glass color can be altered....when I dug into the issue generally, what is going on is really unclear.
The ONLY thing that is clear is that people seem to be having a lot more trouble with new pyrex as compared to old pyrex - be it from a base material change, the glass not being tempered properly, or people now knowing how to use it. When pyrex was still owned by Corningware through the late 1990's, they were not having these kinds of complaints and issues from the public.
Although I did not resonate with the methodology that Consumer Reports used to test the glass dishes, I did agree with their conclusion to be careful with the new pyrex. In following prior Consumer Reports threads I saw that some independent labs cut up and analyzed some new pyrex items, and they discovered that the tempering of the soda-lime glass was not even. That is sobering. Tempering is a process that introduces stress gradients into glass to make it more durable under impact conditions - if you drop it on a floor, for example. But improperly tempered glass can hide little or large tracts of internal stress that you can't see. The dish may be fine at room temperature but if you heat it can break or explode with no warning, sending the glass and whatever was in it everywhere.
Again I'm not sure if this explains why people are having trouble with unstable dishes, or if new users of glass bakeware just don't know how to handle it, or if there indeed was a post-Corningware basic material change from borosilicate glass to tempered soda lime. What I do know is that under conditions of heat, "boro" as it is called is just a very tough glass. It's due to the basic properties of the material - soda lime glass has a coeffficient of thermal expansion anywhere from 90ish to 100ish, whereas borosilicate is half that. As a result, borosilicate glass, which is what pyrex was originally made of, and still is over in Europe, is more resistant to thermal shock than pyrex made from soda lime glass. Borosilicate has withstood the test of time, it started in chemistry labs as a great material to use safely under tough conditions (that is, tough conditions for glass, which does not like to be heated) on things like bunsen burners, test ovens, etc. Somewhere along the line this was recognized and "pyrex" was brought into home kitchens.
In Europe "pyrex" is still made of borosilicate by a company called Arc International (they bought the name from Corningware too). They have not received the hundreds of complaints - they have not had any complaints - about shattering, breaking, or exploding dishes (under low to mid heat conditions) that have been logged in the U.S. about the new pyrex. Their line is really limited, howveer...I could not find another company with some of the nifty storage designs that are sold here in the U.S.
If you are wondering about the possible stability of your old pyrex as I was? It does not seem to be clear when the new soda lime glass replaced the old pyrex borosilicate glass for bakeware, some are saying the change was made in the 1940's, others are saying it happened after the pyrex name changed hands in the late 1990's. Soda lime glass has a greenish tinge and looks like window or car glass, whereas borosilicate has a yellowish sunny tinge to it - this is easily seen in the edges of a dish. However, glass color can be tinted so this is not a foolproof guide. Based on what is happening, it seems like a guideline of glass made by Corningware before 1998 can be relied upon as glass that is stable under the conditions of proper usage. As an alternative, you could also purchase the european made borosilicate pyrex, which is also sold here on Amazon. I don't know if there is more than one company or not, Arc International has a limited line as compared to US made pyrex.
All of this said, the pyrex glassware being made by the new companies is being used by millions and millions of households every day without issue? It may be fine for you? But please be careful if you are going to heat it - be sure to understand how not to subject the pieces to thermal shock by reading the instructions carefully!!! Especially don't take a hot dish out of the oven and set it on a room temperature or cold counter, stovetop, or in cold water, instead place it on a thermally insulating hot pad to cool.