Pyrex, a well-known type of bakeware advertised as oven and microwave safe, has come under fire for shattering without warning, prompting a class-action lawsuit in Chicago alleging the bakeware is dangerous and defective.
Hundreds complaining online have posted photos of shards of glass in ovens, microwaves and on countertops saying the shards came from their Pyrex dishes.
Pyrex has been a kitchen staple for more than a century now and is manufactured by Corelle Brands.
It originally was made of a type of glass called borosilicate that experts said is heat-tolerant and less likely to shatter, but somewhere along the way, that recipe changed and it now is made of the less expensive soda lime glass. It's unclear when the change was made.
"Fifteen minutes in, is it was just like, 'Kaboom!'" one consumer told NBC 5 about her experience with a Pyrex dish. "It was like a bomb had gone off."
Corelle Brands called those type of cases unusual, saying it "produces glassware of the highest quality that is safe to use in conventional, convection and microwave ovens."
NBC 5 Responds found 850 reports of shattering or exploding glass cookware, including Pyrex and other brands, submitted to the Consumer Product Safety Commission over the past seven years.
Corelle Brands said that while "any glass bakeware has a risk of breakage, Pyrex glassware has an exceptional safety record."
So why is it happening? In a demonstration, NBC 5 Responds brought two brand-new baking dishes. One was Pyrex brand, made of soda lime glass, and one was a different brand, made of borosilicate.
Using sand, which is common in this type of demonstration, glass expert Mark Meshulam filled both baking dishes and put them in a standard oven at 450 degrees.
"It has mass," Meshulam said of the sand. "It can absorb heat and really hold it just like a liquid can."
Nearly an hour later, Meshulam took out the Pyrex dish and placed it on a wet stone slab.
Though this is an extreme condition and not advised, Meshulam said it can and does happen. The same thing can happen, he said, when a cool glass dish goes into a preheated oven.
The Pyrex instantly exploded, sending shards of glass flying.
"This was an actual explosion brought about by thermal shock," he said after the demonstration. "It was pretty sobering. Pretty scary."
As soda lime glass heats up, Meshulam said, it grows and expands. If you put it on a cool surface, "that movement is retracting suddenly."
The surface quickly absorbs the heat, creating thermal shock. "The bottom is trying to shrink really, really rapidly, and it just can't."
The baking dish made out of borosilicate was kept in the oven for almost two hours and put on the wet stone slab.
"Instead of breaking catastrophically and spraying shards all over the place, it broke in what looks to me to be three pieces," Meshulam said.
He said that's because borosilicate does not expand nearly as much as soda lime glass when heated.
One thing the maker of Pyrex and its critics can agree on: The best place to put hot glassware out of the oven is onto a cloth potholder.
The other questions may be answered in the class-action lawsuit filed in Chicago containing allegations that Correlle Brands has denied in court.
The company said it does not comment on pending litigation.