Is pyrex just tempered glass, or is there anything else in it?

The heat resistant quality

I always thought it was just tempered glass, but am not 100% sure. Pyrex can go in the microwave and the oven. I use them for lunches.

http://en.wikipedia.org/wiki/Pyrex

Pyrex was originally made of borosilicate glass. This made it able to withstand extreme changes in temperature and so it was used often in laboratory glassware.

Pyrex was bought out years ago by some other company who proceeded to use the more common soda lime glass that may not be as resistant to temperature changes (this is controversial). That’s why the bottom of new Pyrex glassware is imprinted with a slew of warnings, and probably why it is so cheap nowadays.

They still make laboratory glassware of borosilicate glass, but what you find at the local drug store is soda lime glass.

The soda lime glass Pyrex should not be used on stove top, although I don’t see why it can’t be used in the oven or microwave, except that you want to avoid extreme temperature changes that may cause the glass to shatter.

I don’t know that there is much of a difference between any other tempered glass and Pyrex these days. They usually say they are microwave/oven/dishwasher/etc safe if they are. I’d get down to a thrift store or go on EBay and find vintage Pyrex that was made with borosilicate.

“borosilicate Pyrex is composed of (as percentage of weight): 4% boron, 54% oxygen, 3% sodium, 1% aluminium, 38% silicon, and less than 1% potassium.”

Pyrex Wiki

In other words you can get it really hot.

It’s true that boron is no longer included in the formulation of Pyrex cookware sold in the US, but according to Snopes, the stories of Pyrex having been rendered less safe when Corning sold the brand are unfounded. They say that the switch to soda-lime glass happened long before the 1998 sale (all Pyrex cookware has been soda-lime glass since the ‘80s). Snopes also says that the cookware is still made in the US.

Still made in Corning, NY.