Bests & Worsts Reviews from Amazon

I have a little more experience with glass than the average consumer, and I also own "new" Pyrex and a set that's ~25 years old. I make glass beads as a hobby. It's well-known in that world that COE -- coefficient of expansion -- will tell you everything about how glass will perform under temperature changes. Borosilicate, or hard glass, has a COE around 30. Soda lime, or soft glass, has a COE between 90 and 110. When glass expands or contracts, it breaks. To use an example from my glassworking experience, I can put a large rod of borosilicate glass at room temperature directly into the flame of a torch (temperature between 1500 and 2000F), and it won't shatter. Try the same thing with a soft glass rod of the same size, and you'll have shards of hot glass all over your workbench. Also, you can't mix different COEs of glass in the same piece; it renders it structurally unstable. So Pyrex has to be either hard glass or soft glass; it cannot be a mixture of the two. Similarly, I got a set of Pyrex bowls for my wedding that lasted for years before succumbing to breakage (via drop). I still have one of them. I also have a newer set of the same size bowls. One of them chipped on the bottom almost immediately. At that point I wasn't aware of the differences in glass, so I didn't know why, but now I do. Borosilicate is far more expensive than soda lime glass. I'm quite sure the change was made as a cost savings. Personally, I'd gladly pay more (and it would probably be double, at least) for a Pyrex set made of borosilicate. One more tidbit: glass pieces of any size must be annealed in a temperature controlled kiln to ensure structural stability, long life, and shatter resistance. That includes both borosilicate and soft glass. I would never sell a bead to someone that hadn't been annealed; a piece like that can break under any tiny stress. So Pyrex's assurance that they heat-treat their glass is undoubtedly correct, but I'd wager they always have. Caveat: I don't own this set but am considering purchasing it with the knowledge that it needs to be treated like it's made of glass.