I love Pyrex dishes. The high quality glass is suited for a multitude of uses and save for the oven, fridge, and microwave. The glass is extremely strong and highly resistant to breaking. As such, I have a large collection of the dishes in multiple sizes.
One day I was washing several of them and noticed that they came in two distinct colors. Some were more blue, others more green. At first I thought it was related to size, but further investigation showed the difference was almost completely random. As a good science-minded person, I immediate wanted to know why. Down the rabbit hole I went.
Ordinary glass shatters if it’s heated too quickly: Pour boiling water into a common glass tumbler, and it’s likely to fall apart seconds later. The glass on the inside expands when it gets hot, putting stress on the cold glass on the outside. When the stress gets too great, it cracks.
When Pyrex was first introduced, it was always made of borosilicate glass, which solved this problem by adding boron to the silica. This addition changed the atomic structure of glass so it stays roughly the same size regardless of its temperature. Thermal expansion is reduced, eliminating the stress of non-uniform temperatures. Borosilicate glass withstands heat not because it’s stronger, but because it doesn’t need to be stronger.
When World Kitchen took over the Pyrex brand, it started making more products out of prestressed soda-lime glass instead of borosilicate. With pre-stressed glass (also known as tempered glass), the surface is under compression from forces inside the glass. It is stronger than borosilicate glass, making it much more resistant to chipping and breaking.
However, when prestressed soda-lime is heated, it still expands as much as ordinary glass does. It doesn’t shatter immediately, because the expansion first acts only to release some of the built-in stress, but it will eventually break. The temperature change needs to be quick and dramatic, conditions outside of normal household use. Which means these vessels are just as safe as the borosilicate glass. There is no reason to worry if your Pyrex are different colors.
Unless you are a criminal making crack cocaine. I learned that one unfortunate use of Pyrex is cooking crack cocaine. The manufacturing process for crack involves a container of water undergoing a rapid temperature change when the drug is converted from powder form. That process creates more stress than soda-lime glass can withstand, making household Pyrex unsuitable for it. Thus Pyrex brand owners World Foods have accidentally helped fight illegal drug production.
This goes to show that any practical question you ask has an answer if you look hard enough. As with this search, you may also find that the answer is a lot more interesting than you expected. Always be asking questions, and always seeking answers.
-That is all.