Pyrex bakeware is designed to handle normal oven temperatures, but “heat resistant” does not mean completely immune to breaking. One of the biggest dangers to glass cookware is thermal stress, which happens when different parts of the glass change temperature too quickly. If one area expands from heat while another remains much colder, enough stress can build inside the dish for it to crack or suddenly shatter. Pyrex specifically warns users to avoid sudden temperature changes because they can cause immediate or delayed breakage.

Taking a casserole from the refrigerator and placing it into the oven can therefore raise concerns, although the exact guidance depends on the particular Pyrex product. Current Pyrex instructions say its bakeware should always go into a fully preheated conventional or convection oven, because placing glass inside while the oven is still preheating may expose it to intense direct heat from the heating element. Frozen dishes require even more caution and should be completely thawed according to current guidance.