Preparing a casserole ahead of time is a convenient way to make dinner easier, but seeing a Pyrex dish suddenly shatter in the oven can be both shocking and dangerous. Many people assume all Pyrex cookware is completely resistant to heat, yet even heat-resistant glass has its limits. In most cases, the problem isn’t simply the oven temperature—it’s the rapid change in temperature that causes the glass to fail.

One of the most common causes is thermal shock. If a cold glass dish is taken directly from the refrigerator and placed into a hot oven, the outer surface heats up much faster than the inside. That uneven expansion creates stress within the glass, and if the difference is large enough, the dish can crack or shatter unexpectedly.

Small imperfections can also contribute to breakage. Tiny chips, scratches, or cracks from years of use may weaken the glass, making it more vulnerable when exposed to sudden temperature changes. Even placing a hot dish on a cold countertop, adding cold liquid to hot glass, or exposing the dish to direct contact with a heating element can increase the risk.

Not all Pyrex products are made from the same type of glass, either. Depending on where and when the cookware was manufactured, it may be made from different materials with varying resistance to thermal shock. Regardless of the type, manufacturers generally recommend allowing refrigerated dishes to warm slightly before baking and avoiding sudden temperature extremes.

The safest approach is to let refrigerated glass cookware sit at room temperature for a short time before placing it into a preheated oven, inspect it regularly for chips or cracks, and never expose it to rapid temperature changes. Taking these simple precautions can greatly reduce the risk of unexpected breakage and help your cookware last for many years.