Thermal shock occurs when one part of the glass heats or cools much faster than another. As the glass expands unevenly, internal stress builds until the dish can crack or even shatter without warning. This is why moving Pyrex directly from the refrigerator into a hot oven—or from a hot oven onto a cold, wet surface—can be risky, regardless of how durable the dish appears.

One of the most common mistakes is transferring a frozen Pyrex dish straight into a preheated oven. Even older borosilicate Pyrex, which is generally more resistant to temperature changes than many modern tempered soda-lime glass versions, has limits. A frozen meal should be allowed to thaw in the refrigerator or on the counter before baking, or it should be transferred to another suitable baking dish if immediate cooking is necessary.

To extend the life of your Pyrex, allow refrigerated dishes to sit at room temperature for a while before placing them in the oven. Another safe option is to put the dish into a cold oven and let both the oven and the glass heat up gradually together. Likewise, avoid placing hot Pyrex on cold countertops, metal sinks, or wet towels, as sudden cooling can create the same dangerous stress.

Pyrex is designed to be durable and dependable, but it is not indestructible. Treating it with care and avoiding rapid temperature changes can prevent cracks and unexpected breakage. By respecting its limits, your Pyrex dishes can remain reliable kitchen tools for many years, continuing to handle everything from baking and serving to storing leftovers safely and effectively.