Many cooks love their glass cookware! Easy monitoring simplifies the cooking process and the overall versatility of the material, e.g., straight from the freezer to the stove top, reduces the need for additional cookware. But more than a few cooks (my wife included!) report experiencing cracked and even shattered glass cookware during food preparation. Others have heard or read about the phenomenon and fear the possibility enough to avoid glassware altogether.
Is shattering glass cookware a real concern or simply an urban legend? Rather than an academic question, the possible danger arises when selecting or adding glass or ceramic cookware to your collection. The issue is ‘thermal shock,’ or the way in which some materials are prone to damage if they are exposed to a sudden change in temperature–from hot to cold.
Thermal shock can occur with any material that does not conduct thermal energy well, especially glass and ceramic, which also lack high tensile strength. More precisely, it has to do with an uneven expansion within the material at the molecular level.
Not all glass and ceramic cookware are created equal, however. Improving thermal shock resistance of glass and ceramics can be achieved by improving the strength of the material or by reducing its tendency to uneven expansion. Some manufacturers, for example, add boron which prevents shock by reducing the tendency of the glass to expand when exposed to a sudden and extreme changes in temperature.
Pyrex Cookware, which manufactures its heat-strengthened glassware in the USA using a soda-lime formula, strongly suggests that cooks looking to add glassware to their collection closely investigate the origin of the cookware. They contend that nearly all foreign-made glassware is not constructed to avoid uneven expansion and is, therefore, more readily subject to thermal shock and the resulting risks. They also strongly recommend that users read and follow the manufacturer’s directions.
Just for the record, most people who routinely use glass cookware swear by it and consider it a valuable asset to their collection. Also keep in mind that thermal shock can impact non-glass materials as well, like cast iron which may simply crack (high tensile strength), although this normally requires multiple abuses. Thermal shock may also break down nonstick surfaces, degrading performance and causing flaking of nonstick material into the food. Remember to read all instructions and cautions before using your cookware.