Under the right circumstances, glass bakeware will shatter, crack, split and even explode when exposed to thermal shock. Thermal shock is when an object abruptly goes through a drastic temperature change causing it to fracture as it expands or contracts.
While glass bakeware and kitchenware is a healthier alternative to metal, it is not without limitations; like all glass, it can break. Despite breaking as a possibility, glass is preferred because there are no hazardous materials to leach into food and it bakes more evenly.
Regardless of brand, “Pyrex” is the name consumers often use when referring to glass Pyrex®, Anchor Hocking, Bake King or other bakeware and kitchenware because it has been a trusted household name for decades. Pyrex® was valued for years for its sturdiness and ability to withstand rapid, dramatic temperature changes that typically shatter normal glassware. However, with changes in manufacturing, that old-fashioned reliability has changed with Pyrex as well as some other brands.
Pyrex (trademarked as PYREX) is a brand introduced by Corning Inc in 1915 for a line of clear, low-thermal-expansion borosilicate glass used for laboratory glassware and kitchenware. It was later expanded to include clear and opal ware products made of soda-lime glass. In 1998, Corning sold the Pyrex brand name to World Kitchen LLC. World Kitchen stopped the manufacture of borosilicate glass and changed to less expensive, tempered soda-lime glass for kitchenware sold in the United States. Tempered soda-lime glass does not handle heat as well as borosilicate glass but does withstand breakage when dropped better. With some caution, tempered soda-lime glass withstands thermal shock reasonably well. Anchor Hocking and Bake King products are also made from tempered soda-lime glass. The OXO brand uses thermal shock resistant borosilicate glass in the manufacturer of its glass bakingware; the manufacturer’s information states that it can go from freezer to oven without the need to thaw.
To determine the kind of glass used in true Pyrex ware, look on the underside of the dish for these designations:
PYREX® (all UPPER CASE LETTERS plus, in the USA, a trademark notice comprising a capital R in a circle = low-thermal-expansion borosilicate glass either clear or opaque originally made by Corning Inc. Scour estate auctions, thrift stores, antique stores, or purchase in Europe to acquire it.
pyrex® (all lower case letters plus a trademark notice comprising a capital R in a circle) = clear tempered high-thermal-expansion soda-lime glass kitchenware made by World Kitchen.
PYREX (all UPPER CASE LETTERS in an encircled oval with no trademark notice with European country noted) = European license for use on borosilicate glass products manufactured by International Cookware.
Some precautions are necessary for modern-day tempered soda-lime kitchenware. In 2010, Consumer Reports tested some Pyrex and found that taking the newer glass out of a hot oven and placing it on a wet granite countertop yielded poor results with the glass shattering almost instantly. As a result of its investigation, Consumer Reports called on the Consumer Products Safety Commission (CPSC) to look into the problem of shattering bakeware.
Further, Consumer Reports issued ten precautions to consumers to minimize the chances of the glassware shattering:
- Always place hot glassware on a dry, cloth potholder or towel.
- Never use glassware for stovetop cooking or under a broiler.
- Always allow the oven to fully preheat before placing the glassware in the oven.
- Always cover the bottom of the dish with liquid before cooking meat or vegetables.
- Don’t add liquid to hot glassware.
- If you’re using the dish in a microwave, do not use browning elements, and avoid overheating oil and butter.
- Do not take dishes directly from the freezer to the oven or vice versa.
- Never place hot glassware directly on a countertop (or smooth top), metal surface, on a damp towel, in the sink, or on a cold or wet surface.
- Inspect your dishes for chips, cracks, and scratches. Discard dishes with such damage.
As always, it is the consumer’s responsibility to read and save the manufacturer’s instructions for handling any product safely. Without instructions, check for a stamped label on the bottom side of the baking dish. If in doubt, the precautions issued by Consumer Reports will suffice for all glass bakeware and kitchenware.
Reviewed and updated 5/2024, mg.
