Pyrex is the registered trade name for heat-strengthened, or tempered, glass kitchenware. Pyrex items include cake pans, casseroles, tea pots and measuring cups. First manufactured by Corning, Inc., in 1915, Pyrex has been made by World Kitchens, LLC, since 1998. Similar tempered glass bakeware is also made by Anchor Hocking and other manufacturers.
Regular glass and Pyrex are made from the same basic ingredients and with the same initial processes. After the molten glass is blown, pressed or molded into shape, the item is slowly cooled at a constant rate. This cooling process is called annealing and, except for polishing, is the final step in making most regular glass. Pyrex requires another step.
Pyrex tempered glass is made when the cleaned and polished product is reheated to near its annealing temperature and then rapidly cooled on the outside by jets of air while the inside is allowed to cool more slowly. This creates a surface tension that significantly changes the characteristics of the glass. Tempering is done on finished products because once it is tempered, glass cannot be cut or shaped without causing the piece to shatter.
Glass strength refers to impact resistance. Fully tempered glass is four to six times stronger than regular annealed glass.
Regular annealed glass can break into long, hazardous shards with sharp edges. Fully tempered glass breaks in a much safer granular pattern, called dicing, which produces roughly cubical pieces no larger than ¼ inch.
Pyrex and other tempered glass bakeware is heat resistant up to 425 degrees F, making it suitable for normal kitchen use. Regular glass is not recommended for oven use.
Thermal Shock Resistance
Thermal shock resistance measures the ability of glass to withstand (without breaking) sudden changes in temperature, such as might be caused by pouring boiling water into a cold glass, or putting a cold casserole into a hot oven. Although it is not unbreakable, Pyrex tempered glass has much greater tolerance for thermal shock than regular glass.
Pyrex recommends its products not be exposed to sudden extreme changes in temperature. Frozen dishes should be allowed to thaw before baking, hot dishes allowed to cool before refrigerating. Pyrex is not approved for use on the stove top, over direct flame or under heat sources like broilers or toaster ovens. And, of course, neither is regular glass.