Cooking Glass?

Question

Why is that Pyrex glass is used for cooking or baking while ordinary glass is not?

Answer 1

Pyrex is a brand name of borosilicate glass introduced by Corning Glass Works in 1924. Though borosilicates had been produced before, the name Pyrex is widely used as a synonym for the material. Corning spun off its kitchenware division in 1998, and the Pyrex brand is now owned by World Kitchen, Inc.

Pyrex kitchen products produced by World Kitchen are no longer made from borosilicate glass, but from Soda-lime glass and their packaging indicates that they must never be used over a flame, on stove tops, under a broiler, or in a toaster oven. Pyrex is created by adding boron to the traditional glassmaker's "frit" of silicate sand, soda, and ground lime. Since Pyrex melts at a higher temperature than ordinary silicate glass, some new techniques were required to bring Pyrex into industrial production. Borrowing from the welding trade, new burners combining oxygen with natural gas were required.
Borosilicate glass has a thermal expansion coefficient about one-third that of ordinary glass. This reduces material stresses caused by temperature gradients, thus making it more resistant to breaking. Pyrex begins to soften around 821 °C (1510 °F); at this temperature, the viscosity of type 7740 Pyrex is 107.6 poise.
Pyrex is less dense than ordinary glass. When broken, Pyrex tends to crack into large pieces rather than shattering (it will snap rather than splinter).

Answer 2

Three quarters of all Americans have PYREX® glass products in their kitchens. Since its inception in 1915, all of the PYREX glass products are "Made in America" at the PYREX plant in Charleroi, Pennsylvania.

Originally, the glass was developed by Corning Glass Works, Incorporated to make railroad signal lanterns that wouldn’t break under the extreme temperatures that the signal lanterns had to endure. Later in the '60s and ‘70s, NASA used it to make windows on the capsules of the manned Gemini and Apollo missions because PYREX glass was so strong. This is the same quality product today.

The manufacturing process is a very “hot” process, with extreme temperatures. To make PYREX glass, silica the main ingredient (a compound found in quartz and sand), is moved from railroad cars into underground bins and then moved into 200-ton mixing vats. Eight ingredients are added and some broken glass and is cooked in a huge furnace at 3,000 degrees Fahrenheit.

After all the materials are mixed, the molten glass is cut into small portions called “gobs” and dispensed into molds of various sizes. A hydraulic press is used to give the pieces their final shape (like the PYREX measuring cup & baking dishes) and a fire polishing process gets rid of rough edges. The glass then goes through a tempering process that gives the product its strength and durability.

Lastly, the PYREX product, such as the PYREX measuring cup is given its “final dress” by silk screening key measurements and the PYREX brand name.