How Is Pyrex Glass Made?

Pyrex glass is a heat- and chemical-resistant product used in cookware, thermometers, laboratory equipment, eyeglasses and telescopes. The original Pyrex glass first manufactured by Corning Glass Works Company in 1915 was a borosilicate glass made by heating silica sand and boric acid. This Pyrex glass continues to be used in laboratory equipment made by Corning. The American manufacturer of Pyrex kitchenware switched to the less-expensive soda-lime glass in the 1950s. The European manufacturer of Pyrex products still uses borosilicate glass in its kitchen wares.

Molten Glass

Pyrex glass is used for laboratory equipment because of its heat- and chemcal-resistance.

Glass is made in two stages. First, a molten glass composite is manufactured by heating crystalline compounds to temperatures of more than 2,900 degrees Fahrenheit. Heating the compounds to high temperatures melts them and breaks their ordered molecular structure. As the material cools, the molecules are locked in place before they can reform in their original state.

Glass Composite

Pyrex borosilicate glass is about 80 percent silicone dioxide. This is mixed with boric acid, to make the product more heat resistant. Boric acid comprises about 20 percent of the glass composite. The two are heated and oxidized into a substance called boric oxide. Soda lime glass, which is less heat- and chemical-resistant, is made of 70 percent silicon dioxide, 15 percent soda (sodium oxide), 9 percent lime (calcium oxide), and small amounts of other compounds.

Secondary Ingredients

Sodium oxide, or soda, is a ''flux'' added to reduce the melting temperature of the glass. Calcium dioxide, or lime, is added as a stabilizer. Aluminum oxide is added to stiffen the molten glass. Silver compounds then are added to produce colors. Other decorations may also be applied.


Next, the glass composite is ready for the second phase: shaping. The molten glass is in the form of a thick red-orange syrup, which flows to the shaping equipment. The products are put into two-piece molds. Air is forced inside the mold. The glass conforms to the sides of the mold and takes on its shape as it cools. The product is then polished.


Pyrex glass has been made in the United States since 1915, and the products are in 80 percent of American homes, according to World Kitchen, makers of Pyrex kitchen products since 1998. Corning Glass Works Company manufactures Pyrex laboratory products, including beakers and test tubes, in the United States, Canada, Asia, Europe and Latin America.