Pyrex Glass


Corning Glass Company/Corning Glass Works, Corning, New York (1875-to date).     (Now known as Corning Incorporated).

The trademark “Pyrex” was first used c. 1915 for Corning’s borosilicate heat-resistant glassware formula.  VERY large quantities of kitchenware, serving bowls, ovenware, measuring pitchers, etc, as well as glass electrical insulators for telephone and power lines were made.

Pyrex ovenware has been a staple in American households for many decades.

Bottles and other types of heat and chemical-resistant laboratory glassware are also found with the “PYREX” name embossed on the bottom.    Glass tubing, test tubes, funnels and other miscellaneous laboratory equipment and containers were made in quantity.

CD 233 style power line insulator. Marked "PYREX  REG. U.S.PAT.OFF. // 661 / CORNING MADE IN U.S.A."

CD 233 style power line insulator, circa 1930s or 1940s.  Marked “PYREX REG. U.S.PAT.OFF. // 661 / CORNING  MADE IN U.S.A.”


Some “Carnival glass” insulators were made, bearing the PYREX / CORNING brand marking.     The most common Pyrex insulator (found in clear glass) would probably be the CD 128 style.  A search of the internet, such as Google Images pages with  “Pyrex insulator”  and “CD 128″ should bring up examples of that particular shape.

NOTE: In 1998, Corning discontinued the production of their PYREX line of glassware, (concentrating instead on their lines of tech-related products) and the rights to the PYREX brand name was assigned to World Kitchen, LLC.   The  original formula for the glass was evidently changed after 1998, to a  tempered soda-lime glass, and there have been complaints by consumers that this recently-made glassware (ovenware, etc) has shattered and appears to be inferior to previous standards of durability characteristic of the earlier borosilicate formula.

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For more detailed information on Pyrex glass, check Wikipedia’s webpage here.

4 Responses to Pyrex Glass

  1. Janet Mayer says:

    The glass was not tempered correctly. If you contact the company, I bet they’d replace it for you. The glass would have to be tempered at the factory and there is nothing in the environment that could cause this to happen on a piece of glassware that was tempered adequately. Extreme temps will only impact an untempered piece of glass.

    • David says:

      Hi Janet,
      Yes, proper tempering of glass does REDUCE, but DOES NOT competely ELIMINATE the chance of breakage. There have been hundreds of reports in recent years of this cookware shattering. Such things have happened over the years occasionally, but the frequency of the occurrences have greatly increased after the manufacturers changed their formula to a cheaper grade of glass. SO, whether or not the glass was properly tempered, it is a less durable grade of glass, not as strong and resistant to breakage as the original Pyrex formula was.
      The problem seems to be that the more recently-made Pyrex glass (distributed by World Kitchen) and it’s main competitor brand of cookware (manufactured by Anchor Hocking) are now made of a soda-lime glass instead of BOROSILICATE glass which was the original PYREX formula and which much more resistant to sudden temperature changes.
      From Consumer Reports, quote: “U.S. manufacturers of both Pyrex and Anchor Hocking have switched from borosilicate to soda lime glass for their glass bakeware”
      Please check out these webpages:

      Take care, David

  2. L Cross says:

    I had a Pyrex 4 cup measuring cup explode in my cupboard.I have had it for only about 8 months. What would have caused this?

    • David says:

      I don’t know, but assume it is a more recent, perhaps poorly annealed / lower quality glass. Even Pyrex can break if subjected to sudden EXTREMES of heat and cold within a short period of time.