Pyrex insulators were manufactured by the Corning Glass Works, Corning, New York. They were used for communication line and high voltage power lines. The smaller insulators such as 661, 662, 63 were sold to businesses such as Bell Telephone and Western Electric. Power companies and major utilities used the larger insulators. Corning-Pyrex produced insulators from 1924 through the mid-1940's.

"Carnival Glass" insulators were made by Pyrex starting in 1928. This coloring was really a conductive coating of tin oxide. This allowed the electricity to leave the insulator without causing static.

An insulator's value, when in use, is determined by its electrical resistance. Corning experimented with the electrical resistance of its famous Steuben glassware, a line of Pyrex glass made irridescent by a thin coating of tin oxide. This same process was used on Pyrex insulators, because a build-up of electricity on insulators would reach a peak and discharge, causing radio static, making reception impossible. After insulators were produced with the tin oxide coating, the electricity was allowed to "leak noiselessly" off the insulators, reducing static.

The term "carnival glass insulator" results from the association of insulators made by the same process as glassware given out at carnivals or fairs.

In my opinion, the Pyrex stacker insulator is really unique. It consists of two CD 311's supporting a CD 248. All three are held together by an eleven inch wood peg. Each insulator is separated by a 1/16" layer of cork. My stacker is light clear yellow, embossed PYREX PAT. 5-27-19. MADE BY CORNING GLASS WORKS, CORNING, N.Y. U.S.A. Contrary to Milholland's book, all three sections are embossed as above. This patent date was not in any insulator book I own, so I asked Corning about this when I wrote them. This date, May 27, 1919, is for Patent 1,304,623 by Eugene Sullivan and William Taylor to Corning Glass Works. It is for "new and useful Improvements in Glasses".

Corning sent me a copy of the patent, which consists of four pages of formulas and terms. In short, the patent is for a glass to be used in culinary and laboratory conditions and to have high thermal endurance and high stability. It is, therefore, for the glass used in making the insulator to have the above qualities, and not the design of the insulator itself.

Another Pyrex insulator is the sombrero, no CD #. Mine are embossed PYREX T.M. REG. U.S. PAT. OFF. MADE IN U.S.A., and one is rubber stamped AUG. 7, 1941. These were made by Corning in 1941 and used in Washington and Oregon on high voltage lines. They were made in the carnival glass for reasons stated earlier.

Corning stopped production of insulators thirty years ago. They have no inventory, and it is their feeling that it would be impossible to obtain a complete collection of their insulators. I hope not.