1977 >> December >> A Closer Look At Corning Pyrex Insulators  

A Closer Look At Corning Pyrex Insulators
by Jeff McCurty

Reprinted from "INSULATORS - Crown Jewels of the Wire", December 1977, page 4

Upon receiving a request for information on their insulators, Corning Glass Works always seems to be most interested in helping. The Historical Resources Department, headed by George Southworth, with the thorough research completed by Mr. Otto Hubert, has supplied me with quite some fascinating information in regard to their insulators. I now would like to pass it on to Crown Jewels readers with this article. 

Upon asking about their embossing practices, I was informed that Corning Glass Works refers to the embossing on their insulators as "mould markings". The mould markings are used to identify the electrical characteristics of the insulator and assure that the correct insulator size is used on any given power line. 

Because Corning Pyrex insulators have such extended mould markings on each of their insulators, I asked if there were any specific reasons why. "Pyrex" and "Corning Pyrex", both registered trademarks of Corning, apparently indicated the origin of the insulator's manufacture. The size of the insulator itself had something to do with the choice of the marking. Other than this, Corning knows of no reason why some of their insulators were marked "Pyrex" and some "Corning Pyrex". 

As to the placement of their mould markings, Corning did have specific reasons. Many of the larger power type insulators have their markings on the underside of the glass, reading through it. By having the letters placed here, on the insulator skirt, it greatly improved the cleaning of the glass by rainfall, and thus increased the electrical insulation of the unit, which was a most desirable result. The actual location on the insulator was one of convenience. 

Certain Pyrex insulators had additional letters and dot markings used to identify the moulds from which they were made, for mould material control purposes. Corning has been unable to locate the record of the key to these letters and dots 

The Pyrex Stacker (with Pyrex 662 for scale), see photo, which consists of two CD 311's supporting a CD 248, has the most extensive mould marking of any Pyrex unit I know of. All three sections read: PYREX PAT. 5-27-19, MADE BY CORNING GLASS WORKS, CORNING, N.Y. U.S.A. Corning Glass Works supplied the following information: 

"The three part Pyrex brand insulator assembly over a central wood post was designed for and used in a dry climate such as Colorado. The two lower Pyrex sections consisted of a tubular sleeve combined with a drooping skirt, topped by a standard skirted type regular Pyrex insulator which was threaded and was screwed to the central wood post. This type of insulator assembly was replaced by the now common suspension type assembly with three to five or more units connected to a single string. The insulators in the multiple strings consist of three to six or more single so-called cap and pin insulators coupled together with metal cap and rod cemented in place." 

The Corning Glass Works has trademarks "Corning" and "Pyrex", and patents on borosilicate glass, commonly called "Pyrex Glass". The trademark "Pyrex" was registered July 10, 1915 and issued July 13, 1917. The Pyrex Glass Composition Patent was issued May 27, 1919. No one at Corning has any record or recollection as to why the date was marked on the Stacker insulator.


Corning's Radio Antenna Insulators (see photo above) include the patent # 1700066. This was for an antenna insulator to be composed of Pyrex Glass. See Patent copy, lines 65-75. Because no specific design was issued, the patent covers all designs of the Radio Antenna Insulator, 7", l2", and 19". 

Corning states that the "REG. U.S. PAT. OFF." marking on virtually all of their insulators refers to the Pyrex Brand Glass (T.M.) patent of May 27, 1919. See Patent copy. 

One of the controversies in reports of Corning Pyrex insulators has been dates of manufacture. Some past articles including some of my own, have stated different dates, which may have seemed confusing. That is because only one set of dates was stated. Corning has now supplied the following dates of their insulator production. Corning Glass Works made: Power Line Insulators, 1924-1925; Communication Line Insulators, 1926-1941; and Radio Wares Insulators, 1924-1951.

| Magazine Home | Search the Archives |