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Long before the modern era of computers, cellphones, smartphones, fiber-optic cables and the internet, long distance electric/electronic communication consisted primarily of the telegraph and telephone. The electric telegraph (in the United States) was developed by Samuel Morse in 1837, and the first message was sent by Morse in 1838. The telephone was invented by Alexander Graham Bell in 1876.
As time went on, networks of “open wire” telegraph lines, and later, telephone lines, were developed and built throughout the country, and these lines required the installation of insulators. Insulators were necessary by serving as a medium for attaching the wires to the poles, but much more importantly, they were required to help prevent electric current loss during transmission. The material, glass, is itself an insulator (not a “conductor” or “transformer” as insulators are often incorrectly labeled in antique malls and flea markets).
Both glass and porcelain insulators have been used since the early days of the telegraph, but glass insulators were generally less expensive than porcelain, and were normally used for lower-voltage applications. The oldest glass insulators date from about 1846.
The period from 1875 to 1930 might generally be thought of as the “heyday” of the glass insulator. Hundreds of millions of these glass “bells” were produced during this time by many glasshouses, located primarily in the East and Midwest with a few plants in California and Colorado. Many of the glasshouses that made insulators also produced bottles and other glassware. Most insulators are found in some shade of blue-green/aqua-colored glass (typical cheap “bottle glass” or “green glass”) but many, many other color shades are found. Clear glass was used (with some exceptions) primarily after about 1935. Hundreds of different styles were developed, and insulators are found with a wide variety of embossed names, patent dates, and other markings. Many earlier insulators have bubbles, streaking, “snow”, surface creases and other marks of crudeness which was common for this type of glass because quality standards were not usually set as high as tableware. As long as the insulator performedit’s duty adequately, the color and minor imperfections in the glass were of little concern. These “marks of crudeness and age” now add to their value and charm, to collectors of antique insulators.
Old photos from around the U.S. show many telephone, telegraph and electric power poles sporting large numbers of insulators arranged on crossarms. Some telephone poles (such as in large cities) carried as many as 20 or more crossarms, each one laden with six, 10 or 12 (or even more) insulators. Each insulator was attached to the crossarm by being screwed onto wooden (or in some cases) metal pegs or (more properly) “pins”. A steel or copper tie-wire was attached to the insulator, and connected with the communication wire. These “pintype” insulators were an extremely commonplace sight and communication lines with insulators were strung alongside most roads, highways, and railroads.
During the 1960s, 1970s and 1980s many of these lines were dismantled as technology advanced. Today, a few lines using glass insulators are still in service, but are only a tiny percentage compared to the heyday of open wire communication.
Insulators (generally speaking) are still commonly in use, but insulators of the modern era (speaking of the United States in particular) are mostly heavier, high-voltage types used in electric power line transmission and distribution, and are of porcelain (“ceramic”) or polymer (plastic) construction. Most modern telephone lines now make use of insulated sheathed cable, and many are underground.
Today, vintage glass insulators are a collectible item in their own right, often saved, studied and displayed along with antique bottles, tableware and other early glassware. The majority of glass insulators carry embossings (raised lettering), as previously mentioned, including company names, brands, trademarks, or model numbers, patent dates, etc. A small percentage of insulators are entirely unmarked. Perhaps somewhat surprisingly, so many insulators are found that were carefully and rather painstakingly lettered with markings that would not even be discernable by the average passersby below……….only visible to linemen (and birds) !
Collectors of insulators often specialize in a particular glass companies’ products, or in certain styles, shapes or colors of insulators. They might delve into the history of a particular company, what styles where made and when, the markings used, etc. Besides the more typical “pintype” communications/electric power line insulators, other sub-categories include lightning rod insulators, radio wire or guy wire “strain” or “egg” style insulators, inside home wiring knob or spool insulators, and battery rests.
Many, many different cast iron molds were used over the years to produce glass insulators, and the collection & study of old insulators can be compared, in some respects, to numismatics (the study and collection of coins). Most earlier insulators were made by forcibly pressing molten glass into a mold. The mold was momentarily closed, and then within a few seconds was opened and the finished insulator was removed to be placed into a lehr (cooling oven). Modern glass insulators are/were made by mass-production machine pressing methods.
On earlier insulators, many minor variations in the engravings cut into the inside surface of the iron molds resulted in slight differences in the exact appearance, size and placement of the raised lettering seen on the surface of the glass. On some companies’ products, more than one style of lettering can be identified. For instance, the so-called “Script”, “Prism” and “Stamp” (“Typewriter”) styles of font which are seen on many Hemingray insulators.
Sometimes an individual mold can be identified by studying and comparing insulators that were produced from it over a considerable stretch of time. Changes in the mold, such as re-cut (re-tooled) engraving, repair, or the addition or erasure (“blotting out”) of the engraving can be discovered upon very close inspection of the insulator. Thus, the comparison I’ve made to the coin collecting hobby, with the many slight differences in coin die design details, as revealed under close scrutiny by serious collectors.
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On this webpage I’ve compiled a list of glass factories that manufactured (or are believed to have produced) glass electrical insulators. Although primarily listing factories that were located here in the United States of America, I’m also including a few Canadian and Mexican firms. If you have additional information, corrections, or clarifications, please contact me (at the email address listed at the bottom of the page), as I am continually looking for the most accurate data available on these companies. Sources of some of the information is included after each entry if I have it available.
Some of the information is from research originally compiled by N.R.Woodward, creator of the “CD” (Consolidated Design) numbering system used by collectors for identifying and cataloging insulators. Also, a portion of the info is found in various articles in the 2-volume reference book “INSULATORS: A HISTORY AND GUIDE TO NORTH AMERICAN GLASS PINTYPE INSULATORS ” by John & Carol McDougald (1990). Those books are no longer in print, but I would heartily recommend them as an important part of any serious insulator collector’s library.
As an antique bottle collector, I’m interested in the identification marks used by bottle factories. for a list of marks found embossed on bottles (and other types of glassware), and the companies those marks are believed to represent.
NOTE: Dates given are (in most cases) the years during which a factory was in operation. In a few cases, the dates only apply to the time period during which insulators were being manufactured. Some information is questionable or incomplete, and I will update this webpage as I learn more. This list does not include “end user” companies (telephone/telegraph companies, railroads, electric power utilities, etc) who had insulators made for them with embossing such as “A.T.& T.”, “C.P.R.”, “G.E.”, etc, but only lists the actual manufacturers of these insulators.
For a listing of some primary embossings found on glass insulators, and the glass factories that may have produced them, .
American Insulator Company Boston area, MA (c.1883-1886?)
Although insulators made by (or for) this company are marked “AM.INSULATOR CO. N.Y.” (on the base), business offices were located in both New York and Boston according to directory listings. Possibly the actual manufacturing facility was located in the Boston area, and may have been the American Iron Glass Pipe & Plate Company (Iron Glass Works) factory at Haverhill, Massachusetts. In addition, recent evidence has surfaced which proves that the Lyndeborough Glass Company, South Lyndeborough, New Hampshire (1866-1888), produced many of the American Insulator Company-embossed insulators.
[McDougald-1990: Joe Maurath, Jr., Bob Fuqua; Mark Newton]
American Iron Glass Pipe & Plate Company (aka Iron Glass Works) Haverhill, MA (1884-1885?)
There is a possibility this was one of the factory locations where some of the American Insulator Company insulators were produced (besides the Lyndeboro Glass Company, see that entry). At least some of the “National Insulator Co” embossed insulators were almost certainly made here.
[McDougald-1990: Joe Maurath, Jr., Bob Fuqua]
Armstrong Cork Corporation (See page for more information.)
Baltimore Glass Manufacturing Company Baltimore, MD (1895-1897)
Insulators embossed “B.G.M.CO.” were produced by this company.
Bay State Glass Works East Cambridge, MA (1878-1879)
Samuel Oakman is listed as agent. [McDougald-1990]
Beaver Falls Glass Company Beaver Falls, PA (1869-1879)
Insulators marked “B.F.G.Co.” were made by this firm, and several other unattributed styles are believed to have been made here as well.
Boston Bottle Works Boston, MA (c.1872-1877)
Colorado Glass Works (See Valverde Glass Works)
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