General Overview on Glass Insulators – Basic Information

General Overview on Glass Insulators

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.

Telegraph Line pole with glass insulators, along a railroad in Indiana. Most glass insulators have been dismantled in recent years, but these were still in the air as of September 14, 2017!

Telegraph pole with glass insulators along a railroad in Indiana. Most glass insulators have been dismantled in recent years, but these were still in the air when I took this photo on September 14, 2017!  Some of the insulators here include Hemingray No. 40, Hemingray-42,  Hemingray-45 (or Armstrong DP-1), and (the  oldest insulators on this pole) aqua “beehives” which are marked “H.G.CO.”

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, fruit jars and other glassware.

Most insulators are found in some shade of aqua (blue-green) colored glass (typical inexpensive “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 over the years, and insulators are found with a wide variety of embossed names, initials, patent dates, and other markings.


All glass pintype insulators are classified in what is called the “CD Numbering” system of identification. This system was instituted by N.R. “Woody” Woodward, an early pioneer, researcher and author in the field of collecting glass insulators.  The CD (Consolidated Design) numbers basically identify insulators by their shape and profile, regardless of exact embossed markings, glass color, or base type.

For instance,  “CD 154” is the CD number assigned to the most common style of glass insulator ever made, and that most likely to be seen by the average person……..the HEMINGRAY-42.  Several other glass companies besides Hemingray Glass Company made the same basic model, although their versions may have raised markings, color, and base types (smooth base, sharp drip points or round drip points) that  differ.  But all of them would be known as CD 154s.   Here’s a few other commonly found insulators:  Hemingray NO. 9 is classed as a CD 106.   Hemingray-10 is a CD 115.   Hemingray-12 is a CD 113.   Armstrong DP 1  is a CD 155. Hemingray-45 is also a CD 155.  Kerr  T.S. is a CD 129.  There are several insulator websites that discuss this classification system in more detail.

Nearly all insulator collectors who are affiliated with the “organized hobby” of insulator collecting use CD numbers for more clarity when communicating with other collectors by mail, email, text or phone.  CD numbers are used on many ebay auctions, although non-collectors who list insulators on ebay or other internet sales sites may not be familiar with the system.


Two early glass telegraph insulators made by Hemingray Glass Company at Covington, KY. They date from the 1870s or 1880s, and are marked "PATENT DEC 19 1871".

Two early glass telegraph insulators made by Hemingray Glass Company at their Covington, KY factory. They are the CD 132 style, and date from the 1870s or 1880s. The insulators are marked “PATENT / DEC 19 1871” with a “2” on the reverse. In this case the “2” served as the model or catalog number.

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 performed it’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.

CD 145 H.G.CO. / Petticoat insulator made by Hemingray Glass Company

CD 145 “Beehive” style telegraph insulator, marked:  K  / H.G.CO. //  PETTICOAT, made by Hemingray Glass Company, circa 1886-1895, in aqua.

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 bearing six, eight, 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.

Telegraph line pole with glass insulators

Telegraph line pole with glass insulators (possibly CD 145 and CD 133 styles). From undated real photo postcard, Joliet, Illinois, circa 1909.

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 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.


Because virtually all railroads (that were built in or before the 1950s) originally had communication lines strung alongside them (primarily telegraph, but also in some cases telephone), railroad buffs (“rail fans” or “trainspotters”) sometimes become interested in glass insulators since they were such a common sight along old railroad rights-of-way.

Railroads, trains, and telegraph lines and insulators have had a long history of being interconnected. Huge numbers of old photographs of railroads, trains and railroad-related scenes from across the United States show the telegraph lines with insulators in place – in their original “habitat”, so to speak.    And modern model railroading layouts sometimes include miniature telegraph poles with tiny “insulators” to help recreate the “look” of an earlier era.

Today, most railroads in the US no longer have a telegraph line next to the tracks.  But old-timers know what a beautiful sight it once was to look far down the railroad tracks and see a long, long line of telegraph poles, each adorned with glittering glass insulators, especially as seen against a setting sun!

Cleaning Insulators

Most insulators are typically found in a very dirty condition, often coated with a layer of gray or black “train soot”, or dirt and grime accumulated over many, many years of service in the outdoor environment. The underside surfaces in the skirt area are often heavily stained with a coating of stubborn train smoke.  Insulators installed along railroads typically are coated, to some degree, with black soot. Sometimes the layers of soil are so heavy that the true color of the insulator cannot be discerned.

Often, new and casual collectors don’t know how to clean insulators so the true beauty of the glass can be seen and fully appreciated.  Most serious insulator collectors do not keep their insulators in “as found” condition, preferring to clean them, although some collectors do keep a few pieces in their collection in their “originally found”, dirty condition just for more authenticity or “history’s sake”.

There are several ways to clean insulators, but the easiest (in my opinion) is to soak the insulator in a product that contains the active ingredient oxalic acid.  Oxalic aid is usually very effective in breaking down the stubborn layers of train soot which can be VERY difficult to remove with ordinary cleansers, soaps and detergents.  Oxalic acid is an ingredient in certain brands (but not all) of  so-called “Wood Bleach” or “Deck Cleaner”.  (Behr’s markets their version as “All-in-One Wood Cleaner” and it does contain this ingredient).   Usually, a one gallon plastic container of deck cleaner can be mixed with 2 to three gallons of water in a five-gallon plastic bucket. (DO NOT USE METAL CONTAINERS OF ANY TYPE!).  Oxalic acid can also be bought in crystal form (like a fine white powder).  Oxalic acid is technically a poison, so using plastic gloves when handling it is strongly recommended.  (Caution: oxalic acid cannot be used to clean Carnival Glass insulators, as the carnival coating may be dissolved).

The simplest and more inexpensive method is to use the product “Bar Keepers’ Friend” which also contains a diluted form of oxalic acid, and that product is sold in most large general department stores and grocery stores (usually stocked next to the cleaning products “Ajax” and “Comet”).  One can of BKF can be mixed into a 5 gallon plastic bucket of water (slightly lukewarm or room temperature), and the insulators are carefully immersed and left to soak for at least 24 hours. Two days would be better.  Since BKF contains oxalic acid in a more diluted form,  it may or may not be as effective as other products, but it is usually a reasonably good cleaner of glass.  After soaking, remove the insulators, using plastic gloves, and scrub carefully with NON-scratching cleaning pads or OOOO-grade steel wool.   Most insulators will respond well to cleaning with BKF, although some may not.   Some insulator collectors use lye as a cleaner, but I do NOT personally recommend it unless you are VERY, VERY careful and take ALL precautions as directed on the product label.

For a listing of some primary embossings found on glass insulators, and the glass factories that may have produced them, click here .

Detailed article on the “HEMINGRAY No. 9 / PATENT MAY 2 1893”  pony style insulator used primarily on telephone lines. 

Here are a few of my individual webpage articles on glass insulator companies: 

Brookfield Glass Company

Hemingray Glass Company

California Glass Insulator Company

Lynchburg Glass Corporation

Mclaughlin Glass Company

Whitall Tatum Company

Ohio Valley Glass Company

List of Glass Factories that manufactured Insulators

For a list of glass factories/companies in the United States that made electrical insulators, (or are believed to have made them at some time in their history), please click here to go to my page listing those companies in alphabetical order:  Glass Insulator Manufacturers

Please click here to go to my website HOME PAGE.

For a list of marks seen on bottles, fruit jars, tableware and insulators, click here to go to  my alphabetical listing:   GLASS BOTTLE MARKS (this points to page one).

Click here to check out my page on the so-called “Crackle Glass” insulators.

Page discussing recently color-altered glass, including insulators and  other types of collectible glassware: Artificially Purpled Glass.

Click here for a basic summary page on WHAT IS GLASS?.


These are sources of information on other websites. I would encourage anyone who has even a slight interest in glass and/or porcelain insulators to check out these websites!! Lots of good general information on Glass & Porcelain insulators, and the hobby of collecting insulators!! (National Insulator Association)  Great information and articles.  Rick Soller’s informational website—this site covers a lot of insulator-related “specialty” topics!   Elton Gish’s very cool website with extensive information on collectible porcelain insulators of all types.   This link points to one of the pages on, this page illustrating some of the markings seen on porcelain insulators. Great pics of some more unusual colored porcelain pieces!   This site is a project by Shaun Kotlarsky, and is a work in progress, showing pictures and info concerning all kinds and types of insulators!  Check it out!



10 Responses to General Overview on Glass Insulators – Basic Information

  1. Larry says:

    Thanks for taking your time to identify and provide dates for vintage insulators. Your site is very informative and i am going to bookmark the page for reference when identifying the varieties I come across. I am fascinated by old glass insulators and bottles and some of the early ones are really, really cool! I would like to find an early beehive (aqua, like the one pictured towards the top of the page) variety some day. Blessings!

  2. Sandy Bahler says:

    Hi there. Maybe you can help me. I have a dark peach Hemingray insulator marked with T.S. on one surface and on the opposite side there is 18-59: . I cannot find it online. I think TS is pretty common, but the color is a medium peach shade. I also have purple one Hemingray – 9. 8-48: My mom and dad moved out of their house and I found these.

    • David says:

      Hi Sandy,
      I am sorry to report that your “dark peach” Hemingray T.S. insulator has either been stained on the outside surface, OR it has been irradiated (“nuked”) to artificially change the color. Your insulator was originally clear or off-clear in color and is classed as a CD 129 in the “Consolidated Design” system of catalog numbers used by insulator collectors. You can search google with the keywords: Hemingray T.S. CD 129 insulator. Your insulator (with a date code of 59 plus two dots) was made in 1961.
      Your Hemingray-9 is classed as a CD 106, and has been stained with a fake outside purple color. It is also a clear insulator with a superficial coating. The date code (48 with two dots) indicates it was made in 1950.
      By the way, there ARE authentic purple glass Hemingray No. 9 insulators, but they date from the late 1890s or very early 1900s and do not have the same lettering arrangement as your comparatively modern number 9. All authentic purple Hemingray No. 9s have the embossing “HEMINGRAY / NO. 9” on the front and “PATENT / MAY 2 1893” on the reverse.
      I hope this will help.

      • Albert.K says:

        Can you give me some hints as to where I can find insulators/not from antique shops/flea markets. I mean out in the wild, if you know what I mean.

        • David says:

          Most insulators that are found “in the wild” are found along old telephone and telegraph lines or where they once stood. The most commonly-searched areas are along old, abandoned railroad tracks, where telegraph lines once ran. HOWEVER, most railroads nowadays prohibit trespassing on their right-of-ways, so searching along active railroads is discouraged. Other places to look: around abandoned buildings/houses; at yard, garage & barn sales; “junk” and curio shops; trading posts; rock shops (in the West); old trash dumps; near rivers, creeks, ravines where refuse was tossed; thrift stores; estate sales; farm auctions; Craigslist, etc.
          Hope this helps a bit,

  3. Pingback: Glass From The Past: Vintage Utility Pole Insulators | Randy Bowles Stories

  4. Diana Hobbs says:

    Trying to find out the value of a bottle in my possession. It is clear with two embossed hearts with corn husks surrounding them …tin cap…one quart bottle…numbers on the bottom as follows D-9 55-69 M328270. The 8 may be a B can’t tell and there is a 1 in a circle. I can’t find it in any books. It appears it may be a whiskey bottle possibly. The glass is wavy in appearance.

    • David says:

      Hi Diana,
      You have a whiskey bottle or decanter made by Owens-Illinois Glass Company. Please see my page on that company, and scroll down to the COMMENTS section; see the reply by “Carol” where info (link) on whiskey bottle codes liquor permit markings is mentioned. The “D-9” is a distiller code. (Any similar bottle with a “D-code” such as “D-9” or “D-126” can automatically be assumed to have held whiskey or some distilled liquor).
      The “55” is a liquor permit number assigned to Owens-Illinois Glass Company (specifically, their Huntington, West Virginia glass plant)………and the “69” is a date code for 1969, the year your bottle was made. the “I inside a circle (or oval) is Owens-Illinois’ logo/ trademark. For values, I strongly suggest to please check ebay auctions over a period of time and look for Similar bottles and their ACTUAL “Completed Auctions” prices. (Type in relevant keywords to bring up listings). Most of these types of bottles are currently very common and hold minimal interest to antique bottle collectors. Tremendous numbers of these types of bottles were made (many, many different designs) over a long period of time, especially in the 1940s-1970s, and no book will list most of them, (well, perhaps a small sampling), so I would not be surprised if someone cannot find this in a mass-market ‘bottle book’. I hope this helps,
      Take care, David

  5. William Cooley says:

    Cool! My brother just gave me 12. All are green and some are beehives! Very informative!

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