General Overview on Glass Insulators – Basic Information

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.

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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.

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“CD” NUMBERS

All glass pintype insulators are classified in what is called the “CD Numbering” system of identification. This system was created and used 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.

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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.

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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 threaded wooden or metal pegs, known as “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.

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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.

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Insulators (generally speaking) are still commonly in use, but insulators of the modern era (speaking of the United States in particular) are usually 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 discernible 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 “pin type” 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.

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MOLDS

Many, many different cast iron or steel 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.

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TRAINWATCHING & INSULATORS

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!

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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 if using 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 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.

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GLASS INSULATOR VALUES

As I’ve mentioned elsewhere on this site, my main purpose here is to concentrate on the background and history of the glassware being discussed, not on values or prices. This site is not intended as an appraisal source so little attention has been given to values. However, since the subject of values does come up often when people write to ask about insulators they’ve found, I feel that it is best to address the issue in this article.

All insulators, glass and porcelain, in my opinion, have some value. (Other collectors may strongly disagree with me on this point, but that is their prerogative). The value might relate only to the fact that a piece is an authentic relic of bygone days, or an object that can serve as a conversation piece or item of home decor.

The value may stem only from the fact that an insulator is a piece of United States history (or another country’s history) that is no longer being manufactured — a piece of “industrial archaeology”, so to speak.  Or that perceived worth may lie solely in its practical value as an insulator, or that it’s still perfectly good for other uses such as paperweights or candleholders. A heavily chipped, cracked insulator with almost no monetary value to collectors could theoretically still be perfectly useable in the practical sense (as a farmer’s livestock fence insulator, or installed on a short telephone line in a rural area, for instance).

The majority of the most commonly found types of glass pintype insulators have only nominal monetary collector value, perhaps 50 cents to a dollar. That includes the extremely common types such as the Hemingray-42, Hemingray-45 and Whitall Tatum No. 1 and others. However, even within the common types of insulators, slight differences in color, embossed markings or base type can make huge differences in the market value of a specific piece to experienced collectors. A notch above the most common types will be found many insulators that range in the 1 to 5 dollars value bracket. Others are worth hundreds, and some of the rarest known insulators (some “one of a kind”) are worth into the thousands or tens of thousands of dollars.

Again, it depends on a combination of characteristics including degree of scarcity, condition, demand, age, shade of color, attractiveness, and other factors. Damaged insulators, speaking strictly of monetary value, in most cases are worth only a small percentage of the value of an otherwise identical “Very near mint” (VNM) piece. Insulators placed for sale in retail outlets such as antique malls and flea markets will, by necessity, usually be priced higher simply because of factors such as overhead – sellers typically have to pay steep booth rental fees in such places as antique malls.

The most commonly used glass insulator price guide (used by most insulator collectors) has been published for many years, in several editions, first by John & Carol McDougald, and in more recent years by Don Briel.  A quick search of the internet will bring up sites where those books can be purchased.
There are other websites that discuss the general subject of values and prices. One tactic many collectors use is to do keyword searches on ebay or other internet sales sites such as Etsy.  In using those sites it is best to consider only the price of a piece that actually sold (by searching “Completed Items” on the advanced search page), NOT the starting prices, “Buy it now” prices or minimum bids, as these may be set by sellers who honestly have no idea on the actual value of the pieces they are selling, and so come up with a price that is unrealistic.

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For a listing of some primary embossings found on glass insulators, and the glass factories that may have produced them:  Glass Insulator Embossings. 

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

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Here are a few 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

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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

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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.

My webpage 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?.

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RESOURCES / GENERAL  INFO  CONCERNING INSULATORS

These are great sources of information posted on other websites.  I would encourage anyone who has even a slight interest in glass and/or porcelain insulators to check out these websites!!

Insulators.info. Lots of good general information on Glass & Porcelain insulators and the hobby of collecting insulators!!  A treasure-trove of articles and webpages on all aspects of collecting insulators!  This should be your first stop on the internet for insulator-related material!

https://nia.org. (National Insulator Association)  Great information and articles.

https://www.collectinginsulators.com.  Rick Soller’s informational website—this site covers a lot of insulator-related “specialty” topics!

https://www.r-infinity.com/   Elton Gish’s very cool website with extensive information on collectible porcelain insulators of all types.

Porcelain Insulators – Insulators.info   This link points to one of the pages on insulators.info, this page illustrating some of the markings seen on porcelain insulators. Great pics of some more unusual colored porcelain pieces!

https://allinsulators.com/   This site is a project by Shaun Kotlarsky, and is a work in progress, showing pictures and information concerning all kinds and types of insulators!  Check it out!

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