Glass Insulators ~ General Overview ~ Summary, History and ...
Jun. 05, 2025
Glass Insulators ~ General Overview ~ Summary, History and ...
Glass Insulators ~ General Overview ~ Summary, History and Background
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 . The electric telegraph (in the United States) was developed by Samuel Morse in , and the first message was sent by Morse in . The was invented by Alexander Graham Bell in .
You can find more information on our web, so please take a look.
As time went on, networks of “open wire” telegraph lines, and later, 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 .
The period from to 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 . 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.
“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, , text or . 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.
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.
Old photos from around the U.S. show many , telegraph and electric power poles sporting large numbers of insulators arranged on crossarms. Some 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.
During the s, s and s 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 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.
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 s) originally had communication lines strung alongside them (primarily telegraph, but also in some cases ), 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 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.
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 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.
Here are a few individual webpage articles on glass insulator companies:
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.
Link to ZheXi
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?.
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!!
Glass Insulators: Conducting fascination for over 100 years
OK folks, I’ll be honest—I don’t even know where to begin. Asking me to write an article on insulators is like asking a grandparent to write an article on their grandchild. A zillion pages later and I am still writing.
I will start off with just a little bit about me. I started collecting insulators in . I was actually digging for bottles in Ashtabula, Ohio, where I was born and raised, when I dug up an insulator with an date on it. Nobody knew what the date meant, but we knew the insulator was old. I was 10 years old at the time. I looked it up in the libraries but found nothing.
My first insulator, dug in Ashtabula, Ohio in . CD 131 Brookfield, Patent July 25, . See below to learn about CD numbers.
Six years later, I lied to my mother about sleeping over at my best friend’s house. With a new driver’s license in hand, I jumped in my $50 Pontiac Catalina and drove 400 miles to Washington, DC, to look up the patent at the Library of Congress in the National Archives. Gas was about 50 cents a gallon, and I had one loaf of bread, one jar of peanut butter, a moving blanket, and a half-gallon bottle of water (which was glass back then). So, by stealing the peanut butter and bread from mom, filling the water up in the gas station bathrooms, and sleeping in the back seat of the car, the total round trip to DC and back cost about $30. It took a lot of Dr. Kilmer’s and blob top soda bottles sold to the antique shops to get the $30. The July 25th, date was the patent for screw threads in insulators. I still have the insulator.
Now having owned as many as 12,000, and having handled more than ten times that amount, with thousands of hours spent hunting and researching, I feel comfortable writing a little about insulators.
Here is some fundamental information about insulators.
What are insulators?
Insulators are non-electrical conducting objects, usually made of glass or porcelain, intended to insulate the current running in a wire from grounding out, especially in fog or rain. Most often they are mounted on wooden pins on the cross arms of poles. If they insulate properly, the electric signal or current will meet its final destination in a safe and useful manner.
What are insulators made out of?
Most insulators in the U.S. were made of glass or porcelain. There are some composite, gutta-percha, rubber, and even wooden insulators. I will be speaking primarily about glass insulators since they are my specialty (with the exception of very early telegraph insulators which were made of various materials including porcelain).
Materials: Telegraph insulators made out of glass, gutta percha, composition, wood, metal, and one of the earliest plastics ever made.
Ramshorns: Very early ramshorn type insulators. The ramshorn itself is iron. They are set in glass, gutta percha, rubber, and composition.
Porcelain threadless: A very rare and diverse collection of threadless insulators made out of porcelain.
How old are insulators?
Glass insulators emerged in s America with the invention of the telegraph. The early telegraph insulators were mostly threadless, pin-type insulators. There were some glass blocks and ram’s horn types as well. The very first glass insulator, the bureau knob, was used by Samuel F. B. Morse on the line from Baltimore to Washington. The first electronic telegraph message in May of stated “What hath God wrought?”
Left: This is the first glass pin-type insulator first used in the s. CD 780, Bureau Knob. Right: The 780 and the glass blocks were the first insulators used on the telegraph and date back to the s. CD 780 and CD glass blocks.
The threadless insulators are kind of like pontil bottles, with a similar end date of about to when my buddy Louis A. Cauvet patented the threaded insulator. The threadless were also primarily used on telegraph lines, since the was not invented until , when Alexander Graham Bell said to his assistant, “Mr. Watson—come here—I want to see you.”
Threaded insulators were then made by the millions and used throughout the world. Many of the glass houses that made bottles made insulators as well. The last glass insulators were made by Kerr in the early s. Yup, the same guys that made the fruit jars.
Left: This is a pair of transition insulators. Like with some bottles, there are both pontil and non-pontil bottles using an identical mold. These insulators have the same outer mold and only the plunger forming the threadless or threaded pin hole are different. CD 736 threadless and CD 135.5 threaded E.R.W.’S. Right: This is the last glass insulator ever made. How depressing! Waaaaaah! CD 155 Kerr.
What color are insulators?
Put very simply, insulators are made in every color that bottles, china, and windows were made in. Back then, in almost all cases, the color did not matter. A lot of insulators were made from “end of the day” glass; instead of throwing out the batch of glass at the end of the day, glass makers would fill up insulator molds and sell them by the hundreds. Appearance was not a big deal with insulators. I have a few “crystal” insulators made in Sandwich, Massachusetts. Imagine turning one of these babies up-side-down and drinking champagne out of them so you could fit in with the bigwigs?
The shelf above has the complete rainbow of color on it: red, orange, yellow, green, blue, and purple. Like bottles, aqua is the most common color, but some of the rarest insulators are aqua. A few insulators were factory-coated with carnival glass or a flashed amber. The most sought after colors are cobalt blue, yellow, 7-Up green, and purple. Some of the purple insulators were originally made clear, but due to the sun’s effect on the manganese in the glass, they actually turned purple.
What makes a “good” insulator “good”?
Like with bottles, there are many factors that make an insulator “good” or more desirable. Here are a few of the factors:
Condition is a big deal with most insulators. There are fewer mint condition insulators than bottles, due mostly to the industrial use. Rare insulators in mint condition command a premium.
Color is very important. A $10 insulator commonly found in aqua can fetch you $10,000 in a rare color. And there are a few cases like in the CD 701.6 where the aqua insulator is worth five times more than the dark green CD 701.6.
I just had to put this in. The manufacturer of this CD 121 is R. Good Jr. out of Denver, Colorado. Pretty hard to argue about this being a “good” insulator. Ha! CD 121 Good.
Age also adds value. Most threadless insulators are worth more than threaded insulators. However, the ten most valuable insulators in the hobby are threaded. I am not being a wise guy, just being honest.
Embossing is also a major factor. Having just common embossing can add value. Having a rare embossing can add even more value. The CD 150 Brookfield books for about $500, and the CD 150 Barclay books for $5,000!
Rarity obviously adds value. I know of some CDs where I am almost certain that fewer than a dozen exist. They may have made hundreds or thousands, but to my knowledge, very few have survived, and the ones that have are very difficult to find.
Desirability is the big wild card. Some insulators are just more desirable than others. This makes no sense when you look at statistics, numbers, color, or age. Some are just flat out more desirable. I know of about five or ten CD 100.2s and CD 100.6s. They are extremely rare, and yet the CD 141.9, of which I know of about 20 or 25, still fetches two or three times more on the market.
The 100.2 is extremely rare and books for several thousand dollars. The 141.9 is not nearly as rare and books for three or four times more than the 100.2. (Please be careful if buying a CD 100.2. The CD 100 is very similar looking, and the CD 100 is very, very common, and valued at one dollar or less.) CD 100.2 Surge and CD 141.9 Emminger’s.
What is a CD?
CD stands for Consolidated Design and is the numerical designation used to identify glass insulators. U numbers are used for porcelain, and M numbers are used for multi-part porcelain insulators. N. R. “Woody” Woodward invented the CD system in the early s. He was an early collector and researcher, solely responsible for categorizing all of the glass insulators in North America. He partnered with Marilyn Albers to assign CD numbers to the foreign glass insulators. There is some logic to the numbering system with simple pin type insulators starting at CD 100 and ending at CD 350; the threadless claiming the CD 700s; and some of the block types and more unusual shapes reaching the s. When listing an insulator for sale we usually state it as CD#, name, color, condition, and price.
For example: CD 731, Tillotson, aqua with bubbles, mint $0.00
I was asked by N. R. Woodward to take over the CD assignments for the insulator hobby, so now I am responsible for assigning any new CD. We have the National Insulator Association (NIA) at www.nia.org as our national association and Insulator Collectors On the Net ICON at www.insulators.info as a great collectors chat and web site.
This is my favorite insulator which I dug up in New York in . Great condition and 1,444,444 seed bubbles, which I counted all by myself. CD 731 Tillotson.
If you have any questions about insulators, please feel free to contact me. Let me know your time limit, because I can ramble on forever about these stupid things. Happy collecting!
You can reach Dario by mail at Dario DiMare, 318 Main Street, Northborough MA . Give him a call with your insulator questions at (617) 306-. And, send your insulator mysteries and photos to .
This article appeared in the Beachcombing Magazine November/December issue.
Why are glass insulators used on power lines?
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