Aug. 11, 2025
By Dario DiMare
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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.
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.
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.
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.
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.
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.
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 .
Both glass and porcelain insulators have been used since the early days of the telegraph. Still, glass insulators were less expensive than porcelain and generally used for lower-voltage applications. The oldest glass insulators date from about .
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The period from to might generally be considered the "heyday" of the glass insulator. Hundreds of millions of these glass "bells" were produced by many glasshouses located primarily in the East and Midwest, with a few plants in California and Colorado. Many glasshouses that made insulators also made bottles, fruit jars, and other glassware.
Insulators were commonly made from affordable "bottle glass" or "green glass" that had a shade of aqua. However, other color shades were also available. Clear glass was more frequently used after . Over the years, numerous styles were developed, and insulators could be found with many markings, such as embossed names, initials, and patent dates.
Glass insulators came in a few primary forms depending on their specific electrical application. “Pony” insulators, often no more than 3 1/2 inches in height, were used for lower voltage lines. In contrast, “Power” insulators for high-voltage purposes were much larger, sometimes weighing more than 30 pounds. The most common style is the “Signal” variety for communication and secondary power lines. High-voltage “Cable” insulators, distinguished by their U-shape, accommodate thick cable placement. Whereas “Exchange” insulators were designed with multiple grooves to allow a primary line to pass through it and secondary branches to direct wire toward other locations.
Collectors classify all glass pin-type insulators in the "CD Numbering" identification system. This system was created and used by N.R. "Woody" Woodward, an early pioneer, researcher, and author in collecting glass insulators. The CD (Consolidated Design) numbers identify insulators by their shape and profile, regardless of exact embossed markings, glass color, or base type.
The CD 154 is the identification number given to the most common type of glass insulator, the HEMINGRAY-42. This is the type that most people are likely to come across. Although other glass companies also manufacture similar models, their versions may differ, such as raised markings, color, and base types (smooth base, sharp drip points, or round drip points). Nonetheless, all of them would fall under the category of CD 154s.
Here are a few other typically seen insulators:
Hemingray NO. 9 is 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.
Dating a vintage glass insulator is a straightforward thing to do using the CD identification system. These numbers describe the insulator’s shape and profile.
Examples include:
CDs 100-144 Have a side wire groove and no inner skirts.
CDs 145-184 have side wire grooves and one inner skirt.
CDs 185-188 are through pinhole styles.
CDs 185-205 are Transposition styles.
CDs 206-249 are saddle groove styles with an inner skirt.
CDs 250-279 are cable-style insulators.
CD numbers are helpful ways of groping glass insulators together into categories. But it’s important to note that they leave out specifics like embossing, markings, base styles, and color
Insulator collectors who are part of the "organized hobby" of insulator collecting commonly use CD numbers to communicate with each other. These CD numbers are often used in eBay auctions. However, people unfamiliar with the hobby may list insulators on eBay or other internet sales sites without using the CD number system. Several insulator websites discuss this classification system in more detail.
Earlier insulators had bubbles, streaking, “snow,” surface creases, and other marks of crudeness, which was typical for this type of glass because quality standards were not as high as tableware. As long as the insulator performed its 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.
Most glass insulators have embossings that include company names, brands, trademarks, model numbers, patent dates, and more. However, there are a small number of insulators that are unmarked. It may be surprising, but many insulators have been carefully and painstakingly marked with lettering that the average person below does not discern. These markings are only visible to linemen.
Glass pin-type insulators are common and usually only have a nominal value of around 50 cents to a dollar. Some well-known types include Hemingray-42, Hemingray-45, and Whitall Tatum No. 1. However, slight variations in color, embossed markings, or base type can significantly affect the market value of a specific piece for experienced collectors. Some insulators are worth between 1 and 5 dollars, while others are worth hundreds. Some of the rarest insulators, often one of a kind, can be worth thousands or even 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 tiny percent 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 in several editions, first by John & Carol McDougald and in more recent years by Don Briel.
To clean insulators, you can make a paste by mixing baking soda with water and, using a toothbrush, scrub the paste into the crevices of the insulator. There are two types of insulators: threaded and unthreaded. Threaded insulators may have wood residue where the insulator sits on a post. In contrast, unthreaded insulators may have a putty or glue that held them on the post. You can use the toothbrush and baking soda to remove whatever residue you can without damaging the glass.
Oxalic acid cleaners like Zud and Barkeeper's Friend can also be used and are available at your local grocery store. You can make a paste with oxalic acid and water and apply it on the glass insulator where there is a rust stain. Let it sit for a minute or two, then use a toothbrush to scrub it. Rinse the insulator with water and dry it.
You can use white vinegar to remove water residue and accumulation from years of weathering on the pole or lying in a field. Pour white vinegar into a deep bowl and soak the insulator overnight. The next day, use the toothbrush to brush its teeth and threads. Rinse the insulator with clear water to remove the vinegar.
Consider using specially designed display stands or shelves to showcase your glass insulator collection. Some collectors prefer grouping them by color, style, or manufacturer, while others opt for a more eclectic arrangement. When storing, ensure they are protected from accidental breakage using appropriate padding or individual containers.
Insulators can be found in various ways. Antique shops, flea markets, online auctions, and specialized insulator shows are great places to start. Rare antique and vintage glass insulators are widely available on eBay. The platform enables you to review the list of glass insulators for sale, check their many details, and find additional images of the ones that interest you.
Auction houses are a great place to buy untampered antique glass insulators. Some well-known auction houses include Bill and Jill Insulators, Manifest Auctions, and Hanford Auction House.
Connecting with other collectors through clubs, associations, or social media platforms can also provide valuable information, trading opportunities, and a sense of community
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