Glass Insulators: Conducting fascination for over 100 years

By Dario DiMare

rare antique electrical glass insulators

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 1967. I was actually digging for bottles in Ashtabula, Ohio, where I was born and raised, when I dug up an insulator with an 1865 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.

antique glass insulator

My first insulator, dug in Ashtabula, Ohio in 1967. CD 131 Brookfield, Patent July 25, 1865. 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 1962 Pontiac Catalina and drove 400 miles to Washington, DC, to look up the 1865 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, 1865 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 telephone 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).

antique insulators ceramic

Materials: Telegraph insulators made out of glass, gutta percha, composition, wood, metal, and one of the earliest plastics ever made.

rams  horn insulators vintageRamshorns: Very early ramshorn type insulators. The ramshorn itself is iron. They are set in glass, gutta percha, rubber, and composition.

ceramic electrical insulators antique

Porcelain threadless: A very rare and diverse collection of threadless insulators made out of porcelain.

How old are insulators?

Glass insulators emerged in 1840s 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 1844 stated “What hath God wrought?”

blue and grey glass insulators

Left: This is the first glass pin-type insulator first used in the 1840s. CD 780, Bureau Knob. Right: The 780 and the glass blocks were the first insulators used on the telegraph and date back to the 1840s. CD 780 and CD 1000 glass blocks.

The threadless insulators are kind of like pontil bottles, with a similar end date of about 1865 to 1875 when my buddy Louis A. Cauvet patented the threaded insulator. The threadless were also primarily used on telegraph lines, since the telephone was not invented until 1876, 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 1970s. Yup, the same guys that made the fruit jars.

unique antique glass insulators

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.

rainbow color electrical glass antique insulators

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.

purple antique glass insulatorI 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.

fancy antique insulators glass

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 1950s. 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 1000s. 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 as our national association and Insulator Collectors On the Net ICON at as a great collectors chat and web site. 

glass antique insulator with bubbles

This is my favorite insulator which I dug up in New York in 1990. 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 01532. Give him a call with your insulator questions at (617) 306-2420. And, send your insulator mysteries and photos to

This article appeared in the Beachcombing Magazine November/December 2021 issue.


Hi, I have a Cauvet insulator with the date July 25 1865. It does not have “ER” on it. I have been unable to find anything about this insulator. Can you help me? Thanks, J.Cook

John Cook May 19, 2024

What about a 7up green that don’t have any numbers just S.F. ??

Liz May 03, 2024

I see some insulators on old telephone lines here in California along the railroad tracks. Am I allowed to take them? The wires are dangling and clearly no longer in service.

Lisa Alexander February 20, 2024

I have an aqua one, I think its an insulator but it doesn’t match any of your photos. would you want to tell me about what it is if I send you a photo? all I know is its pretty.

sandi October 18, 2023

I just purchased two of the aqua colored ones would love any information you have on them

Tracy Tersigni June 16, 2023

In the DeLand Memorial Hospital and Veterans Museum, we have a permanent exhibit of insulators, all sizes, shapes, and colors. They are like jewels! The founder of our historical society was himself a linesman for Florida Power. I was touring a group today and they asked why the different colors. So, I found your helpful website! Fascinating! Now, next time I will be able to answer. Thank you all!

Suze Peace February 27, 2023

My parents, now passed, had a huge collection. My father who invented the Coleman cooler and lantern was allowed to mold some models out of brass and aluminum. I only have the two large ones left, shown in the picture. Can’t imagine today’s cost of making them. The brass one is very heavy. I have pictures if interested.

Guy Cooper January 29, 2023

Hi, Wayne! Feel free to email Dario with a photo of your insulator! He’s at –Kirsti

Kirsti Scott October 21, 2022

I have a backwards N on a Lynchburg insulator. This one is cracked. I also have. A Lynchburg with a written in correct N that is mint. Just curious. Thanks for your time.

Wayne Bartelme October 21, 2022

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