Glass Insulators: Conducting fascination for over 100 years

By Dario DiMare

You can find more information on our web, so please take a look.

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.

Link to ZheXi

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 .

A. Hemingray insulators were used for a variety of purposes, but the primary goal was always the same: to insulate an object from another object, whether that was a pole, telegraph pole, power line, lightning rod, or battery platform. Hemingray made insulators for all of these different applications, and you can see them all in the database along with their original intended usage. If you're looking for a specific usage type, you can also look it up using the Hemingray Database Advanced Search feature.

A. If the number appears directly after "Hemingray-" (such as Hemingray-42) or appears directly after "№" (the abbreviation for numero, which is Latin for number), that is called the insulator's style number. Some Hemingray styles are comprised of letters, words, or a combination, such as: T.S., E2, № 1 Provo Type, Muncie Type, etc. View all Hemingray insulators by style.

Hemingray insulators produced between and also have mold numbers and date codes, such as "20-46::". Refer to this page for more information on how to decypher these numbers.

A. I clean most* of my dirty insulators in a 2 gallon plastic paint bucket filled with a mixture of water and oxalic acid (wood bleach crystals). A good ratio is about 0.5 pounds of oxalic acid for every 1.25 gallons (20 cups) of water. This mixture lasts for many years (the mixture in my bucket has been used for over 10 years and has cleaned dozens of insulators with no problem). Using rubber gloves, place your dirty insulators carefully into the bucket and let them sit for several days (or weeks if you're not in a hurry). Once they're ready, remove the insulators, rinse them off under cool water, and use 00 or 000 steel wool (or even synthetic steel wool works) to get off any remaining soot. Especially stubborn insulators can go back into the bath for another round.

There are many important notes to observe before working with oxalic acid!

• *Never use oxalic acid to clean carnival glass, flashed amber, opalescent glass, or insulators with metal pieces attached! You will destroy them.
• Add the oxalic acid crystals to the water (not vice versa).
• Use a plastic bucket (not metal). Oxalic acid is corrosive and will eat away at metal.
• Always wear eye, face, and hand protection. Even though it's a mild mixture, be safe.
• Keep the bucket out of reach of children and pets. Always keep a lid on it when not in use, and label the bucket.

This is just one method of cleaning your insulators. For more information on other insulator cleaning techniques, please visit Insulators.info. Please note: I assume no responsibility or liability for any misuse or damage caused by the information above.

A. Those "bumps" are called drip points. Hemingray patented drip points on May 2, as "teats" for helping to draw moisture off of the insulator. Moisture was undesirable as it could cause leakage on the line due to the water acting as a conductor. Hemingray produced two styles of drip points: The first design used from the s through the s are referred to in the hobby as "sharp drip points," abbreviated "SDP". The second style, introduced in the s and used through the s are "round drip points," abbreviated "RDP". Some Hemingray insulators in the s through s were also produced with a "corrugated base" (abbreviated "CB") which appears as a rough, cross-hatch style base. This is not considered a form of drip point, but essentially served the same purpose. Many Hemingrays were also commonly manufactured with a smooth base (abbreviated "SB").

A. Hemingray ceased operations in the 's, so if you're looking to buy Hemingray insulators to start your own collection, I would highly recommend joining the National Insulator Association and looking for an insulator show in your area. This is the best way to get in touch with other insulator collectors. You'll also find that we insulator collectors are very generous and might even give you an insulator or two to get started! We love encouraging new collectors in the hobby. eBay is also a good place to find Hemingray insulators. Though you'll probably be paying more for shipping, if you aren't able to make it to a show, it's often a great alternative. I also occasionally post insulators on my For Sale page.

A. If you have an insulator that is on my Wanted List that you're looking to sell, please contact me!

Otherwise, if your Hemingray insulator is worth $5 or more, eBay is generally a good place to start. On the other hand, if you're looking to sell a box full of very common $1 insulators, you're better off just donating them to a local antique/junk/thrift shop, giving them away to kids, or using them as indoor/outdoor decorations. I recommend performing a search on eBay to see how many other people out there have the same insulator you do and how much they're selling for (and, more importantly, if there are any bids!)

For more information, please visit transparent glass insulators.