Insulators were originally designed to keep the wires linking telegraphs and telephones insulated from the wooden poles that held them aloft. In conjunction with the expansion of rural electrification in the early 20th century, there was a major boom in the manufacturing of insulators, peaking from the 1920s through the 1940s with production in the millions per year.

Commonly made from glass or porcelain in a dazzling array of shapes and colors, including amber, cobalt blue, olive green, and royal purple, insulators are prized for their rarity and physical beauty. In collector’s circles, clear and aqua shades are considered generic because they result from the natural iron content present in most glassmaking materials. Some insulators were embossed with a manufacturer’s name and style number using engraved molds, yet many insulators were produced without any markings whatsoever.

The earliest insulators were non-threaded “ram’s horn” and “glass block” designs, developed simultaneously with the growing telegraph line network initiated in 1844. Soon the pinhole or pintype style was established, allowing a glass insulator to be attached atop a wood or metal pin using cloth coated in a tar-like substance. Threaded insulators were first patented by Louis Cauvet in 1865 as a means for more securely fastening the glass objects to the tops of wooden pins. This design is still considered standard for electrical insulators today.

Use of porcelain and ceramic insulators spread during the late 19th century alongside higher-voltage electrical wires, like those required for home power lines, because the protective properties of porcelain proved superior to glass. Porcelain insulators were produced in a variety of colors to imitate their glass counterparts, ranging from deep blues and chocolate browns to bright yellows and greens.

Insulators are typically identified by manufacturer, including H.G. Co., Pyrex, Brookfield, and Hemingray, as well as the Consolidated Design (CD) coding system. CD numbers describe the production location and shape of pintype glass insulators, with 10 – 99 representing battery rest insulators, 100 – 375 for North American threaded insulators, 376 – 699 for all foreign insulators, 700 – 799 for North American unthreaded insulators, and 1000 – 1199 for all other miscellaneous styles, including glass blocks, spools, and dead-end insulators.

Within these broad categories, shapes are broken into many different variations describing the number of petticoats, drip points, skirt styles, wire grooves, and other features. Favored North American designs include the “Mickey Mouse” shape with two protruding glass ears and a saddle-groove form, as well as those made in lustrous carnival glass. The French created a distinctive “Gingerbread Man” form, with a rounded top and two pointed, upturned arms. The “T-Bar” design was also popular in Europe, and features outstretched, grooved arms, sometimes made with a square top, creating a robot-like resemblance.

Glass insulators were designed in a few basic forms depending on their specific electrical application. “Pony” insulators, often no more than 31/2 inches in height, were used for...

One of the most prolific insulator manufacturers in the U.S. was the Hemingray Glass Co. Incorporated in 1870, the firm operated factories in Kentucky and Indiana, where it also produced other glass items, like bottles, fruit jars, kerosene lamps, and glassware. Hemingray created the largest stylistic variety of any manufacturer and purposefully designed colorful insulators, as opposed to the typical practice of recycling leftover batches of colored glass. The company's famous Hemingray No. 42 remains the best selling insulator of all time.

About our sources | Got something to add?

▼ Expand to read the full article ▼

Best of the Web (“Hall of Fame”)

Christian Willis' well-organized database of Hemingray insulators. Browse over 300 photos by CD number, style, or s… [read review or visit site]

Shaun Kotlarsky's tribute to the Hemingray Glass Company. While known for its insulators, this site also showcases … [read review or visit site]

Bill Meier's reference site by and for insulator collectors includes event listings, reference information, photo g… [read review or visit site]



For an explosion of color and a quick education on the many beautiful varieties of insulators, blown glass bricks, … [read review or visit site]

Alloy Artifacts

Alloy Artifacts

A tool collectors’ dream, this site is a deep repository of photos and info on 20th century hand tools and the co… [read review or visit site]

Vintage Machinery

Vintage Machinery

This collective website, which started as an online discussion forum, also includes a large database of manufacture… [read review or visit site]



Josh and Cam Larios have created this site enabling people to upload and 'tag' photos of historic or artistic manho… [read review or visit site]

A Millers Falls Home Page

A Millers Falls Home Page

Randy Roeder has carved out a niche for himself with his fine website devoted to the history of the Millers Falls C… [read review or visit site]

Clubs & Associations

Discussion Forums

Other Great Reference Sites

Most watched eBay auctions    

Antique Telegraph Wire Insulator Wood Cap Over Aqua Glass Civil War Era Royal Purple Cd 162 Antique Western Glass Telegraph Insulator, Scarce!Nice True Bright Green- Not Aqua Chambers Insulator- W/1870's Patent Dates Nm!"mighty Fine" Hemingray "mickey Mouse" Power Glass Insulator W/ Fangs & Fizz!Dark Emerald Patent Feb 5 1850 Early Lri Lightning Rod Insulator Very Rare!!Dark Olive Green Cd 674 Isorex/248 Foreign Antique Glass T-bar Insulator Rare!Rare Otis Lightning Rod Insulator With Metal ProngsCarnival Cd 322 Corning Pyrex 233 Antique Glass Telegraph Insulator, Scarce!!E.c.m. Co.Stellar Light Purple Light Smokey Cd 208 California InsulatorHemingray High Voltage Triple Petticoat Eared InsulatorVintage California Glass Insulator Purple Amethyst Light Purple Cd 522 6118 (fidenza) Antique Foreign Glass Telegraph Insulator!!Cd 106 Two Tone Purple Hemingray No 9 Patent May 2, 1893- Crude Old Script Nm!Light Honey Amber Cd 162 H.g. Co. Petticoat Insulator Pat. May 2 1893Knowles Cable Insulator Milky Swirls Bubbles The M & E Co Philadelphia Dog EarHemingray H.g. Pat May 2 1893 Medium Aqua Glass Insulator ExcellentAntique/vintage 'b' Dec. 19 1871 Aqua Blue "bullet" Glass Threaded InsulatorH. G. Co. Pat May 2 1893 Insulator Aqua Color Cd 196Carnival Cd 168 Hemingray D-510 Antique Glass Telegraph Insulator, Great Shape!Macomb U-42 Mottled Light Brown Porcelain Insulator MintTelephone Pole Glass InsulatorLight Green Cd 190/191 Diamond Two Piece Transposition InsulatorNo 4 Cable Deep Saddle Groove Power Insulator*mint* Aqua 1893 Glass 14 Lb. Two Piece Muncie Type InsulatorF.m. Locke Victor U-38 White Glaze Porcelain Insulator Mint - Rare Skirt Marked Unique Bubbly Lavender Zicme InsulatorHemingray No 60, Cd-257 Mickey Mouse Vintage Glass InsulatorBeautiful Carnival Glass Hemingray Cd-168 Signal Insulator...nice!Gorgeous Green Postal Glass Insulator With Bonus!Green Cd 143 C.n.r. W/prism Periods InsulatorCd-162 W.f.g. Co. Insulator - Pdark UrpleNice Hemingray Blue Cd-241 Hemingray - 23 Glass Cable InsulatorUnmarked U-425a Cobalt Blue Porcelain Insulator MintDark Olive Green Cd 152 Brookfield Glass Insulator Dead Mint!!Vintage W.g.m. Co.sun Purple Glass InsulatorWhiskey Amber May 2 1893 Petticoat Insulator Light Sage Green Glass Piano Insulator 1800's Great Condition!! Very Rare!Hemingray 53 Cd 202 Dark Aqua Blue, Free Shipping!!Vintage Telephone Pole Glass InsulatorHemingray 50 Two-piece Two-tone Glass Transposition InsulatorTelephone Pole Glass InsulatorSpecial Issue Cobalt Blue W/milk Cd 726 Canada 2000 InsulatorColumbianTough Cd Yellow Amber Bar 1 Side/j Other Side- Superb Cond- Real Hard 1 To Get!Tough Stunning Cobalt Blue U-474 Findlay Porcelain InsulatorTelephone Pole Glass InsulatorBlue Trio Of Powerful Glass Distribution Insulators!Cd 102 Canadian Insulator No Name Light Purple Color Rtic Stainless Steel Can And Bottle Cooler Colster KoozieVintage Pyrex Amber Orange Carnival Glass Insulator No. 63Honey Amber Hemingray Lowex 660 Insulator Cd 219 Mint ConditionSpecial Issue Bubbly Dark Green Cd 740 Style Canada Glass Works InsulatorBlue Aqua Cd 251 Hemingray-61 Made In U.s.a. InsulatorSpecial Issue Bubbly Dark Cornflower Blue Cd 718 Telegraph In Canada InsulatorVintage Hemingray Refrigerator Water Bottle, 1 Qt. 1932, Depression Glass GreenVintage Hemingray 60 Mickey Mouse Clear Glass Insulator W/wood Pin3 Antique/vintage Glass Insulators Stunning Emerald Green Cd 152 Brookfield InsulatorRare "hemi Blue" Cable Insulator Cd 259 Mint Condition