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 Cd 701.6[010] "confederate Egg" Glass Insulator In Olive BlackglassOrange Amber Hemingray-19 Glass InsulatorRoyal Purple Beehive Insulator InsulatorFlat Out Yellow "frog Eyes" Lowex Glass Power Distribution InsulatorBeautiful Almost Mint Cd 162 Deep Amber Hemingray 19 Signal InsulatorStoneware Pottery Spongeware Insulator On Yellow Ware From Central IllinoisAntique Cd162[090] Hemingray-19 7-up Green Glass Insulator.Threadless Olive Amber Blackglass Cd 734.8 Insulator No EmbossingMulford & Biddle U.p.r.r. Threadless Railroad InsulatorCd 742 Threadless Insulator With Amber And Milk SwirlsAntique Star Cd113[010] Yellow Green Glass Insulator With Rare "elmer Ring"Homer Brooke's Pat. Cd-133.1 Aug 14 1882 Brooke's Blue Glass Insulator ExcellentNice! Blue! Chicago Insulating Co. Cd-135 Odd Shaped Glass InsulatorPeach/straw Cd 134 W.f.g. Co. Glass InsulatorGorgeous Purple Great North Western Telegraph Co. Cd-145 InsulatorAntique Cd136 B&o Railroad Insulator Pat. Dec19 1871 Ice Green Steely - Delft Mix Cd 121 W.f.g. Co. Glass InsulatorBeautiful Almost Mint Deep Yellow Amber Cd 152 B Embossed Hat InsulatorBeautiful Purple Whitall Tatum Cd-154 Vintage Glass InsulatorBright Violet Purple 1893 Signal InsulatorThree! Nice Canadian Pacific Railroad Co. Cd-143 Glass InsulatorsPretty Essentially Mint Sca Beehive Insulator H G Co Cd 164 Jade Milk InsulatorBig Beautiful Hemingray Cd-303/310 Two Piece Muncie Type Power InsulatorPurple Cd 145 G.n.w. Tel. Co. Glass Insulator With Ghost EmbossingCd 233 Pyrex Marigold Carnival Glass Insulator No. 661 - Mint!No Name Porcelain Insulator, G Marking On DomePurple Cd 106 InsulatorFlat Top Kerr Green Tint Vvnm Glass InsulatorCorning Pyrex Orange Marigold Carnival Glass Insulator #662 4Glowing Flash Amber Hemingray-62 Glass Insulator@@@ Great Color Locke Insulator - Nice Example @@@Super Glowing Cd 154 Orange Amber Dominion 42Antique Cd121[020] Aqua No Name Glass Insulator With Nice Pleated Skirt. Prism Cd 280 Aqua Glass InsulatorTeal Green No 3 Cable InsulatorBig! Pyrex 441, Cd-327 Glass Power Insulator...great Straw Color!Royal Purple Cd 121 Canada W/blot Outs Toll InsulatorBright 7-up Green Cd 122 Hemingray InsulatorPostal Tel Co W.brookfield Cd 138 [020] Glass Insulator4 Brookfield Cd101 Mushroom Top Olive And Amber Streaks Dark Emerald Green Cd 164 Mclaughlin-20 Glass InsulatorTeal Green Early 1871 Hemingray Bullet 2 InsulatorCd 230 Yellow Amber Lowex 512 With Rock In DomeBeautiful Blue Aqua Cd 302 Hemingray Ohio Powerline Insulator Three Nice Cd-145 Glass Insulators...three Very Nice ColorsAntique Cd136 Brookfield B&o Railroad Insulator 45 Cliff St.ny Light Blue Shop 4Lot Of 17 Glass Dogbone Insulators Radio Strain Insulator Steampunk Knox PyrexAntique Cd102[060] Diamond Glass Insulator In Dark Yellow Green.Nice Ice Blue Hemingray-62 Cd 252 [020] Glass InsulatorBright Cd 235 Corning Pyrex Carnival Glass InsulatorAntique Glass Carnival Insulator Cd 142 Hemingray W.u.t. Co./ts-2 Copper InsertLight Blue Cd 259 Cable InsulatorBrookfield Marked B Cd-145 Light Purple Sea Glass Insulator ExcellentPurple Cd 422 Agee Australian InsulatorVintage Imperial Porcelain White High Voltage Powerline Insulator Dated 1898Ohio Brass Ob U-65 Blue Porcelain Insulator ExcellentVintage Brookline "green" Glass Insulator And Mounting BracketBright Carnival Cd 234 Pyrex InsulatorCalifornia Cd-161 Plum Glass Insulator Mint