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.

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