A Brief History of Porcelain Insulators

From the start it should be noted here that this is a very brief description of the basics on porcelain insulators, there are several excellent publications listed in the Reference Books section that are a "must" for any serious collector. Similar to the glass CD (Consolidated Design) number, porcelain pin-type insulators can be identified by U (Unipart) or M (Multipart) numbers. Uniparts are single pieces of porcelain or multiple pieces that have been glazed together (Glaze-welds). Multipart insulators have two or more sections that are cemented together.

Porcelain insulators had their start when local potteries began making telegraph insulators in the 1850's and 1860's. These crude early pieces were usually threadless and were produced in much lower quantities than their glass counterparts, and few have survived the years. Some more notable examples are the "Elliot" flint insulators (U-980 and U-981), Bennington pottery insulators complete with the unique Bennington glaze (U-982 and U-983), as well as a porcelain version of both the "Teapot" (U-990) and the "Slash top" (U-988).

Porcelain saw very minimal use for either telephone or telegraph after the development of threaded glass insulators. Glass tended to be cheaper than quality porcelain, and was deemed better as it was thought that clear glass would discourage insects from building nests under the insulators reducing their effectiveness. This all would change with the proliferation of electricity.

With the advent of electric power distribution in the 1880's, larger and more reliable insulators were needed to carry the higher voltages of power lines, in the tens of thousands of volts. Glass at that time was not sufficient, and one man in particular, Fred M. Locke of Victor, NY, was convinced that porcelain was a superior insulating medium and began experimenting with clay mixtures that would produce finer insulators. In addition he was working to create new insulator designs that would hold up to high voltage service in all weather conditions. Fred contacted several companies to help him with his experiments, and one offered assistance. That was Imperial Porcelain Works of Trenton, NJ.

Dry Process Porcelain

Several other companies had started making insulators from "dry press" porcelain, so-called because the almost dry porcelain in granular form was pressed (dry) into 2 or 3 piece steel molds. These insulators tended to be porous and they were practical only for lower voltages and in drier conditions. Dry process insulators, where unglazed, appear grainy and may have dark lines where dirt has entered the tiny cracks and holes in the clay. Some typical examples of dry process insulators include U-701, U-744, U-928B, and U-935A, as well as a multitude of smaller signal styles made for low voltage use. Often there is a discernible mold line visible on the insulator body. Fred Locke thought, correctly, that wet process porcelain was by far superior and could withstand higher voltages. See Fred Locke's Biography for additional information.

Wet process porcelain is mixed wet, plunged in a mold, threads plunged, shaped on a wheel or turning machine, then dried, glazed, and fired at very high temperatures. The porcelain, when properly made, lacks the tiny cracks and voids found in dry process porcelain. The key is to remove the air from the clay during mixing.

This was difficult to accomplish and fortunately for Fred, Imperial was expert at doing this. Imperial produced a number of Fred's early insulator designs, including U-937 , The "Helmet". This insulator was developed for use on the Niagara-Buffalo transmission line in 1896. This early line was the first to transmit power from Niagara Falls, and therefore progress was monitored by utilities everywhere as a model. A number of insulators were submitted for use on the line, and all but Imperial's were dry process porcelain. The insulators were tested in brine at 40,000 volts prior to installation, and ultimately only the wet process Imperials passed the test. Niagara helped spell the end of dry process porcelain in high voltage use. Both Fred and Imperial Porcelain continued to struggle with the total elimination of air from the wet clay, which is evident in the bubbles and air pockets that are seen in pieces that have been sliced with a lapidary saw. Vacuum processes were soon developed to eliminate this problem.


A number of other insulator manufacturers tried to make wet process porcelain themselves, including Fred Locke. At this time, a thick solid section of porcelain was difficult to produce; Imperial being the only expert in the technology. As a result, "glaze-welding" came into use. An insulator was produced in 2 or 3 "shells", or pieces. These pieces were glazed and then stuck together and fired. The glaze would "weld" the pieces together, producing one insulator.

Glaze-welding hit its zenith in the late 1890's and early 1900's and was used by all major insulator manufacturers except Imperial. Some examples of glaze-welded insulators are U-928, U-928A, U-926, and U-966. For identification purposes, these are still considered one-piece, or unipart, insulators.

As voltages increased, insulators became larger, and technology developed, glaze-welding fell out of favor and was replaced by larger uniparts, and even larger multiparts. Multipart insulators are made in 2, 3, or 4 sections, or shells, and are cemented together to form one unit. These appeared around 1901 and continue to be produced to this day. Generally, unipart insulators are used for voltages up to 34 KV. Two-piece multiparts are used in the 20 to 40 KV range, three-piece insulators are used in the 40 to 69 KV range, and four-piece insulators are used for the 60 to 80KV range.

In the 1940's a new type of insulator was developed by Lapp Insulator Company: the "Post" insulator. Until this time, insulators has been screwed onto wood or metal pins mounted in the crossarm to secure them. Post insulators are one-piece solid or hollow core porcelain columns with multiple petticoats or skirts from top to bottom. They are mounted directly on the pole or crossarm and bolted in place. Their performance was superior to pin-type insulators, and by the 1960's they found widespread use on power lines everywhere. They have never completely replaced pin-type insulators, except for medium voltage lines between 40 and 70 KV. Several "mud" collectors specialize in these large insulators.

From around 1915 on, the porcelain insulator virtually replaced glass on all electrical distribution, even at low voltages, as the superiority was demonstrated in both insulation quality and strength. To compete, some glass companies (Hemingray and Whitall Tatum) tried to mimic porcelain with amber insulators in the 1930's and 1940's! The smaller porcelain distribution pieces, although typically brown, were also provided in many colors. These were provided as markers (Green for series street light circuits, Yellow to denote a power line on a telephone pole, etc.) or to identify different utility company lines or circuits. An incredible number of colors have been found and will rival the most colorful glass display!

In addition to the North American insulators described above, there is a whole world of other unique and fascinating porcelain. Many foreign countries like Britain used porcelain almost exclusively, even in the telegraph days. Much of the early foreign porcelain was of much better quality than the American counterparts. Many unique styles and colors are available. See the book Worldwide Porcelain Insulators for additional information.

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Last updated Sunday, December 24, 1995