This guide is designed to give you a basic introduction into collecting glass and porcelain insulators. Specifically I will assist you in how to identify them, determine their age, value, rarity and condition so that you will be well prepared prior to spending hard earned cash on a purchase.

 Insulators were first manufactured on a large scale in the mid 1860's to provide insulation to the telegraph lines that were rapidly expanding across the United States, Canada, England and some remote areas of Russia. Later they were needed as electricity and telephones made their way across the American frontier, eventually spreading thru Europe and the rest of the world. Insulators come in many shapes, sizes, colors and were made of many different materials including rubber, porcelain, pottery, glass, composition, fiberglass, metal, wood, mica, etc.

Glass insulators are categorized by collectors using a Consolidated Design (CD) System that was created by N.R. Woodward. Numbers begin with 100 and go up to 1140 for insulators made in North America are recorded in the book "Insulators - A History and Guide to North American Glass Pintype Insulators" by John & Carol McDougald. Insulators manufactured outside North America have also been assigned cd numbers and are recorded in the book "Glass Insulators From Outside North America" (GIFONA) by Marilyn Albers and N.R. Woodward. When a "foreign" insulator's style matches one already assigned a cd number in North America, the number stays the same in both books. Early pintype insulators made in the late 19th century were produced without internal threading and were secured to pins with creosote, burlap, rope, wood fibers, parafin, etc. Threadless examples are currently assigned CD numbers by Ray Klingensmith.

Glass insulator markings. Insulators are usually found marked with a manufacturer or users name embossing. For example, Brookfield marked insulators with their name and others are found only with a user name such as AM TEL & TEL Co which was frequently used on their telephone "toll" lines. Sometimes electrical parts suppliers had their names embossed on insulators they sold but they were made by another manufacturer. A rare example of this is found on a spool marked M.A. Knapp Chicago that was most likely made by Hemingray. 

Generally, most insulators are marked on their crown or skirt. Base embossed examples are uncommon and in some cases extremely rare. Any marked GE Co or Boston are highly desirable.

Rarity. Some of the more common names found on insulators include Armstrong's, Brookfield, California, Diamond, Dominion, Hemingray, Kerr, Lynchburg, Maydwell, NEGM and Whitall Tatum. Due to limited distribution some of these are uncommon in one region of the USA or Canada but could be common in another part. There are probably a couple dozen unique examples of insulators that were created as prototypes that to our knowledge did not gain favor with manufacturer's due to their complex designs making them too expensive for the competitive marketplace. If you were lucky enough to find one it could upon verification be worth thousands of dollars. Some other rare examples were tried on remote power lines and were found to be defective and were discarded as obsolete. The large high voltage insulator marked "Locke 25" is an example of this. There is a unique toll insulator made in Australia marked PMG is another good example, with only a few known to exist in collections today.

Porcelain insulators are assigned numbers based on whether or not they are made as one complete unit (unipart) or two or more parts cemented together (multipart). Unipart porcelain and ceramic insulators are assigned U-numbers and multiparts are assigned M-numbers. North American porcelain insulators are assigned numbers by Elton Gish. There are books available for sale online that can assist you in identification and values, so I will only provide a basic overview. There are a wide variety of insulators made of ceramic, porcelain, stoneware and other related materials. Collectors primarily focus on pin-types, johnny balls (guy wire strains), suspensions and radio strains. Marked examples in colorful glazes or early, classic styles in good condition are often worth a premium. Some names to look for: Johns-Manville, Cook, Fred M Locke, Lima, New Westminster, Varley and Jobson Bros.

Age. Your first clue can be determined by looking for any marking stamped or embossed on its surface. Often you will find a manufacturer's symbol and occasionally the country of origin and its date. For example, if you find a glass insulator embossed "Hemingray - 42" "Made in U.S.A." then you know that you have an American made glass insulator. The no. 42 signifies the manufacturer number. This happens to be the most common style made by the most prolific producer of glass insulators in the world with millions exported around the world. This style was so successful that it has been copied by numerous manufacturers in Mexico, Canada, South America, Spain, and Italy. Some of these copies can be found in spectacular colors that were made in limited quantities and can therefore command a special premium.

Sometimes the approximate age can be determined by a patent date on the insulator. These dates were often recorded to give them prestige and a competitive advantage over other manufacturers. As a result, they were often produced for many years after their original issue date. Sometimes molds were sold to another company and the dates and names were blotted out. Most purple insulators were made prior to World War II due to a shortage of manganese needed for military applications. Exceptions to this rule are found in insulators made in countries outside of North America such as Italy and Colombia and made in the 1950s-60s.

The last glass insulator manufacturer in the USA was Kerr, located in Millville, NJ (they purchased the former Whitall Tatum factory and molds) with the final production ending in 1979. Currently Lapp and Victor are the last of the major ceramic/porcelain pin-type insulator manufacturers still in business in the USA.  Both now import large quantities of their insulators but still retain production of pin-type spools, radio-treated high voltage and other styles.

Value. This is determined by many factors such as condition, manufacturer, quantity made, demand from collectors, color, age, origins, etc. A very near mint (VNM) insulator will be one that has only minor blemishes that hardly detract from its original factory condition. Some manufacturers are more desirable than others due to their limited production or availability, and therefore higher demand. For example, insulators marked Greeley, Chester, O'Brien Patent, EC&M, Tillotson, Liquid, Zicme, etc. Certain styles, such as threadless pin-types and glass blocks are known to be among the earliest styles made. A recent craze among collectors includes finding insulators with abnormal "junk" or other foreign materials inside the glass. Unmelted metallic objects, huge bubbles or heavy colorful swirls now command a premium price to collectors. Due to ongoing fluctuations in values I will not attempt to post values for specific insulators. If you have a question regarding a specific insulator I'd be happy to offer assistance in this area.

Color. This is one of the key factors affecting demand and therefore value of an insulator. Most desirable colors include shades of yellow, red, cornflower blue, light amber, cobalt, olive green and purple. The most common colors of glass insulators are crystal clear, green and shades of aquamarine. However, some insulators in these colors are very rare and can command a premium. The most common porcelain colors are white, dark brown and skytone. Mottled glazes, fingerprints, bright colors, early threadless and old insulators from historical power lines are the most prized.

Radio Antenna Strains. These were used to insulate and secure antenna wires on homes, businesses and even military ships. They are typically found in glass and porcelain but are also found in bakelite, plastic and a combination of materials depending upon their application. They can be found in a rainbow of colors and because they don't take a lot of space one can accumulate a rather impressive collection without taking over too much space in your home. Values on these continue to rise, especially for the rare colors. Occasionally collectors have dug a factory site and uncovered a cache of spectacular examples such as those found at the Alley Agate site.

Most examples of glass radio strains are not marked so finding one marked usually makes them more desireable. Some names to look for are "G" (found on strains made by Zicme), and there is a rare example glass strain made in England by the British Heat Resisting Company in their Phoenix Glass Works for the Royal Air Ministry during WW2. These are marked with an "A (Crown Logo) M" and a part number. Some heavy milky and Bubbly Green examples have been found in Mexico. Pyrex strains have been made and licensed for manufacture and sales globally. Some scarce examples found in England are marked JP for Jobling-Purse, another is marked "Modele Depose" (Registered Design) for use in France.

In the photo, the insulator in the far left is a beautiful cd 154 Ice Blue Cristalerias Vidrart from Argentina, the middle one in the back is a cd 128 Hemingray CSC in a spectacular Opalescent, the far right is a super rare (sim U-1718) Fire Engine Red Bullers cable top porcelain (1 of 3 known) from England, the small dark aqua spool in front is made by Hemingray in the USA and the smoke spool was handmade by Zicme (4 known in Smoke, 2 in Off-Clear) in Bogota, Colombia.

Altered. This is a growing area of concern to collectors as many items are now appearing for sale that have been modified from their original manufacturer specifications. Craftsmen have altered insulators for many purposes by securing objects to the top, by applying colorful stains to an insulator surface, by boiling them to obtain a crackle glass appearance, by radiating or baking them to change the color. That are pretty to look at in the window but are not particularly valuable. Be sure to confirm the authenticity of an insulator's color or shape before you make an expensive purchase, especially if you are collecting them as a potential investment. Beware of any "muddy coffee brown" colored glass insulators as many are irradiated and/or stained. There is no such thing as a crackle glass insulator made by an insulator manufacturer. The cracks would obviously defeat the purpose of the insulator. Instead these are made by hobbyists and other opportunists who enjoy them as a decorative craft item and a few peddle them as authentic.

Books. It is highly recommended that you educate yourself by purchasing the insulator books "Insulators, A History and Guide to North American Glass Pintype Insulators" by Carol & John McDougald and Glass Insulators From Outside North America (GIFONA) by Marilyn Albers & N.R. Woodward. I am working on a supplement to GIFONA and would welcome any research catalogs, photographs, or information on worldwide manufacturers of glass or porcelain insulators. All contributions will be acknowledged in the book.

Reproductions and Commemoratives. There are a growing number of reproduction insulators showing up in the marketplace. The latest trend seems to be in the area of miniature insulators as they are easier to make and in many cases not easily mistaken for an authentic insulator. The NIA celebrates (and helps fund) it's annual national show by the producing and selling a commemorative glass insulator (1970-today). This is easily identified by the embossing on its base. There are other commemoratives being produced that are not officially sanctioned by the NIA yet look strikingly similar to an original. Some are marked Hemingray-19 and have "NAJ" etched and/or embossed on them. The NIA considers this an imitation insulator as it did not meet their approval primarily because it is too close to the original and could be mistaken for an authentic and expensive original. Some miniature "salesman samples" are modern reproductions. Usually they can be identified by their weak embossing and poor annealing job (often laden with surface cracks but not always). Some threadless "fence post" insulators are fantasy handmade pieces (reportedly from Mexico) most likely were not made for insulating electrical wires.

There is an excellent online resource for obtaining additional information on the National Insulator Association's website (www.nia.org).  Finally, for collectors wishing to be active in the hobby there is a monthly magazine being published called Crown Jewels of the Wire (www.cjow.com).