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How to Identify Glass Insulators

American telegraph, telephone and electrical lines used glass insulators through the last half of the 19th century and the first half of the 20th century to insulate wires on the poles. These heavy pressed blobs of glass were most often clear or green glass, but other less common colors are choice for collectors. Manufacturers of fruit jars often manufactured insulators as well. Purchase insulators by color, condition and scarcity. Identify insulators by shape and style as well as by brand name.

Instructions

- 1
  Look at the side of the insulator to locate the brand name. Common brands are Brookfield, Pyrex, Armstrong and Hemingray. A few insulators are marked on the inside, so examine the inside if you do not see an embossed name or initials on the side. A few are unmarked.
- 2
  Inspect the shape of the insulator and the design. Consolidated Design numbers (CD) developed by N. R. Woodward, an insulator authority, place the shapes into groups by petticoats or skirts and grooves. The skirt or petticoat is visible on the inside edge, and may include no skirt, a single ridge, a double ridge or even a triple ridge. For example, the CD numbers from 100 to 184 have side wire grooves and no skirt or one skirt. Milholland's "Glass Insulator Reference Book" provides CD numbers through 1149. Woodward's 1988 Report, "The Glass Insulator in America" provides a cross-reference between CD numbers and manufacturer's numbers.
- 3
  Check the drip points that look like teeth at the base. Some insulators have sharp drip points (SDP) and some are round (RDP). There are also flat drip points (FDP) and wedge drip points (WDP). The drip points help identify the type and style of the insulator.
- 4
  Research your insulators in books about insulators or on websites such as the National Insulator Association (nia.org) or Insulators.info. Identify by brand, shape, style and color before attempting to sell your insulators. Look at the scarce sizes and shapes to recognize the most collectible insulators. Also study the artificially induced colors so you will not buy a "fake" color.
- 5
  Attend shows and connect with collectors and dealers for help in identifying your insulators. Many collectors and dealers enjoy sharing information and are a good source for identification, selling and trading insulators. Insulator production came from many companies and most areas of the United States. The telephone and telegraph companies also used imported insulators. You may appreciate the help for identifying the scarcer insulators.

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