From Brass Armadillo Employee Christina Errington,
(This is my first blog ever, it was originally posted on www.iantique.com) 
Insulators, what are they, what were they used for, what were they made of, and are they of any value?
Glass insulators were first produced in the 1850's for use with telegraph lines. As technology developed insulators were needed for telephone lines, electric power lines, and other applications such as railroad signals. In the mid 1960's a few people began collecting these antique insulators. Today there are over 2000 collectors. Production for porcelain insulators started in the 1850’s and 60’s. Porcelain insulators were more commonly used for power distribution due to their greater strength and surface resistance; by 1915 they basically replaced all glass insulators on electrical lines. Popularity has increased for porcelain insulators over the past ten years thanks to the rise in glass prices, earlier ‘classic’ porcelain being taken out of service as utility distribution voltage increases and the increase of available information on the web and in books.
What are insulators? According to Webster, to insulate means "to separate or cover with a nonconducting material in order to prevent the passage or leakage of electricity, heat, or sound”, in order to keep electrical and communication lines dry and suspended above the ground they were supported by poles, something was needed to keep the lines off the wet poles, along came the insulator. There are many different types of insulators and patents for even more. Insulators are grouped by material, origin, number of parts and threading characteristics.
Origin is either North America, where the predominate material for insulators has always been glass for communications and porcelain for electrical or foreign, some countries preferred and used only porcelain even in telegraph lines. The number of parts is separated into two categories unipart, consisting of one solid piece, or multipart, any insulator of two or more separately molded parts, either cemented together during or after manufacture, or used together on the line. Another characteristic being threaded, an insulator with internal screw threads which correspond to matching threads on a pin, or threadless, an insulator without threads and having a smooth pinhole; there are also non-pintype insulators, which include spools, dead end insulators, and wire strain insulators.
What are they made of and why are they colored? Most commonly, insulators are made of glass, but they are also made from porcelain. In the Smithsonian you can find an insulator dated back to 1872 made of wood, or rubber from 1940. Glass insulators are most commonly found varying from an aqua blue to a green in color. Depending on the type of glass cullet, sand and chemicals used in making them, they can be found in colors from clear to purple, aqua to emerald green, yellow to red-black and many colors in between, to see a range of these colors visit http://glassian.org/Gallery/color.html.
Whereas glass insulators were colored due to glass cullet, colored porcelain insulators provided 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. Porcelain insulators ranged from browns to blues, greens to yellows, ‘butterscotch’ to grey, and even white. Sadly though a multitude of grays, greens, blues, yellows, and cobalt's can’t be found any longer Certain styles common in one color maybe rare in another and therefore more desirable. The brown insulators tend to be less popular unless found in a rare style. Earlier porcelains will have mottled or swirled glazes, when compared to the more recent uniform glazes.
Are they worth anything? There are many factors that determine an insulator's value. Shape, color, embossing, condition, desirability and rarity all affect its value. Most insulators are quite common and have little monetary value. The first step in determining your insulator's value is to determine which insulator you have. Even with a price guide in your hand, you have to determine which of the approximately 460 shapes, 2800 different embossings, and almost 9000 color combinations best describes your insulator, and that’s only counting the glass variations. It is easy to list the common insulators, and also list the very rare insulators, but it is difficult to list all the thousands of collectable insulators that fall between those extremes. A slight difference in the shape or color of your insulator can affect the value immensely.
This information was gathered from the following sites:
http://www.insulators.info/general/profiles/
http://www.insulators.info/general/faq.htm#style,
http://www.insulators.info/porcelain/
http://www.insulators.info/porcelain/color.htm
http://www.insulators.info/porcelain/history.htm
http://www.nia.org/timeline/index.htm