Glass Insulators Value: CD Numbers, Colors and Embossing

Most glass insulators are worth $1-15, as of October 2026. The clear and aqua Hemingray-42, Hemingray-40, Whitall Tatum No. 1 and aqua beehive styles strung on millions of poles are the most common finds. Aqua beehives such as the Brookfield No. 16 sell a little higher, typically around $25. Unusual colors, threadless types and rare styles bring hundreds, and a few have sold for over $7,000.

Insulator collecting runs on three things: the CD number, which names the shape, the embossing, which names the maker and variant, and the color. Common things are common here, and the National Insulator Association's own list of the most common North American insulators reads like a list of what most people find in a barn. The skill is spotting the one in the box that is not on that list.

Quick answer

• Hemingray-42 (CD 154), Hemingray-40 (CD 152), Whitall Tatum No. 1 and similar, clear or aqua: about $1-5.
• Aqua beehives (CD 145) and other common 1880s to 1920s styles: about $5-25.
• Off colors such as light green, ice blue, straw and sun-colored amethyst: roughly $10-50.
• Strong colors such as cobalt, amber, yellow, emerald and deep purple on scarce styles: roughly $100-1,000+.
• Threadless insulators from before the 1870s and rare styles: hundreds to thousands; a CD 718 Thames sold for over $7,200 on eBay in 1998.

How do you identify and value a glass insulator, step by step?

Name the shape, read the embossing, then judge the color and condition. The order matters, because a shape difference can change the value twentyfold.

1. Find the CD number. CD stands for Consolidated Design, a system for cataloguing the shapes of glass insulators regardless of maker, devised by collector N.R. Woodward. Threaded glass pintypes run from CD 100 into the 300s, and threadless types sit in the 700s. Many later insulators have a style number embossed, such as "No. 42," that a cross-reference chart converts to a CD.
2. Read all the embossing. Read the front and back of the skirt and the dome. Note the maker (Hemingray, Brookfield, Whitall Tatum, Armstrong, Kerr, McLaughlin, Gayner, California, Boston Bottle Works), the style number, any patent date, "Made in USA," and numbers or dots on the skirt. According to insulators.info, there are about 2,800 different embossings, and small differences separate common variants from scarce ones.
3. Look at the base. A smooth base (SB) is flat. Round drip points (RDP) are BB-sized bumps around the base edge, and sharp drip points (SDP) are small cones. Ralph Hemingray and James Gill patented drip points on May 2, 1893, and they are typical of 20th century Hemingrays.
4. Check the pinhole. Look inside. Threads mean a threaded pintype, the vast majority. A smooth pinhole with no threads means a threadless insulator, the earliest type, mostly from before the 1870s and much more valuable.
5. Name the color under daylight. Hold it to a window. Aqua, clear, light green and light blue are common. Note any tint: straw, ice blue, sun-colored amethyst, green, cobalt, amber, yellow, olive, purple or milk glass.
6. Check whether the color is natural. The NIA warns that some insulators are coated with glass stains or heat-treated to change their color, and that good stains can be hard to see. A coating usually shows up with a careful scratch on the base with a knife; deep, even purple on clear glass is often irradiated.
7. Grade the condition. Look at the dome, the threads, the skirt and the base for chips, flakes, cracks and bruises. Small base flakes from installation are common and expected; chips on the dome or skirt cost more.
8. Compare with sold results. Search the CD number, the embossing and the color together.

What are the most common glass insulators?

The National Insulator Association's field guide lists the styles collectors voted the most common in North America. If yours is one of these in clear or aqua, it is a $1-15 insulator.

Hemingray, which made glass insulators for more than half a century, first in Covington, Kentucky and from 1888 also in Muncie, Indiana, where all operations moved by 1919, was the largest maker, which is why so many Hemingrays survive. The insulators.info FAQ says plainly that "most insulators are quite common and have little monetary value," and that is the honest starting point.

What is a glass insulator worth?

Values are for insulators without significant damage, as of October 2026. Small base flakes are acceptable to most collectors at these prices.

The Underpriced AI figure gives the shape of the common market: its tracked sales of an aqua Brookfield No. 16 beehive had a typical sale of $25, a middle half between $14 and $89, and one high outlier at $810. At the top, insulators.info reported a CD 718 Thames Glass Works insulator that sold on eBay for over $7,200 in July 1998, and Electrical Review reported in December 2004 that an 1890s glass insulator bought for a few dollars at a garage sale sold on eBay for $10,313. Sales at that level are rare and specific to the exact piece.

How do color, embossing and CD affect value?

Color is the first thing collectors notice, but it only multiplies the value of the shape. A cobalt common Hemingray style and a cobalt rare beehive are not the same insulator.

• Color. Aqua and clear are the baseline. Off colors add a little. Deep cobalt, amber, yellow, emerald, purple and milk glass on a style that normally comes in aqua are where the real money starts. Natural sun-colored amethyst comes from manganese in the glass turning lavender in sunlight, and is usually pale.
• Embossing. A misspelling, a missing letter, an unusual patent date or a short-lived maker's name can separate a $5 variant from a $100 one. Read everything.
• CD. The insulators.info FAQ notes that a slight difference in shape can change an insulator's value by twentyfold or more. Two beehives that look alike can be different CDs.
• Base and drip points. Some variants are scarcer with smooth bases than with drip points, and the reverse.
• Threadless. The earliest insulators were pushed onto wooden pins rather than screwed on, and the threaded pinhole was patented by Louis A. Cauvet in 1865. Threadless types are scarce and collected seriously.

What changes the price?

A worked example

Say you find a box of fifteen insulators on a farm. Twelve are clear and embossed "HEMINGRAY-42 MADE IN U.S.A." on the skirt, with drip points on the base. Two are aqua beehives embossed "BROOKFIELD" with a patent date. One is a light green beehive with no threads in the pinhole.

The twelve Hemingray-42s are CD 154, the most common insulator in North America: about $1-3 each, so roughly $15-35 as a lot. The two Brookfield beehives are CD 145, common but collected, at about $5-25 each depending on embossing and condition. The light green one with a smooth pinhole is the one to stop on. A threadless insulator is an early type, and its value depends on the exact CD and maker. Photograph the embossing and base, look up the CD in a reference, and show it to an insulator collector before selling it with the rest.

Where should you sell glass insulators?

Common mistakes

• Buying or pricing by color alone. A purple Hemingray-42 is usually irradiated, and a stained cobalt is a common insulator with a coating.
• Skipping the CD number. Without the CD and full embossing, a sold-listing comparison is a guess.
• Ignoring the pinhole. A threadless insulator looks plain but can be the best piece in the box.
• Cleaning with acid or a wire brush. Scratches and etching are permanent. Warm soapy water and a soft brush are enough.
• Treating "crackle" as natural. The NIA notes that crackle patterns are typically made by people, not lightning.
• Pricing from the highest listing. Asking prices for common Hemingrays often run many times their sold prices.

How WorthPic reads glass insulator photos

WorthPic runs on the newest and most capable AI model available, tuned for exactly this kind of identification. From your photos it reads the shape, the embossing on the skirt and dome, the base and drip points, the pinhole and the color, matches the insulator to its CD and maker, and gives an honest value range in US dollars with a most likely figure. It explains what drives the price and which extra photo would settle it, often the embossing in side light or the pinhole. It cannot always tell a natural color from a well applied stain or irradiation, it cannot feel the glass, and it can miss a bruise or a small crack. It identifies and estimates. It does not authenticate, grade or appraise.

What to photograph

• The insulator from the side, against a plain background, in daylight.
• Each line of embossing, front and back, in side light so the letters cast shadows.
• The base, showing smooth or drip points and any flakes.
• The inside of the pinhole, to show threads or no threads.
• The insulator held up to a window, to show the true color.
• Any chip, crack or bruise, close up.

When to get it looked at in person

A box of clear Hemingrays and aqua beehives does not need an expert; a sold-listing search by CD and embossing is enough. A threadless insulator, a strong color on a style that is normally aqua, an unusual maker or embossing, or any color you suspect might be altered is worth showing to an experienced collector. The National Insulator Association lists clubs and regional shows, and its members are the people who know the scarce variants by eye. John and Carol McDougald's reference books and price guides are the standard printed references. For other glass from the same barn or cellar, see our antique bottles guide and our old mason jars guide; for advertising and railroad items, see our vintage signs guide.

Sources

• National Insulator Association: Top ten common insulators: collector vote on the most common North American styles, led by CD 154 Hemingray-42.
• National Insulator Association: Stained and altered insulators: color stains, heat treatment, the scratch test and man-made crackle.
• Insulators.info: Frequently asked questions: most insulators have little monetary value, about 2,800 embossings, shape differences that change value twentyfold.
• Insulators.info: CD 718 Thames sale: sold on eBay for over $7,200 in July 1998.
• Underpriced AI: Brookfield No. 16 aqua beehive: typical $25 across 47 completed eBay sales, middle half $14 to $89.
• Electrical Review: Glass insulator sells for over $10,000: an 1890s glass insulator sold on eBay for $10,313 in 2004.