Glass Insulator Manufacturers



General Overview on Insulators

Long before the modern era of computers, cellphones, smartphones, fiber-optic cables and the internet, long distance electric/electronic communication consisted primarily of the telegraph and telephone.   The electric telegraph (in the United States) was developed by Samuel Morse in 1837, and the first message was sent by Morse in 1838. The telephone was invented by Alexander Graham Bell in 1876.

As time went on, networks of “open wire” telegraph lines, and later, telephone lines, were developed and built throughout the country, and these lines required the installation of insulators. Insulators were necessary by serving as a medium for attaching the wires to the poles, but much more importantly, they were required to help prevent electric current loss during transmission. The material, glass, is itself an insulator (not a “conductor” or “transformer” as insulators are often incorrectly labeled in antique malls and flea markets).

Both glass and porcelain insulators have been used since the early days of the telegraph, but glass insulators were generally less expensive than porcelain, and were normally used for lower-voltage applications. The oldest glass insulators date from about 1846.

The period from 1875 to 1930 might generally be thought of as the “heyday” of the glass insulator. Hundreds of millions of these glass “bells” were produced during this time by many glasshouses, located primarily in the East and Midwest with a few plants in California and Colorado. Many of the glasshouses that made insulators also produced bottles and other glassware.
Most insulators are found in some shade of blue-green/aqua-colored glass (typical cheap “bottle glass” or “green glass”) but many, many  other color shades are found. Clear glass was used (with some exceptions) primarily after about 1935.  Hundreds of different styles were developed, and insulators are found with a wide variety of embossed names, patent dates, and other markings.  Many earlier insulators have bubbles, streaking, “snow”, surface creases and other marks of crudeness which was common for this type of glass because quality standards were not usually set as high as tableware. As long as the insulator performed it’s duty adequately, the  color and minor imperfections in the glass were of little concern. These “marks of crudeness and age” now add to their value and charm, to collectors of antique insulators.

CD 145 H.G.CO. / Petticoat insulator made by Hemingray Glass Company

“Beehive” style telegraph insulator, marked:  K  / H.G.CO. //  PETTICOAT, made by Hemingray Glass Company, circa 1886-1895, in aqua.

Old photos from around the U.S. show many telephone, telegraph and electric power poles sporting large numbers of insulators arranged on crossarms. Some telephone poles (such as in large cities) carried as many as 20 or more crossarms, each one laden with six, 10 or 12 (or even more) insulators.
Each insulator was attached to the crossarm by being screwed onto wooden (or in some cases) metal pegs or (more properly) “pins”. A steel or copper tie-wire was attached to the insulator, and connected with the communication wire.  These “pintype” insulators were an extremely commonplace sight and communication lines with insulators were strung alongside most roads, highways, and railroads.

Telegraph line pole with glass insulators

Telegraph line pole with glass insulators (possibly CD 145 and CD 133 styles). From undated real photo postcard, Joliet, Illinois, circa 1909.


During the 1960s, 1970s and 1980s many of these lines were dismantled as technology advanced. Today, a few lines using glass insulators are still in service, but are only a tiny percentage compared to the heyday of open wire communication.

Insulators (generally speaking) are still commonly in use, but insulators of the modern era (speaking of the United States in particular) are mostly heavier, high-voltage types used in electric power line transmission and distribution, and are  of porcelain (“ceramic”) or polymer (plastic) construction.    Most modern  telephone lines now make use of insulated sheathed cable, and many are underground.

Today, vintage glass insulators are a collectible item in their own right, often saved, studied and displayed along with antique bottles, tableware and other early glassware.  The majority of glass insulators carry embossings (raised lettering), as previously mentioned,  including company names, brands, trademarks,  or model numbers,  patent dates, etc.  A small percentage of insulators are entirely unmarked.  Perhaps somewhat surprisingly, so many insulators are found that were carefully and rather painstakingly lettered with markings that would not even be discernable by the average passersby below……….only visible to linemen (and birds) !


Collectors of insulators often specialize in a particular glass companies’ products, or in certain styles, shapes or colors of insulators.  They might delve into the history of a particular company, what styles where made and when, the markings used, etc.   Besides the more typical “pintype” communications/electric power line insulators, other sub-categories include lightning rod insulators, radio wire or guy wire “strain” or “egg” style insulators, inside home wiring knob or spool insulators,  and battery rests.


Many, many different cast iron molds were used over the years to produce glass insulators, and the collection & study of old insulators can be compared, in some respects, to numismatics (the study and collection of coins).  Most earlier insulators were  made by forcibly pressing molten glass into a mold. The mold was momentarily closed, and then within a few seconds was opened and the finished insulator was removed to be placed into a lehr (cooling oven). Modern glass insulators are/were made by mass-production machine pressing methods.

On earlier insulators, many minor variations in the engravings cut into the inside surface of the iron molds resulted in slight differences in the exact appearance, size and placement of the raised lettering seen on the surface of the glass.  On some companies’ products, more than one style of lettering can be identified.  For instance, the so-called “Script”, “Prism” and “Stamp” (“Typewriter”)  styles of font which are seen on many Hemingray insulators.

Sometimes an individual mold can be identified by studying and comparing insulators that were produced from it over a considerable stretch of time.  Changes in the mold,  such as re-cut (re-tooled) engraving,  repair, or the addition or erasure (“blotting out”) of the engraving can be discovered upon very close inspection of the insulator.  Thus,  the comparison I’ve made to the coin collecting hobby, with the many  slight differences in coin die design details, as revealed under close scrutiny by serious collectors.

On this webpage I’ve compiled a list of glass factories that manufactured (or are believed to have produced) glass electrical insulators.    Although primarily listing factories that were located here in the United States of America, I’m also including a few Canadian and Mexican firms.  If you have additional information, corrections, or clarifications, please contact me (at the email address listed at the bottom of the page), as I am continually looking for the most accurate data available on these companies.  Sources of some of the information is included after each entry if I have it available.

Some of the information is from research originally compiled by N.R.Woodward, creator of the “CD” (Consolidated Design) numbering system used by collectors for identifying and cataloging insulators.  Also, a portion of the info is found in various articles in the 2-volume reference book “INSULATORS: A HISTORY AND GUIDE TO NORTH AMERICAN GLASS PINTYPE INSULATORS ” by John & Carol McDougald (1990).  Those books are no longer in print, but I would heartily recommend them as an important part of any serious insulator collector’s library.


As an antique bottle collector, I’m interested in the identification marks used by bottle factories. Click here for a list of marks found embossed on bottles (and other types of glassware),  and the companies those marks are believed to represent.


NOTE: Dates given are (in most cases) the years during which a factory was in operation. In a few cases, the dates only apply to the time period during which insulators were being manufactured. Some information is questionable or incomplete, and I will update this webpage as I learn more. This list does not include “end user” companies (telephone/telegraph companies, railroads, electric power utilities, etc) who had insulators made for them with embossing such as “A.T.& T.”, “C.P.R.”, “G.E.”, etc, but only lists the actual manufacturers of these insulators.

For a listing of some primary embossings found on glass insulators, and the glass factories that may have produced them, click here .

American Insulator Company
Boston area, MA (c.1883-1886?)

Although insulators made by (or for) this company are marked “AM.INSULATOR CO. N.Y.” (on the base), business offices were located in both New York and Boston according to directory listings. Possibly the actual manufacturing facility was located in the Boston area, and may have been the American Iron Glass Pipe & Plate Company (Iron Glass Works) factory at Haverhill, Massachusetts. In addition, recent evidence has surfaced which proves that the Lyndeborough Glass Company, South Lyndeborough, New Hampshire (1866-1888), produced many of the American Insulator Company-embossed insulators.

[McDougald-1990: Joe Maurath, Jr., Bob Fuqua; Mark Newton]

American Iron Glass Pipe & Plate Company (aka Iron Glass Works)
Haverhill, MA (1884-1885?)

There is a possibility this was one of the factory locations where some of the American Insulator Company insulators were produced (besides the Lyndeboro Glass Company, see that entry).  At least some of the “National Insulator Co” embossed insulators were almost certainly made here.

[McDougald-1990: Joe Maurath, Jr., Bob Fuqua]

Armstrong Cork Corporation
(See Whitall Tatum Company page for more information.)

Baltimore Glass Manufacturing Company
Baltimore, MD (1895-1897)

Insulators embossed “B.G.M.CO.” were produced by this company.

Bay State Glass Works
East Cambridge, MA (1878-1879)

Samuel Oakman is listed as agent. [McDougald-1990]

Beaver Falls Glass Company
Beaver Falls, PA (1869-1879)

Insulators marked “B.F.G.Co.” were made by this firm, and several other unattributed styles are believed to have been made here as well.

[McDougald-1990: Ora Beary]

Boston & Sandwich Glass Company
Sandwich, MA (1825-1888)

Considered by some glass collectors as the premier glass company of the entire 19th century in America, this factory made some of the finest glassware, both hand-blown and pressed, ever produced in the United States. They are believed to have manufactured some insulators in the 1850s-1860s period, including CD 701.1 threadless types.

[McDougald-1990: Ray Klingensmith]

Boston Bottle Works
Boston, MA (c.1872-1877)

Bushwick Glass Works (aka Brookfield Glass Company)
Brooklyn, NY (c.1864-c.1906)
Old Bridge, NJ (c.1906-1921)

This manufacturer made insulators marked W. BROOKFIELD,  BROOKFIELD,  B,  as well as many others with markings of the end user company, such as A T & T.  For more information on this prolific glass company, please click here .

Cadiz Glass Works 
Cadiz, OH (1884-1887)

Cadiz Glass Works started making glass in July of 1884, and evidently lasted only about a year or two before being shut down. The works were purchased in March of 1887 and renamed the Lythgoe Glass Works. That operation lasted only about 4 months and the plant was closed again. Rumors of possible re-starts flared up on rare occasions over nearly another decade, with an article appearing as late as 1897 that there was a possibility  the plant might soon be making glass, but nothing ever came of it.

Several scarce and unusual styles of insulators were evidently made there, none of them marked.  A rare fruit jar is known, marked “CADIZ JAR” on the front.

California Glass Insulator Company/California Glass Works
Long Beach, CA (1912-1916)

Insulators marked “CALIFORNIA” and “C.G.I.CO.” were produced here. Click here for more info on the California Glass Insulator Company.

Canada Glass Company
Hudson, Quebec (1864-1877)

[McDougald-1990: Morgan Davis]

Canada Glass Works (aka Foster Brothers)
St. Johns, Canada East (Quebec) (1854-1860)

Cohansey Glass Manufacturing Company
Bridgeton, NJ (1870-1900)

“Electrical insulators” were displayed at the Franklin Institute 1874 Awards. Type(s) are unknown.

[Adeline Pepper-Glass Gaffers of New Jersey (1971), pg. 215; Alice Creswick-The Fruit Jar Works]

Colorado Glass Works
(See Valverde Glass Works)

Corning Glass Works (now Corning Incorporated)
Corning, NY (1875-to date)

Maker of the “Pyrex” brand insulators. The great majority of glass Pyrex insulators for the U.S. market were made between about 1922 and 1951. A tremendous variety of industrial glass as well as glassware for home use has been produced by this company for well over a century.

[McDougald-1990: Jeff McCurty]

Crystallite Products Corporation
Glendale, CA (1935-1940)

Maydwell glass insulator , CD 154Manufacturer of the units embossed “MAYDWELL” which were made for Maydwell & Hartzell, a distributor of products for electrical utility companies.

Denver Flint Glass Company
(See Valverde Glass Works)

Diamond Glass Company/Diamond Flint Glass Company
Montreal, Quebec, Canada (1891-1913)

Starting as Foster Brothers Glass Co. (c.1857-1878), then known as Excelsior Glass Company (1878-1883), later as North American Glass Company (1883-1891), the Diamond Glass Company was formed in 1891. In 1902 the name was changed to Diamond Flint Glass Co. and (by this time a conglomerate of several glass plants) it then merged into the Dominion Glass Company in 1913.  The small pony style CD 102 insulators with a “Diamond” emblem on the skirt are the most commonly seen units from this concern.

[Steve Goodell]

Dominion Glass Company
Montreal, Quebec; Wallaceburg, Ontario, Canada (1913-1976)

Dominion 42 glass insulator in orange amber, CD 154 style Dominion was a re-organization of the Diamond Flint Glass Company, a large conglomerate with factories at several locations in Canada. Other glass plant locations included Toronto, Ontario; Hamilton, Ontario; and Redcliff, Alberta. Most insulators made by Dominion were produced at the Wallaceburg site. Insulators were also made for short periods of time at Redcliff, Alberta (1942) and at Point St. Charles, Quebec (1943). The last insulators made by Dominion are reported to have been produced in 1967, although production of other glass items (bottles, lamp chimneys, etc) continued afterward. Most insulators made by this company are marked “DOMINION” or with a “D in a diamond”.  The “D in a diamond” was used on their containers until at least 1976. The company became known as Domglas after about 1978.  (Pictured: Dominion-42 in orange-amber).

[McDougald-1990: Morgan Davis, Eric Halpin; Steve Goodell]

Duquesne Glass Company
Carnegie, PA (1905-1907)

There has been alot of uncertainty concerning the true source of the insulators embossed “DUQUESNE GLASS CO.” Three types are known, all small telephone line styles: CD 106.1, CD 106.3, and CD 113.2. According to information uncovered by researcher Bob Stahr, the actual source of these insulators was the above-mentioned firm, located in Carnegie, PA, now a part of metropolitan Pittsburgh. The Duquesne Glass Company was incorporated in January of 1905, and was in operation for approximately two years. The glass was manufactured at a plant owned by the H. L. Dixon Company. In May of 1907 the plant itself was sold by Dixon to Superior Steel Company, and Superior gained occupancy in November of that year.

Another glass factory with the same name (i.e. Duquesne Glass Company) was located in Paden City, West Virginia, and was in business from about 1903 (some sources state 1905) to c. 1919. That operation was principally a bottle house, and did not produce insulators.

On a related note, information that has circulated for many years within the insulator-collecting world had mistakenly attributed the “Duquesne Glass Co.” insulators to a much earlier factory called the Duquesne Glass Works (note “WORKS” in name) of Belle Vernon, PA, which operated under various owners from about 1834 to 1886 or later.

[Bob Stahr]

Ellenville Glass Works
Ellenville, NY (1837-1896)

[Insulators reportedly made in 1886-info from a Web article]

H. C. Fry Glass Company
Rochester, PA (1901-1933)

Fry Glass made (at least) 5 different types of glass insulators that are known so far (4 pintypes) and all are unique styles and very rare. Most known examples were recovered from the factory dumpsite. Colors included opaque cobalt blue, milky opalescent, dark purple blackglass, etc. The actual manufacture of these insulators occurred in the 1920s.

[McDougald-1990: Ray Lanpher]

Gayner Glass Company
Salem, NJ (1898-1957)

Although glassman John Gayner was involved in several ventures in the glass manufacturing field in Salem as far back as 1879, the official incorporation of the “Gayner Glass Works”, as such, did not occur until 1898. Insulators were evidently only produced during the period of 1920 to 1923. Most of the insulators they made are embossed “GAYNER” on the skirt area. Their main line of glass products consisted of various kinds of bottles and fruit jars.

R.Good Jr./Robert Good Jr.
(See Valverde Glass Works)

Hamilton Glass Company
Hamilton, Ontario, Canada (c.1864-1898)

[McDougald-1990: Eric Halpin, Morgan Davis] [Colin MacIntosh (Canadian Insulators) states dates as 1864-1895 with a note that Diamond Glass Company absorbed Hamilton Glass in 1894].

Harloe Insulator Company
Hawley, PA (c.1902-1906?)
Elmer, NJ (Mar 1903-Oct 1903)

This company made two unusual “no-tie” style insulators— the CD 109.5 and CD 206.5 styles. Harloe’s embossed marking is a logo consisting of the letters “H I Co” intertwined, above the words “Hawley, PA. U.S.A.”. Because of the embossing, Harloe insulators are often called “Hawleys” by collectors, although the actual company name was never “Hawley”.  Ending date of insulator production at Harloe is uncertain but evidently in the 1905-1906 period. Harloe operated the Elmer, NJ “lower works” for a short while after the Sterling Glass Company ended it’s operation there. (Note: This company should not be confused with an earlier glass company called the “Hawley Glass Company” which was located in Hawley, PA and in operation c.1872-1885, which produced fruit jars and bottles).  [McDougald-1990: Bob Harding; Ray Klingensmith]

Hemingray Glass Company
Cincinnati, OH (1848-1852)
Covington, KY (1852-c.1890?)
Muncie, IN (1888-1933)
Muncie, IN (1933-1967) [Owens Illinois Glass Co.]

Hemingray is the best known and was the most prolific glass insulator manufacturer in the world. Markings used by Hemingray include “HEMINGRAY”, “H.G.CO.”, “PATENT MAY 2 1893”, “PATENT DEC 19 1871”, “KIMBLE”, “LOWEX” and others. More information here.

Houston Glass Works/ Houston Glass Company
Houston, Texas (1903-1905)

Although no absolute proof has been discovered (yet), the chances are very high that this short-lived, obscure firm produced an enigmatic CD 102 pony insulator marked with an “HGW” monogram on the skirt. Only one example has been found to date, in a shade of light green, and it was purchased at a flea market in the Houston area.

Indiana Glass Company
Dunkirk, IN (1907-2002) & Sapulpa, OK (19??-to date)

Indiana Glass Company (subsidiary of Lancaster Glass Corporation) has produced a tremendous variety of glassware throughout it’s long history.  In 1967, the very last insulators carrying the Hemingray name were produced at the Indiana Glass Co. glass plant in Dunkirk, using molds and machinery shipped over from Muncie.  For a site with more information on Indiana Glass, click here .  Also, for a webpage on a very popular item made by Indiana Glass for many years, click here for my page spotlighting their “Hen on nest” dishes.

[Bob Stahr]

Kearns & Company (G. W. Kearns; Kearns, Herdman & Gorsuch)
Zanesville, Ohio (1864-1886)

A check exists circa 1870 that indicates telegraph insulators were then being produced. Nothing is known about what type(s) these might have been.

[McDougald-1990: Ray Klingensmith, Bob Henrickson; Alice Creswick-The Fruit Jar Works]

Kerr Glass Manufacturing Corporation
(See Whitall Tatum Company webpage.)

King City Glass Works
Fairmount, IN (1890-1897)

Insulators marked “K.C.G.W.” were manufactured at this factory. The plant was bought by Marion Fruit Jar & Bottle Company in 1897.    It is very likely that some insulators were produced by the M F J & B Co., re-using old molds from the King City operation.

Lancaster Glass Works
Lancaster, NY (1849-1908?)

Year this operation closed is uncertain. McKearin quotes two sources, stating it could be either 1890 OR 1908. Some of the firm names used include Reed, Shim & Co.; James & Gatchell; James Glass Works; and the Lancaster Cooperative Glass Works.

[McKearin & McKearin-American Glass(1941)]

Louisville Glass Works
Louisville, KY (1855-1873)

John Stanger, along with several other glassmen, started this factory in 1850 at the SE corner of Clay and Franklin Streets in Louisville.  First known as the Kentucky Glass Works, it was being referred to as the Louisville Glass Works by 1855.  Primarily, their products consisted of bottles, flasks, fruit jars, lamp chimneys, window glass and similar items, but personal research in period newspapers confirms that telegraph insulators were being produced at this factory in February of 1866. Type(s) manufactured there are not known at this time.  For more detailed information on this operation, please see this article published in 2005: Louisville Glass Factories of the 19th Century .

Leonard Glass Works
Detroit, MI (c.1883-1886)

Evidence has recently surfaced that indicates Leonard made insulators for the Chicago Insulating Company, making it likely that this firm produced many, if not all, of the insulators marked with the “Chicago Insulating Co.”embossing, i.e. CD 135 and CD 109.

[Dan Culver]

Lynchburg Glass Corporation insulator - LYNCHBURG 44

Lynchburg Glass Corporation
Lynchburg, VA (1923-1925)

Insulators embossed “LYNCHBURG” were made here. Although, according to production records, over 4 million insulators were made within less than 2 years’ time, as a result of various operational problems this factory ceased glass production in early 1925. Pictured: Lynchburg No. 44 (CD 154) in one of the green shades found in this CD. This style was a direct competitor to the ubiquitous Hemingray-42 style, made by Hemingray Glass Company. See more info on the Lynchburg  page.

Lyndeborough Glass Company
South Lyndeborough, New Hampshire (1866-1888)

The celebrated Lyndeboro’ glass factory, long known by Northeast region antique collectors as a maker of many beautiful bottles and whimseys, has recently been confirmed as the source of many (if not all) of the base-embossed American Insulator Company electrical insulators.      [Mark Newton]

Marion Fruit Jar & Bottle Company
Fairmount, Indiana (1897-1904)

A possible producer of insulators. (See King City Glass Works). The MFJ&BCo was bought by Ball Bros. Glass Co. in 1904 and the plant was closed in 1910.  [Alice Creswick-The Fruit Jar Works]

Massachusetts Glass Company
Somerville, MA (c.1867-1871)

[Predecessor of Boston Bottle Works]

See “Crystallite Products Corporation” entry.

S. McKee and Company
Pittsburgh, PA; Jeannette, PA (1834-1908)

This company produced threadless insulators, although the related “McKee Glass Company” (1854-1951), a separate operation, made large quantities of tableware as well as bottles, window glass and other glassware. Please click here for more info on the McKee companies… more

McLaughlin Glass Company
Vernon, CA (1920-1935)

William McLaughlin worked at Robert Good, Jr.’s glass factory at Valverde…….more .

Mt. Pleasant Glass Works
Mt. Pleasant, NY (1844-1870?)

[McDougald-1990: Ray Klingensmith]

National Insulator Company
(See American Iron Glass Pipe & Plate Company)

Newburgh Glass Company
Newburgh (New Windsor), NY (c.1867- c.1872)

This concern was also known as the “New Windsor Glass Works” (in at least one source, i.e. The Telegrapher, 1867). Although Newburgh Glass Company appears to have made primarily bottles, it has recently been discovered (from brief articles in that same trade newspaper) that they also manufactured telegraph insulators, including at least some, if not many, of the insulators marketed by L.G. Tillotson in the late 1860s.

New England Glass Company
Cambridge, MA (1818-1888)

This factory was principally a producer of fine tableware and upscale glass items. Evidence strongly suggests that threadless insulators were made in the c.1846-1850 period for Ezra Cornell, who was involved in the construction of early telegraph lines in New York and other areas in New England.   [McDougald-1990: Ray Klingensmith]

Insulator from N.E.G.M.CO.- New England Glass Manufacturing Company

New England Glass Manufacturing Company
Boston, MA (1898-1900)

Insulators marked “N.E.G.M.CO.” were made by this firm.  Although the factory was reportedly listed only in the 1899 Boston directory, it is believed that actual glass production started in 1898, and might have lasted into early 1900 before the new directory was printed. Shown: CD 162 style “signal” insulator in aqua.

New Granite Glass Works
Stoddard, NH (1861-1871)

An early trade card from this glassworks advertised insulators, both telegraph and lightning rod styles.  Evidence from excavation at the former factory site indicates they made at least some,if not all, of the unmarked dark olive green “blackglass” CD 738 threadless style insulators.     [McDougald-1990: Ray Klingensmith, quoting Ken Wilson; description re Heckler auction item]

Novelty Glass Company
Elmer, NJ (1901-1903)

Maker of many, if not most, of the “KNOWLES” embossed insulators. Many of the insulators that were made by Novelty are seen in shades of medium & darker greens.  A number of the “STAR” embossed insulators were produced at this factory. Although Novelty Glass was dissolved in 1903, the factory itself may have produced insulators as late as 1907 under a succeeding firm’s ownership.

Oakman Manufacturing Company
Mercer Street, Boston, MA (1890-1897)

[McDougald-1990: Joe Maurath,Jr.]

Ohio Valley Glass Company
Pleasant City, OH (1902-1905)

Maker of the insulators marked “O.V.G.CO.” Please click  here  to go to my page on this glassworks.

Opalescent Glass Company
Kokomo, IN (1888-to date)

Insulators (types unknown at present) were reportedly made here from 1888 to 1896.   [Paul Crist, Bob Stahr]

Owens-Illinois Glass Company
Toledo, OH [head office] (1929- )

Owens-Illinois was the product of a merger in 1929 of two large glass companies, the Illinois Glass Company (of Alton, Illinois), and the Owens Bottle Company which was headquartered in Toledo, Ohio. After the merging, the total number of plants owned by Owens Illinois numbered more than 20 in several states, with each assigned a plant code number. In 1933 Owens Illinois bought the Hemingray Glass Company plant in Muncie, Indiana, and it then became Owens Illinois plant #26. Glass insulators were made by O-I at Muncie (carrying the Hemingray name) until 1967. (More info under Hemingray).

Pacific Glass Works
San Francisco, CA (1862-1876)

Either this factory OR the San Francisco Glass Works was in all probability the manufacturer of the E.C.& M.CO. insulators.  Merged with SFGW to form the San Francisco & Pacific Glass Works in 1876.

Richmond Glass Works (Virginia Glass Mnfg. Company)
Richmond, VA

This factory produced some of the threadless “egg” style insulators, as evidenced by specimens found during excavation at the factory site. They were most likely made during the mid- to late 1850s.[McDougald-1990: Ray Klingensmith]

Sandwich Cooperative Glass Company
Sandwich, MA (May 1890-November 1890)

The Electrical Glass Corporation (director, James Pennycuick) bought the old Boston & Sandwich factory (which had closed in 1888) in December of 1889 and it was later known as the Sandwich Cooperative Glass Co. For several months in 1890, glass insulators were evidently made here, but the exact types are unknown. It is speculated that some of the Pennycuick-style threaded insulators may have been made at this factory, including “Diamond-P” ; “C.E.L.CO.”; and “Pettingell-Andrews” embossed CD 134s, and perhaps other unembossed, and as yet unattributed, examples. It isn’t clear how long, or where, the Electrical Glass Corporation made insulators previous to this period of time, assuming they did produce others. [McDougald-1990: Joe Maurath, Jr.]

San Francisco Glass Works
San Francisco, CA  (1865-1876)

This factory was the likely source of most, if not all,  of the E.C.& M.CO. insulators.  SFGW merged with the Pacific Glass Works (started 1872) in 1876 to form the San Francisco & Pacific Glass Works (1876-c.1901).  Insulators might date from after that merger as well.

St. Johns Glass Company
St. Johns, Quebec, Canada (1875-1877)

[McDougald-1990: Morgan Davis]

Standard Glass Insulator Company
Boston, MA (1893-1894)

Star Glass Works
New Albany, IN (1869-1879)

Sterling Glass Company
Elmer, NJ (June 1902-March 1903)

This firm manufactured insulators marked “STERLING” and/or “£”, as well as some of the “star” insulators. [McDougald-1990: Ray Klingensmith]

Thames Glass Works
New London, CT (1863-1865)

Only one intact insulator (CD 718, a threadless type) is known, along with about 3 to 6 known broken/partial examples (the exact number of known partials is unclear, there being some discrepancy among published reports). The insulators are embossed “THAMES GLASS WORKS” on the front of the crown, and “NEW LONDON, CT” on the rear crown. They are made of a very dark olive amber blackglass.

Valverde Glass Works (aka Robert Good,Jr.; Colorado Glass Works; Denver Flint Glass Company)
Denver, CO (1896-1899)

[McDougald-1990: Don Reinke, Mike Miller]

Virginia Glass Manufacturing Company
(See Richmond Glass Works)

CD `121 glass insulator from W.G.M.CO. - Western Glass Manufacturing CompanyWestern Flint Glass Company/Western Glass Manufacturing Company
Denver, CO (1899-1900); (1900-1909)

The markings “W.F.G.CO.” and later, “W.G.M. CO.” are found on insulators from this factory. The WFG’S have been found in many odd shades of color, but the WGM’s are usually found in various shades of purple.   [McDougald-1990: Don Reinke, Mike Miller]

Whitall Tatum Company/Armstrong Cork Corporation/Kerr Glass Mnfg. Corporation
Millville, NJ (1922-1938); (1938-1969); (1969-1978)

The first glass factory in Millville, New Jersey was started in 1806 by James Lee… more

E. Wormser & Company / Wormser Glass Company
Pittsburgh, PA (c.1854-1875; 1875-c.1927)

This factory is believed to be the source of certain unembossed insulators, including CD 120s, and 133.4 “bullets”. The great majority of examples are found in shades of SCA (sun colored amethyst), ranging from a light pink to a medium-dark purple (similar to a typical Whitall Tatum No.1 in purple). A few CD 120s in clear glass exist but are very rare. Aqua examples are known but are extremely rare. A receipt dated September 5, 1884 indicates that 1000 insulators (unspecified type) made by Wormser Glass Co. were sold to the Central District & Printing Telegraph Company. The factory address at that time is shown as “Laughlin Station, B. & O. R.R. Pittsburgh, Pa.” I also saw a business letterhead for sale on ebay some time ago which was dated 1906. Toulouse (Bottle Makers and their Marks-1971) indicates this company closed c. 1929. Alice Creswick gives ending date as 1927.    [Brent Burger]



Vidriera Monterrey
Monterrey, Mexico

Vidriera Monterrey is now known as the Vitro Corporation, known for large quantities of bottles and jars they currently import to the United States.

Cristaleria, S.A.
Monterrey, Mexico

Cristales Mexicanos S.A.


RESOURCES / GENERAL  INFO  CONCERNING INSULATORS Lots of good general information on Glass & Porcelain insulators, and the hobby of collecting insulators!! (National Insulator Association)  Great information and articles.  Rick Soller’s informational website—this site covers alot of insulator-related “specialty” topics!   Elton Gish’s very cool website with extensive information on collectible porcelain insulators of all types.   This link points to one of the pages on, this page illustrating some of the markings seen on porcelain insulators. Great pics of some more unusual colored porcelain pieces!


Please click here to go to my HOME PAGE.

Click here to go to the GLASS BOTTLE MARKS pages, (page one).

Click here to check out my page on the so-called “Crackle Glass” insulators.

Page discussing recently color-altered glass, including insulators and  other types of collectible glassware: Artificially Purpled Glass.

Click here for a basic summary page on WHAT IS GLASS?.

10 Responses to Glass Insulator Manufacturers

  1. Larry says:

    Thanks for taking your time to identify and provide dates for vintage insulators. Your site is very informative and i am going to bookmark the page for reference when identifying the varieties I come across. I am fascinated by old glass insulators and bottles and some of the early ones are really, really cool! I would like to find an early beehive (aqua, like the one pictured towards the top of the page) variety some day. Blessings!

  2. Sandy Bahler says:

    Hi there. Maybe you can help me. I have a dark peach Hemingway insulator marked with T.S. on one surface and on the opposite side there is 18-59: to buy a Cannot find it online. I think TS is pretty common, but the color is medium peach shade. I also have purple one Hemingray – 9. 8-48: My mom and dad moved out of their house and I found these.

    • David says:

      Hi Sandy,
      I am sorry to report that your “dark peach” Hemingray T.S. insulator has either been stained on the outside surface, OR it has been irradiated (“nuked”) to artificially change the color. Your insulator was originally clear or off-clear in color and is classed as a CD 129 in the “Consolidated Design” system of catalog numbers used by insulator collectors. You can search google with the keywords: Hemingray T.S. CD 129 insulator. Your insulator (with a date code of 59 plus two dots) was made in 1961.
      Your Hemingray-9 is classed as a CD 106, and has been stained with a fake outside purple color. It is also a clear insulator with a superficial coating. The date code (48 with two dots) indicates it was made in 1950.
      By the way, there ARE authentic purple glass Hemingray No. 9 insulators, but they date from the late 1890s or very early 1900s and do not have the same lettering arrangement as your comparatively modern number 9. All authentic purple Hemingray No. 9s have the embossing “HEMINGRAY / NO. 9” on the front and “PATENT / MAY 2 1893” on the reverse.
      I hope this will help.

      • Albert.K says:

        can you give me some hints as to where i can find insulators/not from antique shops/flea markets. i mean out in the wild,if you know what i mean

        • David says:

          Most insulators that are found “in the wild” are found along old telephone and telegraph lines or where they once stood. The most commonly-searched areas are along old, abandoned railroad tracks, where telegraph lines once ran. HOWEVER, most railroads nowadays prohibit trespassing on their right-of-ways, so searching along active railroads is discouraged. Other places to look: around abandoned buildings/houses; at yard, garage & barn sales; “junk” and curio shops; trading posts; rock shops (in the West); old trash dumps; near rivers, creeks, ravines where refuse was tossed; thrift stores; estate sales; farm auctions; Craigslist, etc.
          Hope this helps a bit,

  3. Pingback: Glass From The Past: Vintage Utility Pole Insulators | Randy Bowles Stories

  4. Diana Hobbs says:

    Trying to find out the value of a bottle in my possession. .it is clear with two embossed hearts with corn husks surrounding them …tin cap…one quart bottle…numbers on the bottom as follows D-9 55-69 M328270 the 8 may be a B can’t tell and there is a 1 in a circle can’t find it in any books . It appears it may be a whiskey bottle possibly . The glass is wavy in appearance

    • David says:

      Hi Diana,
      You have a whiskey bottle or decanter made by Owens-Illinois Glass Company. Please see my page on that company, and scroll down to the COMMENTS section; see the reply by “Carol” where info (link) on whiskey bottle codes liquor permit markings is mentioned. The “D-9” is a distiller code. (Any similar bottle with a “D-code” such as “D-9” or “D-126” can automatically be assumed to have held whiskey or some distilled liquor).
      The “55” is a liquor permit number assigned to Owens-Illinois Glass Company (specifically, their Huntington, West Virginia glass plant)………and the “69” is a date code for 1969, the year your bottle was made. the “I inside a circle (or oval) is Owens-Illinois’ logo/ trademark. For values, I strongly suggest to please check ebay auctions over a period of time and look for Similar bottles and their ACTUAL “Completed Auctions” prices. (Type in relevant keywords to bring up listings). Most of these types of bottles are currently very common and hold minimal interest to antique bottle collectors. Tremendous numbers of these types of bottles were made (many, many different designs) over a long period of time, especially in the 1940s-1970s, and no book will list most of them, (well, perhaps a small sampling), so I would not be surprised if someone cannot find this in a mass-market ‘bottle book’. I hope this helps,
      Take care, David

  5. Cool!My brother just gave me 12.All are green and some are beehive!Very informative!

All comments are moderated, so will not appear on this site immediately. Please, no posts asking about value of an item. I simply don't have the time, energy or knowledge to answer many of the questions submitted here. Some may be answered directly by email, others posted on the site. Thank you for your patience and understanding!

This site uses Akismet to reduce spam. Learn how your comment data is processed.