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.
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.
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.
From undated real photo postcard, Joliet, Illinois, circa 1909.
" data-medium-file="https:// data-large-file="https:// class="wp-image-1563 size-large" title="Telegraph pole with glass insulators, near Joliet IL, circa 1909" src="https:// alt="Telegraph line pole with glass insulators " width="640" height="780" srcset="https:// https:// sizes="(max-width: 640px) 100vw, 640px" / During the 1960s, 1970s and 1980s many of these lines were dismantled as technology advanced.
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.
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.