Blue Sea Glass

By Rebecca Ruger

hawaiian blue sea glass

The thing about cobalt beach glass that makes it such a crowd favorite is its accessibility—it’s rare, and we whoop when we find it, but it’s not nearly so elusive as the red or the pink or the orange. Cobalt glass is absolutely attainable, without having to purchase from sea glass sellers.

blue sea glass on beach 

Cobalt glass was possibly first made in Egypt around 2000 BC, but wasn’t used or made regularly until much later in England and the United States. Most blue glass is given its color from cobalt oxide or a copper oxide added to the molten glass. (Cobalt, the element, is a lustrous, silvery-blue metal, which is also magnetic.) It only requires a small amount of cobalt oxide to produce a deep, rich blue (as little as 5 ounces per ton of glass).

cobalt blue beach glass bottles

While cobalt oxide produces a deep royal blue, there are other compounds of cobalt which produce different colors. Cobalt aluminate makes turquoise glass; cobalt silicate produces violet-blue glass. Cobalt oxide added to borosilicate glass (a special type of glass in which boron oxide is added to the mix; think Pyrex) produces a purple or red glass.

Bristol Blue Glass, finger bowl and port decanter, inscribed ‘I Jacobs Bristol’, circa 1805. Photo Credit ©Victoria and Albert Museum, London

We know that much of blue glass created certainly in the 18th and 19th centuries was used for medicine bottles, but earlier, because the color blue was often associated with wealth and prosperity, it was also adapted by glass makers in Bristol, England, possibly as early as mid 17th century in what became known as Bristol Blue Glass, which was quite possibly the first cobalt glass made en masse for domestic—and not utilitarian—use. Bristol Blue Glass refers to any of the glass produced in the city of Bristol over a span of hundreds of years, until about the 1920s. At one time the small city may have housed as many as 17 glassworks factories. Isaac Jacobs, a Jewish immigrant from Germany was arguably the most notable glass maker of Bristol Blue Glass in the 18th century. At only 17 years of age, he began working in his father’s glass factory and helped increase the popularity of the Bristol glass. (In some research he is actually credited with being the inventor of Bristol Blue Glass.)

blue stained glass window

Beautiful blue glass was used in many decorative household pieces, such as vases, sculptures, and tableware. And, stained glass can also be a source of many shades of blue sea glass.

Merry Combs’ M in a Circle Cobalt find

Bounce forward a few hundred years and cross the ocean where some of the more probable makers of your found glass in the US will be Phillips, Bromo-Seltzer, Vicks, and Noxzema—even Avon produced many household glass items from tumblers to salad plates to gravy boats. You may regularly run across the "M" mark, or "M inside a circle" mark on a piece of cobalt glass. With no other identifying marks on your sea glass, you still may safely assume this to be part of any of the aforementioned blue medicine bottles. Emerson Drug Company, maker of Bromo-Seltzer, needed a reliable source for the large quantities of glass bottles it used and wisely founded its own Maryland Glass Company in 1907. The glassmaker was soon specializing in producing all kinds of cobalt jars and bottles, including Phillips Milk of Magnesia, Bromo-Caffeine, Vicks Vapo-Rub, and Noxzema, too. The "M in a circle" can be found on a great number of this type of glass made from the 1920s through the 1960s, sometimes with little or no other identifying marks. The plain"M" was sometimes used when other identifiers were also used.

blue bottle cornflower

Other sources of blue glass include many types of bottles in varying shades of blue. Some were colored blue to indicate that the contents might be harmful if swallowed or taken in large doses.

blue glass insulator waste

In some areas, blue glass was used to make glass lightbulb insulator bases. The waste glass from the manufacturing process can be found along the shores of Lake Erie near the General Electric plant that once stood here and in beaches of Barcelona near a light bulb plant located there in the 20th century. The glass insulators themselves can be found on beaches around the world wherever the lightbulbs were used.

cobalt blue sea glass stack


This article appeared in the Glassing Magazine September 2017 issue.

Watch videos about all the colors of sea and beach glass

Learn more about sea and beach glass colors:

where does sea glass get its color


    Hi, Mike! Those pieces are waste glass from the General Electric plant along the southern shore of Lake Erie. Did your friend find the glass around there?

    Kirsti Scott August 07, 2021

    What are the larger pieces that seem to be pitted? We have a friend that found one a similar size and one even larger.

    Mike August 07, 2021

    Hi! The piece with the metal cap is the glass insulator from the bottom of a lightbulb!

    Kirsti January 18, 2021

    There is a photo of a round piece of glass in your above article that has a metal round cap on top…what was this piece used for? Insulator? Mine has a tiny hole and looks like metal. I have a similar piece and in trying to identify

    L Davidson January 21, 2020

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