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What Is Cobalt Glass?

By Alex Newth
Updated May 17, 2024
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Cobalt glass is the deep blue combination of glass and cobalt metal, with the blue coloring resulting from the cobalt inclusions. Very little cobalt is added to molten glass to produce the color; a glass structure with 0.5 percent cobalt will give it a powerful blue color, with manganese and iron commonly added to mute the color. Aside from an appealing appearance, cobalt glass can be used as an optical filter for flame tests, because it filters out the contaminating colors cast from iron and sodium. Smalt, or powdered cobalt glass, is used as a pigment for paint and pottery.

When glass is made from heating sand and other sources of carbon to very high temperatures, the heat causes the carbon to become a molten substance. Before the glass can cool and solidify, cobalt can be added to the mixture to give the glass a deep blue color. Cobalt is one of the strongest pigmenting metals, so only a very small amount is needed to make the blue coloring occur. Most glass will need just 0.5 percent cobalt to produce a striking color.

Cobalt is such a strong coloring agent that it can easily make a powerful blue color, but some manufacturers may want a fainter shade of blue. To achieve this, and to ensure there is enough cobalt in the glass mixture, iron or manganese is added to the glass. Aside from making the glass harder, this also mutes the cobalt’s blue.

While cobalt glass looks ornate and is a prized material for many collectors, it also has a practical application for scientists performing flame tests. A flame test is a test in which a substance is put over a flame, and the change in the flame’s color determines what metals are in the substance. If there is no optical filter, sodium and iron contaminate the flame color, making it look too yellow. Using cobalt glass as an optical filter balances out the color and makes it easier for scientists to view the metal ions.

Another function of this glass is as smalt, or cobalt glass that has been crushed to a powder. Smalt is used in pottery and painting to lend its powerful blue pigment to these mediums. This often was used in the past, but synthetic pigments are more often used in contemporary times, because they are cheaper to make and reproduce the same color each time. Some hobbyists and professionals still use smalt, even though it is more expensive.

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Discussion Comments

By runner101 — On Sep 01, 2011

@sinbad - You are right - cobalt sea glass is rare or like you mentioned that is what those who are avid sea glass finder hobbyists say.

I find different glass pieces as a friend of mine is always looking for new items to make unique jewelry with and most recently she tried sea glass. As it turns out we found the cobalt blue sea glass jewelry items were priced higher than others secondary to its "rarity."

After reading this article and comments though, I wonder how rare "true" cobalt glass sea glass is!

By Sinbad — On Aug 31, 2011

My husband used to live on the beach and one of his favorite things to do was to go hunting for "sea glass." This is the glass fragments that come from broken bottles in the ocean that have been smoothed over by years and years of being tossed in the saltwater and sand.

I was curious about this habit of his and found out there were lots of people who enjoy this as well; enough so that there are conferences of sort for collectors.

One thing that my husband always mentioned was that finding blue, red, or yellow glass was extremely rare. When I looked up information on this habit it turns out he was right.

Now I wonder if the broken cobalt blue glass sea glass is even more rare.

By amsden2000 — On Aug 30, 2011

@Almita - Telling them apart is sort of a two step process for me.

First off -- any cobalt glassware that is opaque or clouded is not made with cobalt pigment. It is the beach glass you talked about.

Next, hold the glass up to a light. If you see any hint of green -- it is not cobalt glass. This is hard to miss, so you should definitely check each jar thoroughly against a light.

If the jars that are left are really dark colored -- look through them for a purple tint. The purple tint is a sign of true cobalt -- the darkest shade of cobalt. Any of the blue glassware that passed the first two tests are cobalt too, but lighter shades of it.

Happy hunting!

By Almita — On Aug 30, 2011

My mom collects cobalt blue glass bottles, does anyone know where to buy the real kind? I know that there are a lot of bottles that are just made with blue beach glass, not the actual mineral.

I wanted to buy mom a cobalt jar to match the rest of her collection, but I'm worried I'll get a "fake" beach glass one. If anyone knows any tips on telling them apart, please share.

There are always shelves of cobalt blue colored glassware in the thrift stores I visit, but they all look the same to me. Do cobalt glass jars really come in that many shades or are those fake?

By manykitties2 — On Aug 29, 2011

Cobalt glass is very collectible and you would be surprised at the huge variety of pieces available through auction houses online. Cobalt glass seems to retain its value through time because of the quality workmanship that goes into making it.

I have a collection of cobalt glass figurines that I use to decorate throughout my house. Some of my favorite pieces are of animals that have additional white artwork done on the glass.

There are some lovely cobalt glass birds that you can buy with gorgeous white paint detailing. They vary in intricacy, but I find the best looking ones have small flowers and snowflakes on them.

By Sara007 — On Aug 29, 2011

My grandmother had a huge collection of cobalt glass in her china cabinet and I was always fascinated by the rich blue color of everything. She had some gorgeous vases, glasses and figurines.

My grandmother always told me she collected cobalt glass because the glass color reminded her of the lithodora she planted in her gardens. These small flowers have a very rich cobalt blue coloring.

I now have her cobalt glass collection in my own home as my grandmother left me it after she passed away. I am really happy to have such a wonderful reminder of her in our home.

By jmc88 — On Aug 28, 2011

@stl156 - The only element that I have ever heard of that is added to make glass stronger is lithium, which only helps make the glass stronger to a certain extent.

Usually in the glass making process they make the glass stronger by tempering or strengthening the glass. They do this by blowing cold air on the glass and also adding various chemicals that prevent cracking and significantly strengthen the stress the glass can take.

By TreeMan — On Aug 28, 2011

@ stl156 - That is an excellent question. For whatever color glass is, as long as things have not been done to it in the glass making process, the elements in the ground determine their color tint. As the article states cobalt makes a powerful blue color but can also make a slight tint of violet. Other elements that create colors include iron, which creates green and brown tints, sulfur, which creates a bit of an amber color, and gold, which creates a reddish tint. There are many others that I do not know of, but I do know there are elements, such as sodium, that are used as de-coloring agents that simply help to get rid of the colors in the glass.

By stl156 — On Aug 27, 2011

I have always assumed that when glass is created it all comes out the same color, but coloring is later added. This article definitely showed me that it is a matter of what elements there are in the ground that create the distinctive color. My question now is what elements, besides cobalt; create what colors and which elements create stronger glass?

By Emilski — On Aug 26, 2011

I have always been attracted to cobalt glass because of its distinctive blue color. I once bought a cobalt glass pitcher for a friend, due to its blue color, because blue is her favorite color, and she loved it.

Anyone who is a fan of blue should invest in cobalt glass. They make for great decorations and since they are naturally blue they are not seen as cheesy or cheap. It can definitely add color to anyone's room, assuming blue is preferred.

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