04-19 cobalt blue bottles

[Image above] The deep blue color of these bottles is due to the inclusion of cobalt oxide in the glass. Credit: cobalt123, Flickr (CC BY-NC-SA 2.0)

By Becky Stewart

When humans first started making glass about 4,000 years ago in Mesopotamia, the final color was a mystery to glassmakers until they reached the end of production. But once people started to understand that mixing sand from one location with soda ash and lime from others could produce a finished product with different hues, colored glass began to demand a premium.

Yet what exactly about a particular combination of sand, soda ash, and lime determines whether it will form clear glass or one of the many stunning colors for which glassmakers are now known? As with so many questions on Earth, the answer comes down to chemistry (with a bit of physics thrown in).

The evolving understanding and science of colored glass

In the early days of glassmaking, people approached the creation of glass using principles from alchemy, the predecessor of the modern fields of chemistry and metallurgy. They considered glass to be a metallic substance, and its ability to imitate other precious stones and minerals through shaping and coloring led early alchemists to believe it may hold the key to the transmutation of other substances.

Though they never succeeded in turning lead into gold—which, fun fact, we now know is  possible through the hazardous and expensive process of nuclear transmutation—alchemists did determine which metallic compounds and other minerals produced specific colors in glass and at what temperatures. When you hear something described as “cobalt blue,” you probably know what shade is being referred to.

The graphic below demonstrates some of the main chemicals used today to color glass. Notice that in all cases, the colored glass remains transparent because the glass does not strongly absorb or reflect the light. This transparency makes glass unique compared to other types of materials.

Credit: Compound Interest (CC BY-NC-ND 4.0)

While these material combinations and their corresponding colors are well documented today, the first extensive documentation of colored glass production is credited to the 8th-century Persian glassmaker Abu Musa Jabir Ibn Hayyan (known as Geber in Western texts). Scholars consider Jabir ibn Hayyan the first true chemist, and his book Kitab al-Durra al-Maknuna (The Book of the Hidden Pearl), written sometime in the late 8th or early 9th century, contains everything he knew about the production of colored glass.

Today, we know the reason these material combinations affect the finished glass’s color is due to their impact on the glass’s structure. A glass’s structure can be modified in several ways, such as by introducing chemical impurities (typically metals and metal oxides), defects, and phase separations, among other factors. These changes to the glass structure affect the material’s optical adsorption edge, or the region in which an electron has enough energy to move between energy states.

Different wavelengths of light have different energies, so glass with a modified structure will not be able to absorb or transmit all light equally. Some of the wavelengths that pass through are refracted or bent, while the small range of wavelengths that cannot pass through are reflected. The collective result of this reflected, refracted, and transmitted light gives the glass its color.

Novel coloring schemes: Dichroic and fluorescent glasses

In some cases, glass color is determined based on the interplay between the nanostructure of the glass and the ambient light conditions under which it is viewed. For example, dichroic and fluorescent glasses.

Dichroic glasses appear one color under reflected light and a different color under transmitted light. Perhaps the most famous example is the Lycurgus cup, an ancient Roman vessel. The cup’s interesting optical effects relate to its nanostructure. It remains a subject of debate whether the creator understood the reason for its color-changing property.

Fluorescent glasses appear to be a different color depending on whether they are viewed under visible or ultraviolet light. Uranium glass is a popular early 20th-century production style that fluoresces a brilliant citrine color under ultraviolet light but appears transparent yellowish green under visible light. Leaded glass will also fluoresce slightly blue under ultraviolet light, although it appears clear under visible light.

For centuries, alchemy was thought of as akin to magic, but as this history shows, their understanding of material properties enabled greater control over the design and production of colored glasses. The chemists and physicians (and materials scientists!) of today owe them a debt of knowledge. These combinations of materials and the colors they produce have been well documented, and today it is possible to buy colored glass in bulk quantities for artistic or scientific uses.

Practical applications of colored glass

In the time since Jabir ibn Hayyan’s book, colored glass has found a wide range of uses from the mundane (storing wine in dark bottles to safeguard against the effects of ultraviolet radiation in light) to the sublime (see Part 1 and Part 2 of CTT’s stained-glass window series, which is publishing this month).

There are many other applications of colored glass in research and industry. If, like me, you work primarily with computer screens, you may have prescription lenses that are tinted yellow to block the blue light those screens give off. Meanwhile, neodymium glass, which is purple under sunlight but blue under fluorescent light, is used in high-powered lasers because it can absorb light of specific wavelengths and re-emit it in in a spatially coherent and amplified form.

Colored glass also plays a big role in the art world, where the scientific principles described herein are used more than people may realize. Learn more by reading the May 2020 Bulletin feature story “Coalescence of glass art and glass science,” co-written by ACerS Fellow John C. Mauro.

Interested in even more novel applications of colored glass? Be on the lookout for the upcoming May 2024 Bulletin, which publishes May 2. It will dive into the exciting world of chalcogenide glasses, which are opaque under visible light but transparent in the infrared!

Further reading

Harding, F.L. (1972). “The development of colors in glass.” In: Pye, L.D., Stevens, H.J., LaCourse, W.C. (eds) Introduction to Glass Science. Springer, Boston, MA. https://doi.org/10.1007/978-1-4757-0328-3_13

Orzel, C. “The (mostly) quantum physics of making colors,” Forbes, 01 July 2019.

Science in art: Dichroic glass” University of Delaware Engineering, 11 February 2016.

Since glass is a solid, how can we see through it? Why can’t we see through wood?,” Popular Science, 02 April 2002.