Critical Metals: The Chemistry of Light

How can metal make light? From lightbulbs to lasers, many technologies have used the luminescent properties of rare earth metals. Discover their illuminating history and science.


IUPAC Periodic Table of the Elements (2003) by IUPAC Periodic Table of the ElementsScience History Institute

The rare earth metals include 17 elements. The structure of their electron shells gives many of these elements unusual magnetic and optical properties. 

That electron structure also means they have similar chemical properties, so they are challenging to separate and identify.

Some rare earth elements are luminescent, meaning they give off light when activated by electromagnetic radiation. This exhibit explores the history of the rare earth metals europium (Eu), yttrium (Y), cerium (Ce), and neodymium (Nd), and the radioactive metal thorium (Th).

The history of the rare earth elements is also linked with tungsten (W), another metal that glows incandescently. 

Carl Auer von Welsbach Cigarette Trading Card (1890/1930)Science History Institute

Our story begins in a chemistry laboratory at the University of Vienna in the early 1880s. An unpaid laboratory assistant named Carl Auer has been studying the rare earth elements. 

Remember, the similarities between these metals make them tricky to separate and identify.

Auer’s research proves that a metal believed to be an element was in fact an alloy (mixture) of two previously unknown elements. This discovery makes his scientific reputation. But he makes his fortune when he figures out the first industrial application for rare earth metals.

Thomas Edison Holding Incandescent Lamp (1912-06-01) by Unknown PhotographerScience History Institute

The 1880s were a time of rapid industrialization. Cities were growing. Factories were expanding. Inventors like Thomas Edison were racing to develop better ways to light up streets and buildings. The quest to find a long-lasting and affordable lamp drove research and experimentation with many unusual metals.

Testing Gas Mantles (1920/1929) by Welsbach Gas Light CompanyScience History Institute

Auer realized that the incandescent properties of the rare earth metals might make them ideal for lighting. When heated properly, these metals glow rather than burn. Their incandescence can turn heat into light more efficiently than candles or gas fires.

The Welsbach Incandescent Gas Burner (1889) by Welsbach Gas Light CompanyScience History Institute

In 1885, Auer patents a method for creating incandescent light that could build on the coal gas system already installed in many cities. He soaks a cotton net in a chemical solution made up of dissolved metals: magnesium oxide, lanthanum oxide and yttrium oxide.

Auer puts this net over the smokeless burner invented by his chemistry teacher Robert Bunsen. The cotton burns away, leaving a solid lattice that glows incandescently when the flame is turned on.

Exterior View of Welsbach Gas Light Company Facility (1920s) by Welsbach Gas Light CompanyScience History Institute

Auer forms a company and goes into production. But the combination of magnesium, lanthanum, and yttrium gives off a sickly green light that consumers don’t like. Auer’s first company goes bankrupt in 1889. Auer tries a new formula in 1890. This thorium and cerium mix produces a warmer, whiter light, and Auer is soon back in business.

Modeling and Hardening High Grade Mantles (1890/1930) by Welsbach Gas Light CompanyScience History Institute

By 1910, more than 900 women worked at the Welsbach factory in Gloucester City, New Jersey. They sewed, trimmed, tested and packed lighting “mantles” for shipment across the United States. 

Revolving Furnaces, Thorium Nitrate Department (1890/1930) by Welsbach Gas Light CompanyScience History Institute

Thorium and cerium were extracted from ore that was shipped to New Jersey from North Carolina, Brazil, and India. The leftover sand was used as landfill at many places in Gloucester CIty and Camden, New Jersey. But thorium is a radioactive metal, and the Welsbach factory left behind a toxic legacy.

Tour of the Welsbach & General Gas Mantle Superfund Site (2017-06-24) by United States Army Corps of EngineersScience History Institute

Today the former Welsbach factory is a Superfund site. The U.S. Environmental Protection Agency and the Army Corps of Engineers have spent more than $300 million removing radioactive soil and monitoring groundwater safety.

General Electric National Mazda Drawn Tungsten Filament Incandescent Light Bulb (1917) by General Electric CompanyScience History Institute

Although Welsbach sold more than 5 billion mantles worldwide between 1890 and the 1930s, electricity ultimately prevailed as the main source for industrial lighting.

Metallurgical research at General Electric produced a tungsten filament lamp that came to dominate the market in the 1920s. Tungsten, rather than rare earth metals, became the most common metal used in 20th-century lighting.

Light Bulb in the Dark (2017) by Amos777eligius via Wikimedia CommonsScience History Institute

In an incandescent bulb, electrons are squeezed through a long, thin coil of metal. This filament gets hot and bright. An inert gas inside the glass globe keeps the filament from burning. But incandescent bulbs waste a lot of heat while making light.

Two Fluorescent Tubes (2013) by Dmitry G. via Wikimedia CommonsScience History Institute

Fluorescent light offers a cooler, more energy efficient alternative. A current of electrons creates ultraviolet radiation as it shoots through a tube filled with mercury vapor. This radiation hits the phosphors, metallic compounds that coat the tube’s inside. Each phosphor converts UV radiation into specific wavelengths of visible light.

We crucible tested over 50,000 phosphors last year. Any one of them might solve a problem for you. (1967) by GTE Sylvania IncorporatedScience History Institute

Industrial chemists experiment with different metallic compounds to develop new phosphors.

Examples of rare earth phosphors include yttrium oxide, to produce red light, and yttrium silicide for blue hues.

How Can a Rose be a Rose be a Rose on Color TV? (1964) by GTE Sylvania IncorporatedScience History Institute

The color and intensity of light depends upon the phosphor. In the early 1960s, phosphors using the rare earth element europium became available. Finding a new red phosphor bright enough to balance existing blue and green phosphors was a major limit on the growth of color TV.

Boatswain’s Mate 2nd Class Kerik Vargas Switches his Lights to High Efficiency Light Bulbs (2011-05-12) by United States Navy and Mass Communication Specialist 2nd Class Mark LogicoScience History Institute

Compact fluorescent lightbulbs use europium phosphors to generate light with 75% less electricity than a traditional tungsten incandescent bulb. The United States Congress enacted the first U.S. standards for lighting efficiency in 2007.

During the 2010s, as lightbulb makers shifted production and homeowners replaced burned out bulbs, residential electricity demand per household declined for the first time ever. 

Q-LINE RGB Laser Pointers (2012-04-02) by BOY via Wikimedia CommonsScience History Institute

In addition to incandescence and fluorescence, metals can make light through Light Amplification by Stimulated Emission of Radiation—the laser! 

Lasers are devices that use energy to stimulate molecules to emit light in specific wavelengths. The molecules of the lasing medium are often held in crystalline forms, like ruby or garnet.

Fishing with a Green Laser (1969) by GTE Sylvania IncorporatedScience History Institute

Unusual optical properties make some of the rare earth metals particularly useful for lasers. A garnet crystal made of yttrium and aluminum atoms can produce focused beams of light.

Researchers at Bell Labs in 1964 discovered that by “doping” this crystal (replacing about 1% of the yttrium atoms with neodymium atoms) they could create an infrared laser.

Fishing with a Green Laser (1969) by GTE Sylvania IncorporatedScience History Institute

Nd:YAG lasers have been used in military weapons systems, ophthalmology, cancer treatment, and manufacturing—they’ve even been used for finding fish!

By Michael RougierLIFE Photo Collection

One important application of laser light was the development of fiber optics. Thin, flexible fibers made of highly purified glass can channel laser light. When combined with signal processing technologies, fiber optics make it possible to carry a lot of digital information at the speed of light.

However, fiber optic signals degrade over long distances. After traveling through about 60 miles of fiber optic lines, laser light fades. Researchers in the 1980s discovered a solution in the fluorescent properties of erbium ions. 

Today, erbium fiber amplifiers are used to intensify the signal in high-capacity fiber optic lines that carry internet data and long-distance telephone calls around the world. 

The difficulty of separating rare earth metals still affects how widely they are used in industrial applications. Solvent extraction is the primary process used for separating rare earth elements today. But this process creates a lot of toxic solvent waste as a byproduct.

Studying Reaction Chemistry with Lasers (2022) by Felice MaceraScience History Institute

Researchers hope to develop alternatives to solvent extraction by better understanding chemical compounds. Neodymium and yttrium lasers provide pictures of how compounds vibrate and move, information that can be useful in finding metals separations that are more efficient and less toxic.

This is Too Bad, Makes Night & Day Alike! (1901-09-14) by Welsbach Gas Light CompanyScience History Institute

Credits: Story

Written and curated by Roger Turner, Science History Institute.

Research and scientific advice by Eric Shelter, Marta Guron, and Ranadeb Ball, University of Pennsylvania, and Leighton Jones, Northwestern University.

Special thanks to:
Zepler Institute, University of Southampton
Funding for this exhibit was provided by the Center for Sustainable Separations of Metals under NSF Grant #CHE-1925708. 

Credits: All media
The story featured may in some cases have been created by an independent third party and may not always represent the views of the institutions, listed below, who have supplied the content.