ions

rare-earth colorants

Lanthanide ions absorb in narrow, host-insensitive pickets rather than broad bands — subtle lilacs and pinks, and glass that genuinely changes color with the light source.

Rare-earth colorants — the lanthanide series elements — work by the same basic principle as transition metal colorants: ions dissolved in the glass absorbing visible light by electronic transitions. But the physics differs in one crucial way, and that difference defines the whole character of the rare-earth family.

In transition metals, the absorbing electrons (d orbitals) are on the outer surface of the ion, fully exposed to the electric field of the surrounding glass. The glass host therefore strongly influences the color — which is why nickel goes violet in potassium glass and brown in soda glass. In rare-earth ions, the absorbing electrons (f orbitals) sit inside filled outer electron shells that act as a shield. The glass barely touches them.

The result: narrow absorption bands at nearly fixed wavelengths, almost indifferent to the host glass composition. A transition metal spectrum is a broad hump that shifts with the glass; a rare-earth spectrum is a picket fence that stays put. This host-insensitivity is both the scientific signature and the practical asset of the family.

The colors are subtler than the transition-metal palette — delicate lilacs, pinks, and pale greens rather than saturated blues and ambers — but the sharp, fixed bands produce effects that broad-band colorants simply cannot achieve, including glass that genuinely changes color depending on the light source it is seen under.

Artifact stories

The alexandrite effect

Neodymium (Nd³⁺) is the family’s most striking member. It absorbs strongly in the yellow-green region (near 580 nm, which includes the sodium D-lines) and has additional absorption in the green, leaving a narrow window on either side. The perceived color depends on which wavelengths the illuminant emphasizes.

Under daylight or cool fluorescent light, which is rich in blue and green, neodymium glass reads as a cool blue-lavender or lilac. Under incandescent light, which is rich in red and yellow and weak in blue-green, the same glass reads as warm pink or rose. The glass has not changed; only the light source has, and the picket-fence transmission spectrum responds differently to each illuminant’s spectral power distribution.

This behavior is called the alexandrite effect, named after the alexandrite mineral (a chromium-bearing chrysoberyl) that undergoes a dramatic green-to-red color shift by the same mechanism. Neodymium glass marketed under trade names like “alexandrite glass” is the glassmaker’s implementation of a cousin of metamerism — strictly, this is color inconstancy (one object shifting appearance with the illuminant), where metamerism describes two different objects that match under one light and diverge under another. In neodymium’s case, the effect is not just interesting: it means that the glass looks genuinely different in every photograph taken of it under different lighting, making it a collector’s favorite.

Didymium eyewear

Didymium is the name given to the naturally occurring, incompletely-separated mixture of neodymium and praseodymium in rare-earth minerals — the name comes from the Greek for “twin,” because the two elements were once thought to be one. Before their separation was achieved in the 1880s, didymium was used as-received, and it remains useful today in its mixed form.

Didymium glass has a sharp absorption band centered near 589 nm — the sodium D-lines. This wavelength is exactly the brilliant yellow-orange flare that erupts when molten glass containing sodium (essentially all soda-lime glass) contacts a hot torch flame. The sodium vaporizes and emits intensely at 589 nm, which is bright enough to make it difficult to see the work clearly.

Didymium eyewear — safety glasses with didymium-filtered lenses — subtracts this flare via the narrow-band absorption while transmitting the rest of the spectrum nearly intact. The glassworker sees the hot piece in color, without the yellow glare. This is a precision notch filter worn as personal protective equipment, and it is standard equipment at every glassblowing torch and glass kiln. The same optical property that makes neodymium glass shift color with the light source makes didymium glass a natural sodium-flare filter — the narrow band position is the asset in both applications.

The rest of the family

Erbium (Er³⁺) gives a soft, distinctive pink used in decorative glassware. Erbium is also the active ion in erbium-doped fiber amplifiers — the same 4f transitions, now used at 1.55 µm in the near-infrared to amplify telecommunications signals rather than to color decorative glass. The decorative colorant and the amplifier medium are the same ion in the same electronic configuration.

Praseodymium (Pr³⁺) produces a pale green. In the didymium mixture it contributes to the overall absorption profile alongside neodymium.

Cerium (Ce³⁺/Ce⁴⁺) is the family’s nearly invisible member: essentially colorless in ordinary light but a strong UV absorber. UV-blocking container glass and ophthalmic lenses exploit this. Cerium is also an oxidizing redox buffer (see redox in the furnace), and in photosensitive glass formulations it acts as a photosensitizer — absorbing UV and generating electrons that reduce silver ions, enabling the light-triggered development of silver colloids by a process analogous to silver stain.

Uranium glass (vaseline glass) is an honorary member — uranium is an actinide rather than a lanthanide, but it has historically been grouped with this family in the colorist’s practice. The uranyl ion (UO₂²⁺) gives the characteristic yellow-green color of vaseline or uranium glass, and the glass fluoresces a vivid green under UV light — re-emitting absorbed UV photons as visible green rather than simply transmitting around them. Uranium glass predates the discovery of radioactivity; collectors still prize it for the fluorescence. Modern pieces use depleted uranium at low levels.

← back to the map