ions

rare-earth colorants

Lanthanide ions absorb in narrow bands that the host glass barely shifts, giving pale lilacs and pinks, and glass that 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 absorb visible light by electronic transitions. The physics differs in one respect, and that difference accounts for how the rare-earth family behaves.

In transition metals, the absorbing electrons (d orbitals) are on the outside of the ion, 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 shield them. The surrounding glass has little effect on them.

The result: narrow absorption bands at nearly fixed wavelengths, almost indifferent to the host glass composition. A transition metal spectrum is a broad band that shifts with the glass; a rare-earth spectrum is a set of narrow bands that stay in place. This insensitivity to the host is what distinguishes the family and what makes it useful.

The colors are paler than the transition-metal palette (lilacs, pinks, and pale greens rather than saturated blues and ambers), but the narrow, fixed bands produce effects that broad-band colorants cannot, including glass that changes color depending on the light source it is seen under.

Artifact stories

The alexandrite effect

Neodymium (Nd³⁺) 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 is the same in both cases; its narrow-band transmission spectrum weights each illuminant’s spectral power distribution differently.

This behavior is called the alexandrite effect, named after the alexandrite mineral (a chromium-bearing chrysoberyl) that shifts from green to red by the same mechanism. Neodymium glass marketed under trade names like “alexandrite glass” shows an effect related to metamerism. Strictly, it is color inconstancy (one object shifting appearance with the illuminant), whereas metamerism describes two different objects that match under one light and diverge under another. In practice the glass looks different in every photograph taken of it under different lighting, and it is 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 is the wavelength of the bright yellow-orange flare produced when molten glass containing sodium (essentially all soda-lime glass) contacts a torch flame. The sodium vaporizes and emits strongly at 589 nm, bright enough to make it difficult to see the work clearly.

Didymium eyewear (safety glasses with didymium-filtered lenses) absorbs this flare in its narrow band while transmitting the rest of the spectrum nearly intact. The glassworker sees the hot piece in color, without the yellow glare. The lenses are a notch filter worn as personal protective equipment, and they are standard equipment at every glassblowing torch and glass kiln. The narrow, fixed band that makes neodymium glass shift color with the light source is also what makes didymium glass a sodium-flare filter.

The rest of the family

Erbium (Er³⁺) gives a pale pink used in decorative glassware. Erbium is also the active ion in erbium-doped fiber amplifiers, where the same 4f transitions are used at 1.55 µm in the near-infrared to amplify telecommunications signals.

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

Cerium (Ce³⁺/Ce⁴⁺) is 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: it absorbs UV and releases electrons that reduce silver ions, so that silver colloids can be developed by exposure to light, by a process analogous to silver stain.

Uranium glass (vaseline glass) is included here by convention: 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 green under UV light, absorbing UV photons and re-emitting the energy as visible green light. Uranium glass predates the discovery of radioactivity; collectors still seek it out for the fluorescence. Modern pieces use depleted uranium at low levels.

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