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color in glass

Pure glass is transparent because nothing in it can absorb a visible photon. Glassmakers color it by adding something that can, and there are four distinct kinds of thing to add.

Pure silica glass is transparent across the visible spectrum because it contains nothing that can absorb a visible photon. The electronic band gap of silica is about 9 electron volts, large enough that the energy threshold for electronic absorption falls in the ultraviolet, well above the visible range. At the other end of the spectrum, the Si–O bond vibrations absorb in the infrared. The visible band lies between those two absorption edges. Glass is also amorphous — structurally homogeneous at the scale of visible wavelengths — so there is nothing to scatter light either.

Window glass is refined to keep that range free of absorbers. To color glass, a glassmaker adds something that absorbs or scatters visible photons, whose wavelengths range from about 400 nm (violet) to 700 nm (red). What you perceive as the glass’s color is the transmitted spectrum: white light enters, the chromophore subtracts certain wavelengths, and the remainder reaches your eye. The four mechanisms differ in what does the subtracting.

The four mechanisms

Dissolved ions

The oldest and most common mechanism is metal ions dissolved directly in the glass melt, much as a dye dissolves in water. Glassmakers have used this for thousands of years: cobalt for blue, manganese for amethyst, iron for bottle green, copper for turquoise.

The physics is called d–d electronic transitions for transition metals: the outer electrons of ions like Co²⁺, Cr³⁺, or Fe²⁺ are split into energy levels by the surrounding oxygen atoms, and they absorb photons whose energy matches the gap between those levels. Because the energy levels depend on the ion’s oxidation state and the exact geometry of the surrounding oxygen coordination, the same element can give different colors in different glasses — or in the same glass under different furnace conditions.

Rare-earth ions work by a related but distinct mechanism: their absorbing electrons are shielded inside the ion’s electron shell, so their transitions are sharper and less sensitive to the glass host. Neodymium gives a characteristic violet-pink; erbium gives pink; erbium-doped fiber amplifiers exploit the same ion’s sharp 4f transitions at a different wavelength (~1.5 µm in the near-infrared) to amplify telecommunications signals.

→ Transition metal colorants · Rare-earth colorants

Metal colloids and nanoparticles

The second mechanism requires no ions: it uses metallic particles — gold, copper, or silver — precipitated inside the glass at sizes of tens of nanometers. At that scale, the conduction electrons of the particle oscillate collectively in resonance with the incoming light wave, absorbing strongly at a specific wavelength. This is called surface plasmon resonance.

Gold nanoparticles absorb green and blue light and transmit red — producing the wine-red of gold ruby glass. Copper nanoparticles give a similar but warmer red. In both cases the melt has no color: gold ruby glass is colorless when it comes out of the furnace. The particles only form during a controlled reheat called striking, and the glassmaker judges the color as it develops.

Silver behaves differently in this family: rather than ruby color through the body of the glass, it produces a yellow surface stain when silver ions migrate into the glass and reduce to nanoparticles near the surface — the basis of silver stain in the medieval stained glass tradition.

→ Colloidal color · Silver stain

Semiconductor band-gap crystals

The third mechanism produces the most saturated colors glass can make. Cadmium sulfoselenide (a mixture of CdS and CdSe) can be precipitated as tiny crystals inside the glass. These crystals are semiconductors, and like all semiconductors they have a band gap: photons with energy above the gap are absorbed; photons below it pass through. Because the gap sits in the visible range, the glass has a sharp absorption edge within the visible spectrum.

By adjusting the sulfur-to-selenium ratio, glassmakers can tune the cut-off across the yellows, oranges, and reds, producing the saturated signal colors used in traffic lights and railway semaphores. At very small crystal sizes — below about 10 nm — quantum confinement effects shift the edge further, and the color becomes size-dependent rather than composition-dependent. Glass was the medium in which quantum dots were first experimentally demonstrated: Ekimov’s 1981 work used copper chloride (CuCl) nanocrystals in glass, with cadmium-based systems following shortly after.

→ Selenium ruby glass

Structure and scattering: color without a chromophore

The fourth mechanism produces color — or white opacity — without any chromophore at all. The source of the optical effect is physical structure rather than chemistry: particles or layers that redirect light rather than absorbing it.

→ Continue reading below in Structure and scattering

Structure and scattering

When glass contains a fine dispersion of particles or phase-separated droplets whose size is comparable to the wavelength of light, it scatters rather than transmits. The result can be milky opacity, selective coloring, or the two-color effect called dichroism, depending on particle size, composition, and whether you are looking at transmitted or reflected light.

Opal glass

Opal glass achieves its milky, opalescent appearance through light scattering from a dispersed second phase embedded in the base glass. The second phase can be fluoride microcrystals (calcium or sodium fluoride, precipitated by adding fluoride compounds to the batch) or phase-separated droplets formed when an initially homogeneous melt separates into two glassy regions on cooling — a process called spinodal decomposition or nucleation-and-growth depending on the conditions.

The particles scatter short wavelengths (blue, violet) more strongly than long ones, following approximately Rayleigh-like behavior for very small particles. This gives opal glass its characteristic blue cast in reflected light and a warm, yellowish-orange transmission — the same physics that makes the sky blue and sunsets orange. At larger particle sizes the scattering becomes more uniform across wavelengths, producing the neutral milky white of dense opaline tableware.

The effect is structural: without the dispersed particles the glass is clear.

Dichroic glass

Modern dichroic glass — used in jewelry, art glass, and architectural panels — gets its angle-dependent color from thin-film interference rather than scattering or absorption. Extremely thin metal oxide layers (titanium dioxide, silicon dioxide, and others) are deposited on the glass surface in a vacuum chamber, building up a stack of optical layers. When light reflects off and transmits through these layers, the waves interfere with each other: certain wavelengths reinforce in reflection while their complements pass through in transmission. The result is a glass that appears one color in reflected light and a complementary color in transmitted light — and shifts across the spectrum as the viewing angle changes.

This is a different mechanism from the dichroism of the Lycurgus Cup, the Roman cage cup that appears green in reflected light and red when lit from behind. The Lycurgus Cup’s effect comes from gold-silver nanoparticles (colloidal mechanism, above) that both scatter and absorb. The thin-film interference of modern dichroic coatings and the nanoparticle scattering of the Lycurgus Cup produce superficially similar two-color effects through different physics.

Redox: the cross-cutting control

All four mechanisms depend, directly or indirectly, on the oxidation state of the elements involved. Iron in glass can be Fe²⁺ (blue-green absorber) or Fe³⁺ (a weak violet absorber that leaves pale yellow-green), or a mixture of both, depending on the oxygen level in the furnace atmosphere and the composition of the batch. Copper can exist as Cu²⁺ ions (turquoise) or Cu⁰ nanoparticles (ruby colloid). The same element in a different oxidation state can give a different color, or none.

Redox control is therefore not a fifth mechanism; it is the variable that governs which mechanism is operating and how efficiently. It is also the tool for decolorizing glass: when the iron in cheap raw sand would otherwise tint the melt green, glassmakers add oxidizing or reducing agents — or small amounts of complementary colorants like selenium or manganese — to cancel the tint and restore transparency.

→ Redox in the furnace

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