chapters · 03 the color rod
Every color here is computed from absorption spectra through the CIE 1931 observer, under the lamp at the foot of the screen. The same lesson sits on the walk between its neighbors.
builds on01 the batch
Each ion absorbs its own bands
Transition-metal ions are the most common colorants. Their outer d-electrons can be lifted between energy levels by light of particular colors, and the surrounding glass sets the spacing. Cobalt takes the orange and red. Copper takes the red end. Chromium takes violet and red, which leaves a green window. Pick a rod to see which part of the spectrum it removes.
Read: transition metal colorants → Rare-earth colorants →
Now blow it
A rod's color is not the color of the finished piece. Melted onto a gather and blown out, the same glass becomes a wall that is thin at the rim and heavy at the foot, and every part of it is colored by its own thickness. This is the windowpane edge from the batch, spread over a whole object.
The bowl is drawn in half section, the way a glass drawing shows it. On the left the near half is cut away: the cut wall is tinted by its thickness, and behind it you see the inside of the far wall. On the right is the bowl as you would see it held up to the light. It uses the rod and amount chosen above.
Glassblowers get around this by casing. They put the color on as a thin skin of strong glass and blow clear glass inside it, so every part of the piece crosses about the same amount of color however unevenly the clear behind it was blown. Switch to cased and the foot stops going dark.
the same rule in a cane
Cut to clear. A cased skin can be ground through in the cold shop to the clear glass beneath, which is how overlay glass gets its windows: the color is only half a millimeter deep.
Model. The bowl is a solid of revolution 170 mm across and 100 mm tall. Its wall runs from 1.1 mm at the rim to about 6 mm at the base and sits on a 6 mm floor, all scaled by the slider; the 8 mm foot is a separate gather and stays fixed. On the right each point's color is Beer–Lambert through the glass a straight ray crosses there, front wall and back. On the left the cut wall is tinted by its thickness at right angles to the surface, and the floor and foot by the whole stack a ray straight up from the table would cross. Each rod is dosed so a 3 mm solid wall passes about 60% of daylight at amount ×1, since colorants differ in strength by an order of magnitude. The cased skin is 0.5 mm at six times that concentration, so face on it holds the same colorant as a 3 mm solid wall. The cane is 12 mm across with the same 0.5 mm skin, and the slice is 3 mm thick. Refraction at the curved surfaces, which bends real outlines, is ignored.
explain it back5 questions. Answer each in your head, then open it.
What in a cobalt ion absorbs the light, and what sets which colors it takes?
Its outer d-electrons, lifted between energy levels by light of particular energies. The surrounding glass sets the spacing of those levels, so the same ion absorbs differently in different hosts.
Chromium absorbs at both ends of the spectrum. Why is chrome glass green?
Cr⁶⁺ takes the violet edge and Cr³⁺ takes blue and orange-red, so what passes is the window between them, in the green.
A solid-color bowl is palest at the rim and darkest at the foot. Why, and how does casing fix it?
Each point is colored by the glass a ray crosses there (Beer–Lambert), and the rim is blown thinnest while the foot is heaviest. Casing puts the color in a thin skin of strong glass, so every part crosses about the same amount of color however the clear inside was blown.
Why does solid cane look darkest down its middle while cased cane looks darkest at its edges?
A ray through the middle of solid cane crosses the most glass. In cased cane the color is only in the skin: a ray down the middle crosses the skin twice, but a ray near the edge runs along the skin for a long way.
Neodymium glass is lilac in daylight. Predict what happens under a tri-phosphor tube, and why.
It turns blue-gray. Its absorption bands are narrow, and a tri-phosphor tube puts its light in a few narrow lines that the bands hit unevenly, so a different share of the lamp survives. The showroom stage shows it.
go further
- Transition-metal colorants essay
- Rare-earth colorants essay
- Mixing Lab lab