The Palette/blue/Made in a lab or a works
Cerulean blue
The sky blue that stays blue under artificial light, which is why it was invented for painters working indoors.
- Hex
- #1D6FB8
- RGB
- 29, 111, 184
- Pigment index
- PB35
- First used
- 1805, commercially from 1860
- From
- Germany
What it is
Cobalt(II) stannate, CoO·nSnO₂, cobalt and tin oxides fired together.
How it is made
Cobalt and tin oxides, fired. The tin is what shifts the hue towards green and pushes the opacity up, and it is also why this is one of the more expensive blues on the shelf.
Andreas Höpfner made it in 1805; it was sold as an artist's colour from around 1860 by George Rowney in England, under the name cerulean, from caeruleum, which had meant Egyptian blue two thousand years earlier. The name was recycled, and the reuse causes a certain amount of confusion in old texts.
Cerulean blue, Shade of blue (color). Wikipedia's description, not ours.
The history
Its selling point was unusual and specific: it does not change colour under artificial light. Most blues go dead or violet under candle and early gas light, which was a real problem for a painter finishing work in the evening or for a picture hung in a lamplit room. Cerulean holds.
It became the standard sky blue of late nineteenth and twentieth-century painting, and it is worth looking at why. It is opaque, slightly green, and relatively light in value, which is exactly what a sky near the horizon is. Painters who reach for ultramarine for a sky end up with a deep transparent blue that reads as space, not as atmosphere, and then have to fight it with white.
The modern colour-picker 'cerulean' (the flat #007BA7 of a thousand websites) and the pigment are related the way a photocopy is related to a painting. The pigment is greyer and cooler than the name suggests to anyone who has only met it as a hex.
Where it is used
Skies, water, pale distance, and any passage where a blue needs to be opaque and light without being chalky. Cézanne and Monet both used it heavily.
In brand work its usefulness is that it sits comfortably at a light value while still reading as a real colour, which most blues do not. Tinting ultramarine or phthalo up towards a pale blue gives you something washed out; cerulean is already there and still has body. If your palette needs a pale blue that does not look like a tint of something else, this is the family to look in.
Go and look at it
Cézanne's Provençal skies
Musée d'Orsay, Paris; Courtauld, London
Opaque pale blue laid in patches against warm ochre rock. The sky is built, not glazed.
How it behaves
- Made of
- Cobalt(II) stannate, CoO·nSnO₂, cobalt and tin oxides fired together.
- Opacity
- Opaque. It covers what is under it.
- Lightfastness
- Permanent. It will outlive the thing it is painted on.
Hazard
Cobalt compound; avoid breathing the dry powder.
- Holds its hue under artificial light, which is what it was made for.
- Expensive, and very often replaced with a phthalo-plus-white imitation labelled 'hue'.
- Low tinting strength. It will not overwhelm a mixture, which makes it easy to control.
The numbers
All derived from the hex, never typed beside it
- HEX
- #1D6FB8
- What a browser and a design file want.
- RGB
- 29, 111, 184
- The same three numbers in base ten. Light, not ink.
- HSL
- 208°, 73%, 42%
- Hue, saturation, lightness, how a person describes a colour.
- CMYK
- 84, 40, 0, 28
- Arithmetic only. A real separation depends on the press, the paper and a profile.
- LAB
- 46, 3, -45
- Perceptual. L* of 46 is how light this actually looks, which HSL gets wrong.
This hex is an approximation, and a page about pigment has to say so. Cerulean blue is a substance. A ground mineral scatters light from thousands of separate grains at slightly different angles, and no single sRGB value is that event. It is a flat average of a photograph of it, and the real thing is outside what a screen can emit. Use the number to specify a screen colour. Do not use it to judge the pigment.
Where it sits on the wheel
Hue 208°
Its opposite, by the diagram
The arithmetic opposite, 180° round in HSL. Useful, and not the same as the opposite a painter mixing pigment would name, that wheel was built for paint.
Lighter and darker
Towards white
Mixing with white does not only lighten, it dulls and cools. This is why a pale tint of a brand colour so often looks chalky, and why a designed light shade beats a generated one.
Towards black
Mixing with black kills the hue as it darkens. A better dark is usually the colour plus its neighbour round the wheel, which is what every painter does instead.
Can you set type on it
Handgloves
White on it
5.23:1 · AA
Handgloves
Black on it
4.02:1 · Large text only
Handgloves
It on paper
4.96:1 · AA
Handgloves
It on ink
3.50:1 · Large text only
4.5:1 is the floor for body text and 3:1 for large text, by WCAG. Treat it as a minimum you may not go under rather than as a target: plenty of pairs pass at 4.6:1 and are still unpleasant to read for a page, and the checker will not tell you that.
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