The Palette/blue/Made in a lab or a works
Phthalo blue
Found as a blue crust in a failed industrial batch, and now the strongest blue anybody has. One part in a thousand is still visibly blue.
Also called phthalocyanine blue · Monastral blue · Winsor blue
- Hex
- #000F89
- RGB
- 0, 15, 137
- Pigment index
- PB15
- First used
- 1935
- From
- Grangemouth, Scotland
What it is
Copper phthalocyanine, C₃₂H₁₆CuN₈: a large flat organic molecule with a copper atom in the middle, structurally a cousin of chlorophyll and haemoglobin.
How it is made
Phthalic anhydride or phthalonitrile is reacted with urea and a copper salt at high temperature. The molecule assembles itself around the copper atom, which is the elegant part: the pigment is a single ring structure rather than a crystal of salts, and its colour comes from the whole molecule.
It was noticed in 1927 and again in 1935 at Scottish Dyes in Grangemouth as an unwanted blue impurity in phthalimide production. ICI worked out the structure and put it on the market as Monastral blue. Once again the pigment arrived as a contamination that somebody bothered to look at.
Phthalo blue, Synthetic blue pigment from the group of phthalocyanine dyes. Wikipedia's description, not ours.
The history
This is the most important pigment of the twentieth century and almost nobody outside the trade knows its name. It is lightfast, chemically inert, cheap to make and absurdly strong, and it displaced Prussian blue, indigo and most of the blue trade within a generation.
It is probably the blue on the thing nearest to you right now. Blue plastic, blue printing ink, blue car paint, denim-coloured dye, the cyan in a four-colour press, blue ballpoint, mostly phthalocyanine chemistry. When you specify a cyan in CMYK you are specifying this.
Its strength is a genuine hazard in practice. A painter who squeezes out a normal-looking amount of phthalo blue and starts mixing will make greens and greys for the rest of the afternoon that all have the same cold bias, because a trace of it survives every wipe of the brush. Experienced people treat it like a stain rather than a colour.
Where it is used
Process cyan, printing ink, plastics, automotive coatings, textiles, and most modern blue paint that is not sold as cobalt or ultramarine. Also the base of nearly every convincing modern black-green and teal.
For a designer the relevant fact is that the entire CMYK blue range is built on this one compound, and that is why screen blues and printed blues disagree in the specific way they do. The cyan primary is not a theoretical colour; it is a particular molecule with particular limits, and the gap between an RGB blue and what a press can reach is in part the gap between a phosphor and a phthalocyanine.
Go and look at it
The cyan plate of any four-colour print job
Any printed thing within reach
Look at a magazine photograph under a loupe: the cyan dots are this pigment.
Yves Klein's IKB
Museums worldwide
Klein's blue is ultramarine, not phthalo, but the binder problem he solved is the one phthalo's brute strength usually hides.
How it behaves
- Made of
- Copper phthalocyanine, C₃₂H₁₆CuN₈: a large flat organic molecule with a copper atom in the middle, structurally a cousin of chlorophyll and haemoglobin.
- Opacity
- Transparent. It stains rather than covers, so it can be glazed over another colour.
- Lightfastness
- Permanent. It will outlive the thing it is painted on.
- Overwhelming tinting strength. Use a tenth of what looks right.
- Comes in a red shade and a green shade (PB15:1, PB15:3) and the difference matters in mixing.
- Almost completely inert. It survives acid, alkali, heat and solvent.
The numbers
All derived from the hex, never typed beside it
- HEX
- #000F89
- What a browser and a design file want.
- RGB
- 0, 15, 137
- The same three numbers in base ten. Light, not ink.
- HSL
- 233°, 100%, 27%
- Hue, saturation, lightness, how a person describes a colour.
- CMYK
- 100, 89, 0, 46
- Arithmetic only. A real separation depends on the press, the paper and a profile.
- LAB
- 16, 44, -65
- Perceptual. L* of 16 is how light this actually looks, which HSL gets wrong.
This hex is an approximation, and a page about pigment has to say so. Phthalo 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 233°
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
14.69:1 · AAA
Handgloves
Black on it
1.43:1 · Fails
Handgloves
It on paper
13.94:1 · AAA
Handgloves
It on ink
1.24:1 · Fails
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.
Read more, elsewhere
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