Cerulean Blue

What color is cerulean blue?

The story of cerulean blue

Cerulean blue was a watercolor before it was anything else, and before that it was a Swiss magazine editor's side project that nobody bought for seventy years.

Albrecht Höpfner made the first batch in 1789 by roasting cobalt and tin oxides together, and published the recipe in the nature magazine he edited, under a title that translates as some small chemical experiments by the editor. That was the whole announcement. The pigment was known as Höpfner blue and sat in the background for decades while painters got on with Prussian blue and, from 1802, the new cobalt blue. A paler, chalkier cobalt was not an obvious thing to want.

London changed its mind at the end of the 1850s. The Times carried an advertisement in December 1859 for caeruleum, a new permanent color for artists, and the color merchant George Rowney had it on sale in the early 1860s, though the firm of Roberson had quietly been buying the same blue from a German maker before him. For its first decade it was sold only as a watercolor. Oil painters did not get it until the 1870s, and that is when it took off.

The Impressionists wanted it because it did something no other blue did: it stayed pale and slightly green at full strength, which is what daylight looks like near the horizon. Monet had it on his palette by 1877. When the National Gallery in London analyzed his Gare Saint-Lazare from that year, they found a nearly pure patch of cerulean in the shadows under the station canopy and suggested it was there because it was greenish enough to stand in for Prussian blue. Berthe Morisot painted the blue coat in Summer's Day, from 1879, with cerulean alongside ultramarine and cobalt. Van Gogh made his own approximation from cobalt blue, cadmium yellow and white, which is a mixing lesson in one sentence.

What you buy today is still Höpfner's cobalt stannate, PB35, and it is one of the pricier tubes on the rack because cobalt is a pricey metal. The name has had a life of its own as well. The United Nations put cerulean on its emblem at the end of the Second World War because the designer, Oliver Lundquist, wanted the opposite of red, the color of war. Pantone made it the first ever Color of the Year in 2000. There is even a speech about a cerulean sweater in The Devil Wears Prada.

Here is my take. Cerulean is not a blue you need on day one. It is the blue you buy after you have painted a hundred skies with ultramarine and noticed they all look like the same evening. It does one thing, the pale daylight blue where the sky meets the ground, and it does it with a grain that looks like weather. A magazine editor's afternoon experiment turned out to be the most honest sky color we have.

Pigment facts

These describe the pigment in general. Every brand's tube behaves a little differently, so test yours on scrap paper.

How to mix cerulean blue

You cannot mix a convincing cerulean, and I have tried. A green-leaning blue with a little white gets you the hue, but the chalky, granulating body is the pigment's own. If you want a stand-in, thin a phthalo blue a long way and accept that it will dry smoother and stain harder.

One pairing matters more than the rest: cerulean with a red earth. Cerulean and burnt sienna make a soft, granulating cloud gray; cerulean with Venetian Red makes the warmer gray for the shadow side of a cumulus. That pair has painted most of the skies I have ever needed.

Using cerulean blue in watercolor

Use cerulean where the sky meets the ground. Real skies get paler and slightly greener toward the horizon, and cerulean is that color straight from the pan. Wet the paper, run ultramarine or cobalt along the top and cerulean along the bottom, tilt once, and leave it. The granulation does the rest.

It is also the quiet hero of distance. A hill five miles away is not green, it is a gray-blue, and cerulean with a touch of Venetian Red gets that in one go.

Because it is opaque, it will sit on top of a dried wash rather than glaze into it. That makes it useful for painting a pale sky back over one that went too dark, and useless for clean layered glazes. Keep the transparent blues for glazing and let cerulean do the covering.

Something I learned the hard way: cerulean dries chalky and lighter than it looks wet, and if you fuss with a wash it turns to dust on the paper. Put it down in one pass, a little stronger than feels right, and walk away.

Cerulean blue vs cobalt blue (and phthalo blue)

Cobalt blue is the older sibling and the one people mix it up with, since both are cobalt pigments. Cobalt is a straight, neutral blue, semi-transparent, with a soft granulation. Cerulean is paler, greener and opaque. Put them in one sky and the order is obvious: cobalt overhead, cerulean at the horizon.

Phthalo blue is the modern blue people buy instead, and it is a different animal. It is transparent, it stains, and it is far stronger, so a brush loaded with phthalo will take over a sky that cerulean would only have tinted. Phthalo is the better mixing blue and cerulean is the better sky.

If you can only carry two, take cobalt and cerulean; they share a family and a temperament. Cobalt blue is the sky overhead, cerulean is the sky where it touches the sea, and phthalo is the deep water under both.

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Mel, founder of Tobio'sI make portable watercolor kits and I named the company after my cat. I write these pages for the beginner I used to be, one pigment at a time.