Cerulean
Cerulean (), also spelled caerulean, is a shade of blue ranging between azure and a darker sky blue. The first recorded use of cerulean as a colour name in English was in 1590. The word is derived from the Latin word caeruleus, "dark blue, blue, or blue-green", which in turn probably derives from caerulum, diminutive of caelum, "heaven, sky"."Cerulean blue" is the name of a blue-green pigment consisting of cobalt stannate (Co2SnO4). The pigment was first synthesized in the late eighteenth century by Albrecht Höpfner, a Swiss chemist, and it was known as Höpfner blue during the first half of the nineteenth century. Art suppliers began referring to cobalt stannate as cerulean in the second half of the nineteenth century. It was not widely used by artists until the 1870s when it became available in oil paint.

Pigment characteristics
The primary chemical constituent of the pigment is cobalt(II) stannate (Co2SnO4). The pigment is a greenish-blue color. In watercolor, it has a slight chalkiness. When used in oil paint, it loses this quality.Today, cobalt chromate is sometimes marketed under the cerulean blue name but is darker and greener than the cobalt stannate version. The chromate makes excellent turquoise colours and is identified by Rex Art and some other manufacturers as "cobalt turquoise".Cerulean is inert with good light resistance, and it exhibits a high degree of stability in both watercolor and acrylic.

History
Cobalt stannate pigment was first synthesized in 1789 by the Swiss chemist Albrecht Höpfner by heating roasted cobalt and tin oxides together. Subsequently, there was limited German production under the name of Cölinblau. It was generally known as Höpfner blue from the late eighteenth century until the middle of the nineteenth century.In the late 1850s, art suppliers begin referring to the pigment as "ceruleum" blue. The London Times of 28 December 1859 had an advertisement for "Caeruleum, a new permanent colour prepared for the use of artists." Ure's Dictionary of Arts from 1875 describes the pigment as "Caeruleum . . . consisting of stannate of protoxide of cobalt, mixed with stannic acid and sulphate of lime." Cerulean was also referred to as coeurleum, cerulium, bleu céleste (celestial blue). Other nineteenth century English pigment names included "ceruleum blue" and "corruleum blue". By 1935, Max Doerner referred to the pigment as cerulean, as do most modern sources, through ceruleum is still used.Some sources claim that cerulean blue was first marketed in the United Kingdom by colourman George Rowney, as "coeruleum" in the early 1860s. However, the British firm of Roberson was buying "Blue No. 58 (Cerulium)" from a German firm of Frauenknecht and Stotz prior to Rowney. Cerulean blue was only available as a watercolor in the 1860s and was not widely adopted until the 1870s when it was used in oil paint. It was popular with artists including Claude Monet, Paul Signac, and Picasso. Van Gogh created his own approximation of cerulean blue using a mixture of cobalt blue, cadmium yellow, and white.

Notable occurrences
In 1877, Monet had added the pigment to his palette, using it in a painting from his series La Gare Saint-Lazare (now in the National Gallery, London). The blues in the painting include cobalt and cerulean blue, with some areas of ultramarine. Laboratory analysis conducted by the National Gallery identified a relatively pure example of cerulean blue pigment in the shadows of the station's canopy. Researchers at the National Gallery suggested that "cerulean probably offered a pigment of sufficiently greenish tone to displace Prussian blue, which may not have been popular by this time."Berthe Morisot painted the blue coat of the woman in her Summer's Day, 1879 in cerulean blue in conjunction with artificial ultramarine and cobalt blue.When the United Nations was formed at the end of World War II, they adopted cerulean blue for their emblem. The designer Oliver Lundquist stated that he chose the color because it was "the opposite of red, the color of war."In the Catholic Church, cerulean vestments are permitted on certain Marian feast days, primarily the Immaculate Conception in diocese currently or formerly under the Spanish Crown.

Other colour variations
Pale cerulean
Pantone, in a press release, declared the pale hue of cerulean at right, which they call cerulean, as the "colour of the millennium".The source of this colour is the "Pantone Textile Paper eXtended (TPX)" colour list, colour #15-4020 TPX—Cerulean.

Cerulean (Crayola)
This bright tone of cerulean is the colour called cerulean by Crayola crayons.

Cerulean frost
At right is displayed the colour cerulean frost.
Cerulean frost is one of the colours in the special set of metallic coloured Crayola crayons called Silver Swirls, the colours of which were formulated by Crayola in 1990.

Curious Blue
Curious Blue is one of the bright tone colors of cerulean

In nature
Cerulean cuckooshrike
Cerulean kingfisher
Cerulean flycatcher
Cerulean warbler
Cerulean-capped manakin

See also
The Devil Wears Prada (film) § Cerulean sweater speech
Pusher (The X-Files episode) § "Cerulean blue is a gentle breeze"
List of colors
Pigment
Blue pigments

Explanatory notes
References
External links
A page on Cerulean Blue
Cerulean blue at ColourLex