Cerulean blue
A pale, opaque, distinctly green-leaning blue — the colour of a sky near the horizon rather than overhead. Cerulean is the specialist among blues, and painters keep it on the palette for one job that nothing else does as well.
Origin
A cobalt stannate, first prepared by Albrecht Höpfner in 1805 and brought to market as an artists' colour by George Rowney in England around 1860. The name comes from the Latin caeruleum, which in Roman usage referred to blue pigments generally — Pliny uses it for Egyptian blue — so the modern name is a borrowed antiquity rather than a continuous tradition. Manufacture calcines cobalt and tin oxides together at high temperature, and tin makes it costly; some modern versions substitute chromium, sold as cerulean blue chromium, which is greener still and cheaper.
In art history
Because it appeared late and cost a great deal, cerulean has a narrow but distinct history: it belongs to the Impressionists and to everything downstream of them. Monet used it in skies and in water, Signac and the Neo-Impressionists valued it because its opacity holds a discrete dot of colour without bleeding into its neighbours, and it recurs through twentieth-century landscape and coastal painting. In this atlas it appears in seven eras, all of them after 1860 — one of the clearest examples in the database of a pigment acting as a date stamp. If a painting contains cerulean, it was not painted before the middle of the nineteenth century.
In the studio
The characteristic that defines it is opacity. Most blues are transparent; cerulean is chalky and covering, which means it sits on the surface as a solid block of colour rather than glowing from underneath. That makes it wrong for glazing and right for the flat, matte, atmospheric passages where a transparent blue would go muddy. Its green bias makes it the natural blue for a horizon, for sea, and for skin in cool light. It is entirely lightfast, unaffected by alkalis — so unlike Prussian blue it can be used in fresco — and safe in every mixture. Its drawback beyond price is a certain deadness: overused, it flattens a picture.
Afterlife
Genuine cobalt-tin cerulean is still manufactured in small quantities and is among the more expensive artists' pigments. Most tubes sold today under the name are either the chromium version or a phthalo-and-white imitation, and the difference matters more here than with most substitutions: the imitations are transparent, and transparency is the one thing cerulean exists to avoid.
Where it appears
Every era and artist in this atlas whose palette includes this pigment.