Rublev Colours Cerulean Blue Pigment - High-Quality, Durable Artistic Pigment

Rublev Colours Cerulean Blue pigment is composed of cobalt stannate. Cerulean blue is made by calcining cobalt and tin oxides. Cobalt blue pigments are the most durable blue pigments commercially available. They have excellent chemical and heat stability and can be used in chemically aggressive environments and exterior durable applications without color fade.

This pigment is easily dispersed, does not need grinding for dispersion in water-based paint binders, and can be stirred using most laboratory and production equipment to achieve complete dispersion.

For a detailed explanation of the terms in the table above, please visit the Composition and Permanence page.

Origin and History

The name “Cerulean blue” comes from the Latin caeruleus, meaning dark blue caelum, which in turn probably derives from caelulum, diminutive of caelum, “heaven sky”.

Cerulean blue is cobalt stannate introduced as a pigment in the 1860s—very stable and lightfast greenish-blue with limited hiding power. Cerulean blue is fairly true blue (not greenish or purplish), but it lacks the opacity and richness of cobalt blue. It was not recommended for use in watercolor painting because of chalkiness in washes. In oil, it was particularly valuable to landscape painters for skies.

The compound was known from the early 19th century, but was first introduced as an artist’s pigment by George Rowney. Gettens and Stout say the date was 1860, but Mayer gives it as 1870 (Mayer, R. The Artists Handbook of Materials and Techniques. Smith, Edwin, 3rd ed., 1973, p. 46). In 1887, the author of a Winsor and Newton publication was hardly enthusiastic about the pigment. He comments, “By daylight, it is a light greenish-grey blue with little depth or richness of colour. Unless used with care, it is apt to produce a chalky effect, and it washes in a very indifferent manner” (Taylor, J. Scott, Modern Water-Colour Pigments 1887, p. 58). Cerulean blue is used in both oil and watercolor; it covers well but is not strongly staining. It suffers less from metameric effects than the other cobalt blues.

Source

Cerulean blue or cobalt tin blue (CoO.nSnO₂) is an inorganic pigment that is the reaction product of calcining a mixture of different amounts of both cobalt (II) oxide and tin (III) oxide. They are homogeneously, ionically entwined, forming a crystalline spinel-type matrix. The constitution may also include Al₂O₃, Fe₂O₃, and/or NiO as modifiers to adjust color hue and other properties as needed in specific applications. Cerulean blue is cobaltous stannate and is made by mixing cobaltous chloride with potassium stannate. The mixture is thoroughly washed, mixed with silica and calcium sulfate, and heated.

Permanence and Compatibility

All cobalt blue pigments are chemically inert, absolutely insoluble, have good hiding power and excellent heat stability, and show excellent lightfastness and weather resistance.

Water-Based Paint

In most water-based paints, Cerulean blue is a semi-transparent pigment with moderate tinting strength. When it dries, it appears lighter and less saturated. Although pigment particles are very fine, they flocculate, giving a grainy appearance in watercolor. Differences in how the pigment is ground and mixed result in significant differences in its performance.

Oil-Based Paint

In oil-based paint, Cerulean blue is a semi-transparent pigment with moderate tinting strength. Differences in how the pigment is ground and mixed can result in significant variations in its appearance.

Oil Absorption and Grinding

Cerulean blue absorbs a moderately high amount of oil, about 55 grams of linseed oil per 100 grams of pigment, to make a paste. Some manuals note that the pigment works better as a watercolor than in oil and is highly valued on that account among artists’ moist colors. Grinding it artists’ use in oil will require 35 percent dry pigment to 65 percent by weight of poppy seed or walnut oil, either of which is preferred by some artists to linseed oil. Cerulean blue has a neutral blue hue that, under incandescent light, appears more or less greenish-blue.

Physical Properties

While not formally published as a Mohs-tested mineral, cerulean blue (cobalt stannate) is best characterized as having a Mohs hardness of approximately 6–7, aligning with its oxide crystal structure and observed abrasiveness in pigment processing. Mohs hardness is formally defined for bulk crystalline minerals, not finely ground pigment powders. As a result:

• Pigment handbooks typically report oil absorption, density, refractive index, and tinting strength, but not hardness.

• Hardness values are instead inferred from Tin dioxide (SnO₂, cassiterite), Mohs 6–7, and Cobalt oxides: generally ~5–6.

• The resulting mixed oxide falls squarely in the upper mid-hardness range.

Practical Implications for Artists’ Materials

• Grinding & Milling: Cerulean blue is relatively abrasive, comparable to ultramarine and many cobalt pigments, and noticeably harder than earth pigments.

• Tool Wear: Extended use can accelerate wear on steel palette knives, muller plates, and milling equipment.

• Paint Film Behavior: The hardness contributes to its granular texture, moderate tinting strength, and excellent durability.

• Conservation Context: Its chemical and physical stability are consistent with its long record of permanence.

Toxicity

Cerulean blue is not considered toxic; however, care should be taken in handling the dry powder pigment to avoid inhaling the dust. All toxicological studies showed no signs of toxicity to humans or the environment.

In animal studies, cobalt blue pigments showed no acute toxicity. No acute irritant effect was observed in tests assessing acute skin and mucous membrane irritation. In studies on rats to assess carcinogenic potential, no statistically significant results were observed. Soluble cobalt compounds and cobalt metal may have a sensitizing effect. However, none have been reported during many years of handling cobalt blue pigments.

Ecology

Since cobalt blue pigments are inert and practically insoluble in water, they do not pose an environmental hazard. The pigment can be removed mechanically from effluents. On controlled dumpsites, no dissolved heavy metals are released into the seepage water.

For more information on how to handle pigments safely, please visit How to Safely Handle Art Materials and Pigments.