Rublev Colours Cobalt Zinc Blue Pigment/Color

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Water-Based Paint
In most water-based paint, cobalt 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 lead to considerable differences in its performance.

Oil-Based Paint
In oil-based paint, cobalt blue is a semi-transparent pigment with moderate tinting strength. Differences in how the pigment is ground and mixed can lead to considerable differences in its appaearance.

Color Names
Current Names: English: cobalt blue
French: bleu , bleu de
German: Blau
Italian: azzurro di
Spanish: azul de

Origin and History
Since ancient times, smalt blue has been used to color glass and ceramics. Cobalt salts, which give smalt its characteristic blue color, were identified in the 18th century. Techniques for manufacturing cobalt blue, a chemically pure salt of cobalt and aluminum oxide, were developed in 1802.

Cobalt blue is cobalt aluminate blue spinel (Pigment Blue 28 or PB 28) with the chemical structure CoAl2O4 of which there are three basic modifications (Pigment Blue 36:1, Pigment Blue 36 and Pigment Blue 72), in which parts of the cobalt are substituted either alone or in combination with chromium and zinc. These variations differ in their tinting strength and hue.

Color Formula Color Index
Cobalt aluminate blue spinel CoAl2O4 Pigment Blue 28
Cobalt chromite blue-green spinel Co(Al,Cr)2O4 Pigment Blue 36
Zinc cobalt chrome aluminum spinel (Zn,Co)(Cr,Al)2O4 Pigment Blue 36:1
Cobalt zinc aluminate spinel (Co,Zn)Al2O4 Pigment Blue 72

The basic cobalt blue color, Pigment Blue 28, is produced by high-temperature calcination of cobalt (II) oxide (CoO) and aluminum oxide (Al2O4). The variations are produced by a partial substitution of the cobalt by chromium and zinc, either alone or in combination with each other. The result is a large variety of cobalt blue pigments that differ in their tinting strength or hues (reddish or greenish blue), depending on the exact chemical composition. The lighter-colored cobalt blue is prepared by addition of zinc (II) oxide to the ingredients used for the basic pigment, forming Pigment Blue 72. Blue-green hues are produced by introducing chromium (III) oxide, partially replacing the aluminum (III) oxide in the basic cobalt blue, forming Pigment Blue 36.

All of them form the crystalline modification of spinel during calcination. The spinels are a class of minerals that crystallise in the cubic (isometric) crystal system with the oxide anions arranged in a cubic close-packed lattice and the cations occupy some or all of the octahedral and tetrahedral sites in the lattice. Spinel minerals form octahedral crystals that are usually twinned. They have an imperfect octahedral cleavage and conchoidal fracture. The hardness of spinel minerals is around 8 (chalk has a hardness of 2-3), specific gravity is 3.5-4.1 and it is transparent to opaque with a vitreous to dull luster.

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

Oil Absorption and Grinding
Cobalt blue absorbs a moderately high amount of oil; about 55 grams of linseed oil per 100 grams of pigment to make a paste. It has been noted in some mannuals that the pigment works better as a watercolor than it does in oil, and is highly valued on that account among moist colors used by artists. Grinding it for artists' use in oil will require 35 percent dry pigment to 65 percent by weight of poppyseed or walnut oil, either of which is preferred by some artists to linseed oil. Cobalt blue has a greenish tone that when viewed under incandescent light is more or less reddish blue.

Cobalt blue is not considered toxic, however, care should be used 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 did not display acute toxicity. No acute irritant effect was shown in tests to determine the acute irritation of skin and mucous membranes. In studies on rats to determine the carcinogenic potential, no statistically significant results were found. Soluble cobalt compounds as well as cobalt metal may have a sensitizing effect. However, none have been reported during the experience of many years of handling cobalt blue pigments.

Since cobalt blue is inert and practically insoluble in water, it does not pose a hazard to the environment. The pigment can be removed mechanically from effluents. On controlled dumpsites, no dissolved heavy metals are released into the seepage water.

Rublev Colours Pigment: Cobalt Blue

Pigment Information
Color: Blue
Colour Index: Pigment Blue 72 (77347)
Chemical Name: Cobalt Zinc Aluminate (Spinel)
Chemical Formula: (Co,Zn)2Al2O4
CAS: 68186-87-8
ASTM Lightfastness Rating
Acrylic: I
Oil: I
Watercolor: I
Specific Gravity: 1.7
Refractive Index: 1.56-1.662
Oil Absorption: 55 g oil/100 g pigment

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