Cobalt Zinc Blue (Colour Index Pigment Blue 72) is a modification of cobalt aluminate blue spinel pigments (Colour Index Pigment Blue 28). Cobalt aluminate 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 applications without fading.

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Cobalt aluminate is identified as the blue pigment on 30 polychromic shards of the 18th dynasty from Theben and Tell el Amarna. The spinel phase has a much lower cobalt/aluminum ratio than modern cobalt blue pigments. It probably contains nickel and zinc, which were found by x-ray spectroscopy. Cobalt salts were first identified in the eighteenth century. The earliest modern history of cobalt aluminate pigments commences with Leithner in Vienna, who appears to have discovered the basic process of calcining cobalt oxide and alumina (aluminum oxide) in 1775. Techniques for manufacturing cobalt blue, the 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 essential 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.

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 the partial substitution of 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 adding 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 crystallize 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 excellent lightfastness and weather resistance.

Cobalt blue is a semi-transparent pigment with moderate tinting strength in most water-based paint. 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.

Cobalt blue is a semi-transparent pigment with moderate tinting strength in oil-based paint. Differences in how the pigment is ground and mixed can lead to considerable differences in appearance.

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 manuals 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 poppy seed or walnut oil, either of which some artists prefer to linseed oil. Cobalt blue has a greenish tone that is more or less reddish blue when viewed under incandescent light.

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 the skin and mucous membranes. No statistically significant results were found in studies on rats to determine the carcinogenic potential. 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. No dissolved heavy metals are released into the seepage water on controlled dump sites.

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