Cobalt Aluminate Blue Pigment

Cobalt Aluminate Blue pigment is cobalt aluminate blue spinel (CoAl₂O₄). It is a dark blue powder produced by high-temperature calcination. This pigment has good UV and visible opacity and is chemically inert, heat resistant, and stable to ultraviolet light. It is non-bleeding and non-migratory. It has exceptional durability and hiding power and is generally used in applications where resistance to heat, light, and weather is needed. It is compatible with most resin systems and polymers and is non-warping. Typical applications include liquid and powder coatings, inks, dispersions, concrete, plastics, and other applications where equivalent pigment chemistry is used.

Cobalt Aluminate Blue is made by calcining at 2400°F a mixture of cobalt (II) oxide, chromium (III) oxide, and aluminum (III) oxide in varied ratios, forming an interdiffused crystalline spinel matrix. Its constitution may also include one or more modifiers, zinc oxide (ZnO), manganese oxide (MgO), silicon dioxide (SiO₂), titanium dioxide (TiO₂), or zirconium dioxide (ZrO₂) to adjust color hue or other properties. 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, 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 Composition and Permanence.

Origin and History

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. Probably it 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.

Source

Cobalt blue is cobalt aluminate blue spinel (Pigment Blue 28 or PB 28) with the chemical structure CoAl₂O₄. 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.

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 partially substituting the cobalt with chromium and zinc, alone or in combination. 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. 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.

Permanence and Compatibility

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

Water-Based Paint

Cobalt Aluminate Blue is a semi-transparent pigment with moderate tinting strength in most water-based paints. 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

Cobalt Aluminate 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.

Oil Absorption and Grinding

Cobalt Aluminate Blue absorbs a moderately high amount of oil, about 34 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 about 70 percent dry pigment to 30 percent by weight of poppy seed or walnut oil, either of which some artists prefer to linseed oil. Cobalt chromite blue has a greenish tone that is more or less reddish blue when viewed under incandescent light.

Toxicity

Cobalt Aluminate Blue is not considered toxic; however, care should be taken when 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. No statistically significant results were found in studies on rats to determine the carcinogenic potential. Soluble cobalt compounds and cobalt metals may have a sensitizing effect. However, none have been reported during many years of handling cobalt blue pigments.

Cobalt Aluminate Blue typically contains a total of the regulated metals lead, cadmium, mercury, and hexavalent chromium below the strictest specification of 100 ppm outlined in the Model Toxics in Packaging (CONEG) legislation.

Cobalt Aluminate Blue is FDA-approved for use as a colorant in all polymers intended to contact food, at a level not to exceed 5% by weight as listed under 21CFR 178.3297.

Cobalt Aluminate Blue is listed in the Council of Europe AP(89)1 Approved Pigments. The migration of the elements antimony, arsenic, barium, cadmium, chromium, lead, mercury, and selenium from Cobalt Aluminate Blue is typically below the limits set forth in the Council of Europe's resolution AP(89)1 on the use of colorants in plastic materials coming into contact with food, 13 September 1989.

Cobalt Aluminate Blue is listed in the Australian Standard AS 2070-1999 Approved Pigments. The migration of the elements antimony, arsenic, barium, cadmium, chromium, lead, mercury, and selenium from Cobalt Aluminate Blue is typically below the limits set forth in the Australian Standard AS 2070-1999, Plastics materials for food contact use (4.1.2 Colourants), published 5 March, 1999.

Cobalt Aluminate Blue is on the list of BfR-approved pigments. Pigments are not regulated by the Bundesinstitut für Risikobewertung (BfR) unless they contain dangerous substances. Cobalt Aluminate Blue is free of dangerous substances and, therefore, does not require BfR registration.

Ecology

Since Cobalt Aluminate Blue is inert and practically insoluble in water, it does not harm the environment. The pigment can be removed mechanically from effluents, and no dissolved heavy metals are released into the seepage water on controlled dump sites.

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