Rublev Colours Cobalt Zinc 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.
Numerous compounds have been called cobalt blue and while today it is commonly assigned to cobalt aluminum oxide, many other meanings have been ascribed to this name. 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 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 the 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 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.
Rublev Colours Cobalt Zinc Blue Watercolor
Note: Colors swatches are shown in mass tone, at the top, mixed with an equal amount of water, and brushed out toward the bottom. All pictures of color swatches on this website are only approximations of the actual color of the watercolor paint. We have taken every care to match the color in these pictures on calibrated color monitors to the actual color. However, because of the wide variance in color monitors, the results you get may vary.
Composition and Permanence| Crimson Ocher | |
| Color: | Blue |
| Binder: | Gum Arabic |
| Additive(s): | None |
| Pigment Information | |
| Pigment: | Cobalt zinc blue |
| Pigment Classification: | Synthetic inorganic |
| Colour Index: | Pigment Blue 72 |
| Chemical Name: | Cobalt zinc aluminate spinel |
| Chemical Formula: | (Co,Zn)Al2O4 |
| CAS No. | 68186-87-8 |
| Properties | |
| Code: | 110 |
| Series: | 5 |
| Opacity: | Semi-Transparent |
| Tinting Strength: | Moderate |
| Staining: | None |
| Granulation: | Low |
| Flocculation: | Moderate |
| Lightfastness: | I |
| Permanence: | A |
| Safety Information: | Based on the toxicological review, there are no acute or known chronic health hazards with the anticipated use of this product. Always protect yourself against potentially unknown chronic hazards of this and other chemical products by avoiding ingestion, excessive skin contact, and inhalation of spraying mists, sanding dust, and concentrated vapors. |
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NotesNote: Some separation of pigment and medium may occur in Rublev Colours Watercolors and is a natural process when no stabilizers are added to paint to prevent this from occurring.