Cobalt is a chemical element with symbol Co and atomic number 27. It is found in the Earth’s crust only in chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal.
Cobalt-based blue pigments have been used since ancient times for jewelry and paints, as well as glass stained glass. Cobalt glass is blue in color. Cobalt aluminum oxide (CoAl2O4) is used in blue tints for the glass and ceramics industries. Cobalt salts produce a rich blue color. Cobalt blue in impure forms had long been used in Chinese porcelain. Louis Leopold Casella created a French version in 1802.
So is cobalt actually black or blue? The answer lies in its chemical and physical properties. Here we examine cobalt’s colors and applications.
Cobalt’s Colors
Cobalt commonly occurs in two oxidation states: +2 (cobaltous compounds) and +3 (cobaltic compounds). In aqueous solution, simple cobalt(II) salts form the pink [Co(H2O)6]2+ ion. Addition of chloride gives the blue [CoCl4]2-. Larger amounts of chloride give the cobalt(II) complex [CoCl6]3-.
Certain cobalt(III) complexes have characteristic colors. Cobalt(III) forms numerous coordination compounds, or complexes. Complexes of cobalt(III) ion with strong field ligands are, as a rule, paramagnetic and octahedral cobalt(III) complexes are invariably high-spin, since the crystal field splitting is weak compared to the pair splitting. The hexaaquacobalt(III) cation, [Co(H2O)6]3+ is purple.
Here is a summary of common cobalt complexes and their colors:
| Complex | Color |
|---|---|
| [Co(H2O)6]2+ | Pink |
| [CoCl4]2- | Blue |
| [CoCl6]3- | Blue |
| [Co(H2O)6]3+ | Purple |
As seen above, cobalt commonly forms blue and purple complexes in solution. But the color of solid cobalt metal depends on factors like purity and oxide layers.
Color of Metallic Cobalt
Pure cobalt is not naturally found. It is generally found mixed with arsenic and sulfur. The mining of cobalt resembles that of copper and nickel. It is refined through smelting and electrolysis.
Pure cobalt is silvery-gray and shiny when freshly exposed to air. But it tarnishes and takes on a grayish black color. The tarnish is caused by oxide layers forming on the surface. This gives cobalt a blackish appearance over time.
But a polished cobalt surface can reveal a blue tint. When viewed under the microscope, pure cobalt has a distinct blue color. This is caused by the specific absorption and reflection of light waves by the metallic cobalt crystal lattice.
So cobalt appears blackish-gray in bulk solid form. But it has an underlying blue color visible under magnification or in thin layers. This gives cobalt a dual black and blue metallic color appearance.
Blue Cobalt Oxides and Salts
Cobalt oxide is formed during corrosion. Cobalt(II) oxide (CoO) adopts a rock salt crystal structure which has a characteristic blue-green color. This provides another source of cobalt’s blue color.
Cobalt chloride hexahydrate (CoCl2•6H2O) is one of the most commonly used cobalt compounds. It dissolves in water to give a characteristically deep blue color. This blue color is explained by the [Co(H2O)6]2+ ion.
The vibrant blue cobalt aluminate (CoAl2O4) is widely used in porcelain glazes. When calcined to 900°C, cobalt aluminate forms a blue solid with the corundum structure. This blue pigment has been used in Chinese porcelain glazes since the Tang Dynasty.
Cobalt silicate (Co2SiO4) yields a distinctive blue-green crystal of olivine structure. Nickel-doped cobalt aluminate and silicate give darker blue colors. Cobalt pigments based on these compounds are valued for their excellent thermal stability.
Cobalt Blue Pigments
Cobalt has been used to produce blue colored glass and glazes since ancient times. But the purified and concentrated form arose in the 19th century with two major cobalt blue pigments:
Thenard’s Blue: This is cobalt(II) aluminum oxide (CoAl2O4) made by calcining cobalt sulfate and alumina at 1000-1100°C. It has excellent thermal stability and weather resistance. The blue color arises from tetrahedral coordinated Co2+ ions in the spinel structure. A greener hue is achieved by substituting nickel ions for some cobalt ions.
Cobalt Blue: This pigment is cobalt(II) oxide-aluminum oxide (CoO·Al2O3) made by calcining cobalt and aluminum salts to 950°C. The blue color comes from small CoO grains dispersed in the alumina matrix. It has lighter tones compared to Thenard’s blue.
These cobalt blue pigments achieved a high level of purity with an intense blue hue. Their introduction led to cobalt blue becoming a prevalent color in porcelain, glassware and other media.
Cobalt Glass and Ceramics
The addition of cobalt compounds gives glass and ceramics a distinctive deep blue coloration. Cobalt blue glass has been produced since ancient Egyptian and Roman times to color jewelry and ornaments. Cobalt glass also yields blue stained glass.
The primary modern blue glass colorant is cobalt(II) oxide-alumina (CoO·Al2O3). Cobalt aluminate (CoAl2O4) and cobalt silicate (Co2SiO4) compounds are also used for glass coloring. The blue color arises from the tetrahedral [CoO4] structural units.
Cobalt blue is known for its stability in molten glass. The color persists even when subject to molten glass temperatures around 1000°C. This heat stability makes cobalt essential for achieving blue colors in glass applications.
The addition of manganese yields violet amethyst colored glass. Nickel turns the blue more greenish. Cobalt blue glass is used for colored bottles and dinnerware. It is also common in art glass and studio glass pieces.
In ceramics, cobalt blue is widely used as a pigment in blue pottery glazes. Chinese porcelain has used cobalt blue glazes since the 14th century. Cobalt aluminate gives the best blue color for high temperature porcelain glazes. Cobalt blue is also used in blue friezes and architectural tiles.
Cobalt Inks and Paints
Cobalt blue pigments are used in inks and paints. Cobalt blue has a warmer and more muted tone compared to the colder ultramarine blue. This gives a more naturalistic effect for landscapes and seascapes.
Cobalt blue watercolors utilize cobalt aluminate or phosphate pigments for their permanence and high tinting strength. Cobalt blue oil paints date back centuries with usage by Old Masters like Rembrandt. Modern cobalt blue paints use nanoparticles for enhanced opacity and color depth.
Cobalt blue is suitable to mix with yellows, reds, greens and violets. It is popular for monochromatic blue paintings. Cobalt blue is also essential for mixing neutral grays and blacks.
Cobalt blue has excellent lightfastness. But it is susceptible to alkali attack so requires an acid resistant coating. Cobalt blue inks and dyes are used for textile printing, color photocopying, and pen inks. The blue color is imparted by cobalt(II) salts or complexes.
Conclusion
In summary, cobalt exhibits both black and blue color properties:
– Cobalt metal has a grayish black appearance from surface oxide layers, but blue under magnification.
– Blue cobalt compounds are formed including cobalt(II) salts, cobalt(II) oxide, and cobalt aluminate.
– Blue cobalt pigments and cobalt blue glass revolutionized the use of blue colors since the 19th century.
So while cobalt can appear blackish in bulk form, its essence is a distinctive blue color from its complexes. Cobalt’s black and blue colors arise from similar origins – the cobalt cation and its electron configurations. This dichotomy makes cobalt both black and blue.
