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, and to impart a distinctive blue tint to glass, ceramics, inks, pottery and enamels. Cobalt has been used in the production of high-performance alloys, and in the preparation of magnetic, wear-resistant and high-strength alloys. The compounds, cobalt silicate and cobalt aluminate give a distinctive deep blue color to glass, ceramics, inks, paints and varnishes.
So is cobalt naturally blue? The short answer is no. In its pure elemental form, cobalt is a hard, silver-gray metal. It does not possess a distinct blue color. However, when cobalt is combined with other elements, like silicate or aluminate, it forms distinctive blue compounds that have been used as pigments throughout history.
History of Cobalt Blue Pigments
The earliest known use of cobalt-based blue pigments dates back to the Tang Dynasty in China (618-907 AD), where it was used in blue and white porcelain. Egyptian blue, the first known synthetic pigment, made by heating sand, copper, natron, and limestone, also contained cobalt.
In the Middle Ages, cobalt was used to color glass and glazes blue in Persia and the Near East. Around 1800, French chemist Louis Jacques Thénard first synthetically produced cobalt blue by heating cobalt(II) oxide with alumina.
In 1802, Thénard recognized cobalt blue as a distinct blue pigment and named it “cobalt blue.” In 1807, German chemist Johann Gottlob Lehmann analyzed the composition of cobalt blue pigment, which contained cobalt, aluminum and oxygen. Cobalt blue, sometimes known as Thénard’s blue or cobalt aluminate, became a popular alternative to costly lapis lazuli blue pigments made from gemstones.
In 1861, French chemist Jean-Baptiste Alphonse Gentele developed a vivid cobalt blue pigment known as cobalt ultramarine, made by heating a mixture of kaolin, potassium carbonate, sulfur, silica and cobalt oxide. It had greater tinting strength than pigments like ultramarine and Prussian blue.
Chemical Properties of Cobalt Blue Pigments
While elemental cobalt in its pure form has a silver-gray appearance, cobalt chemically combined with silica or alumina produces deep blue inorganic pigments.
Some common cobalt blue pigments include:
– Cobalt aluminate (CoAl2O4) – Formed by calcining cobalt oxide and aluminum oxide at high temperatures. Known for its excellent chemical resistance properties.
– Cobalt silicate or smalt (CoO · SiO2) – Made by heating quartz sand and cobalt oxide to very high temperatures. One of the historically important blue pigments.
– Cobalt magnesium silicate – Contains cobalt, magnesium and silicon. Lower in cost compared to cobalt aluminate.
– Cobalt zinc silicate – Contains cobalt, zinc and silicon. Has good tinting strength.
– Cobalt-zinc-aluminum oxide – Mix of cobalt, zinc, aluminum and oxygen. Bright blue pigment used in paints.
– Cobalt titanate – Compound of cobalt, titanium and oxygen. High performance pigment with good heat stability.
The blue color arises when the cobalt ion forms tetrahedral coordination complexes with oxygen atoms in these inorganic crystal lattices. The specific arrangement of the cobalt, oxygen and other metals in the lattice allows absorption of yellow light, leading to a strong blue color.
Modern Uses of Cobalt Blue Pigments
Today, cobalt blue is used in a variety of materials that require a stable vivid blue color with high tinting strength:
– Paints and coatings – Cobalt blues like cobalt aluminate and cerulean blue (cobalt stannate) are used in oil and acrylic paints, automotive and marine paints. They are valued for their chemical inertness.
– Plastics and polymers – As a pigment in plastics, ceramics, floor coverings, rubber products. Provides heat stability.
– Glass and ceramics – Cobalt oxide imparts an intense blue color to glass, porcelain and tiles. Cobalt blue glass was particularly popular in the mid-19th century.
– Inks – Used in printing inks as it is resistant to alkalis, acids, and ultraviolet light. Provides brilliance and depth of shade.
– Cosmetics – Cobalt aluminate is FDA approved for use in makeup, eye products and lipsticks when properly formulated. Provides vivid blue hues.
– Food – Aluminum-silicate cobalt blue lake is permitted for use as food coloring in the EU and USA.
– Art/Hobbies – Artists’ paints, pottery glazes and enamels utilize cobalt blue for its working properties. Safe levels are maintained in products.
While toxicological issues exist with excessive cobalt ingestion, careful processing and control ensures its safe use as a colorant and pigment at approved levels in consumer goods and art materials.
Is Cobalt Naturally Blue in Mineral Form?
In nature, cobalt rarely occurs as a free metal and is usually found in chemically combined forms within specific mineral ores. Some minerals that contain cobalt do appear blue in color, however this is not due to elemental cobalt itself. The blue color arises from cobalt’s chemical bonds with other elements in the crystal lattice.
Some blue cobalt-bearing minerals include:
– Cobaltite (CoAsS) – Contains cobalt, sulfur and arsenic. Has a grayish blue to steel-gray color.
– Linnaeite (Co3S4) – Consists of cobalt and sulfur. Blue to white in appearance.
– Erythrite (Co3(AsO4)2·8(H2O) – Hydrated cobalt arsenate. Violet-blue in color.
So in summary, while a few rare cobalt minerals do possess blue hues, this is not indicative of cobalt in its pure elemental form. The blue color comes from cobalt’s chemical coordination within the mineral crystals, not the cobalt alone. In its raw form, cobalt has a silver-gray appearance until combined with other compounds to produce blue pigments and dyes.
Conclusion
Cobalt is not naturally blue as an isolated element. While often associated with brilliant blue colors due to its use in pigments, glazes and dyes, pure cobalt is metallic gray. The characteristic blue color arises when cobalt combines with oxygen and elements like aluminum, zinc, silicon, and arsenic to form tetrahedral complexes. The specific arrangement of atoms in these compounds causes selective absorption of light in the yellow wavelengths, producing cobalt’s signature blue hue. While occasional cobalt-bearing minerals may possess blue shades, this is again due to cobalt’s lattice position, not the element itself. So in summary, the vibrant cobalt blue seen in pigments and materials requires cobalt to be synthetically combined with other elements – in its raw form, cobalt does not exhibit a blue color.
