Cobalt in Pigments/Ceramics/Enamels

The ability of cobalt-containing minerals to impart colour has been important for thousands of years going back to the Egyptians and Persians. This property has been used in glass, porcelain, ceramics, paints, inks and enamelware.

The pigments used include cobalt in many formulations. In general, the pigments are prepared by mixing the ingredients as oxides or other decomposable salts (sulphates) and then calcining them at 1100°C-1300°C and grinding back to a fine powder. The final colour depends on the application. For instance, in a glaze, further firing occurs which modifies the colour.

The main use of cobalt in these applications is in ceramics. A ceramic article is “a glazed or unglazed object of crystalline or partly crystalline structure (or of glass), produced from essentially inorganic, non-metallic substances; such objects are made from either a molten mass which solidifies upon cooling or which is formed and matured simultaneously or subsequently by action of heat” (ASTM).

Advanced ceramics cover a vast array of applications, going from cutting tools to mechanical seals (the heavy duty, structural ceramics) and from temperature sensors to magnets (the more subtle functional ceramics).

Cobalt is also added to glass as a colour or as a decolouriser. Even 2 ppm gives a tint and 200 ppm produces a dark blue. As a decolouriser, at very low levels, it suppresses the yellowish tint glass would otherwise have as a result of iron contamination.

For increasingly more blue but still transparent glass, one can add up to 2% Co, and really dark blue enamels for decoration can go as high as 7% Co. In some special glasses, other configurations and thus other tints can be obtained. Thanks to its unique combination of solubility, stability, colouring effect, it can be said that there is no real substitute for cobalt in this application. The raw material is usually cobalt oxide.

A lesser known but very important aspect is the behaviour of pigments when suspended in liquids. Many of them will indeed be processed and applied as dispersions, with other solids, additives, etc.

Coming to cobalt pigments uses, there are a variety of possibilities to colour ceramic articles – underglaze, in the glaze, overglaze, in the body, etc. Quality requirements for glaze colours are different from those for body stains. Although body stained tiles for example are considered top quality products in the tile business, more liberties in chemical and physical properties can be taken for the body stains themselves.

On the other hand, pigments for plastics, paints, textiles have to be very fine and well dispersed to ensure surface condition and, in the case of plastics, sometimes transparency.

Paintings with Cobalt Blue are common and range from blue to yellow, blue being the popular colour with the French Impressionists.