Glass, porcelain, ceramics, paints, inks and enamelware have all been known to utilise cobalt compounds in the past and present to create a vivid cobalt blue colour.
The unique combination of colour, solubility and stability, illuminate cobalt compounds as a powerful colouring agent in a variety of applications.
The ability of cobalt-containing minerals to impart colour has been important for thousands of years going back to the times of the Egyptians and Persians.
Cobalt as a colourant
The pigments used for colourants include cobalt in many formulations. In general, the pigments are prepared by mixing the ingredients as oxides before calcining (the treatment of a mineral product at a high temperature) 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.
Cobalt pigments are often used in the decoration of 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” (American Society for Testing and Materials, ASTM).
Cobalt can also be added to glass as a colour or as a decolouriser. Even 2 ppm of cobalt carbonate 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.
Table 1 below shows a list of common colourants that use Co3O4 along with their composition.
Typical Cobalt-Containing Pigments
Table 1 A variety of colours can be created by adding different concentrations of Co3O4 together with other compounds.
For increasingly blue but still transparent glass, one can add up to 2% Co. Darker blue enamels for decoration can contain as much as 7% Co. In some special glasses, other configurations and other tints are utilised. Thanks to its unique combination of solubility, stability and colouring effect, it can be said that there is no real substitute for cobalt in this application. The raw material is usually Co3O4.