In Nature for April 27 two German scientists discuss the special problems posed to environmental systems and public health by certain inorganic yellow and red pigments.
Such pigments have been used in human industries since ancient times in the form of frits and glazes, and incorporated into glasses and ceramics in processes which involve exposure to high temperatures. Most of these bright colours have been derived from heavy metals or transition metals which are capable, when present beyond a certain limiting concentration, of poisoning the environment and adversely affecting human health.
Cadmium-based pigments, which have long been used to obtain many of the most brilliant effects, are particularly hazardous. Cadmium itself is a toxic element and may enter the environment in a bioavailable form through waste disposal sites and incineration plants. There has therefore been a demand for the development of safer inorganic substitutes for our yellows and reds based on cadmium compounds.
It has been found that solid solutions of the perovskites CaTaO2N and LaTaON2 hold as much promise as cadmium pigments for brilliance, tinting strength, opacity, dispersability and fastness to light and heat, while lacking their environmental toxicity. These compounds have brilliant colours ranging from bright yellow to deep red, depending on the ratio of oxygen to nitrogen, which can be adjusted to meet individual needs.
The thermal stability of the oxonitrides in inert atmospheres is much better than that of the alternative cadmium derivatives. For example, in making enamels, oxonitrides can be heated to 720C whereas cadmium selenosulphides are unstable above 650C.
Oxonitride opacity is slightly less than that of selenosulphides in glasses, but their tinting strength is clearly superior. They may prove useful in colouring synthetic plastics materials.