The effect of potassium cations on the shade of the ultramarine blue pigment system has been investigated through analysis of commercial materials, ion exchange reactions of sodium-form ultramarine blue and a detailed study of the effect of adding potassium sources to ultramarine precursor mixtures. Incorporation of potassium cations has the result of producing redder shade material and the replacement of around 20% of the sodium in the standard reaction mixture produces the optimised red-shade pigment. The origin of this hue control may be a direct effect, involving interaction of potassium ions with the ultramarine blue structure and the S chromophore or, more likely, potassium ions aids the stabilisation of S within the sodalite framework during the ultramarine blue formation process. In the commercial process, the role of the potassium source in red shade formation may be performed by the incorporation of potassium-rich feldspar additives.
Booth, D. G., Dann, S. E., & Weller, M. T. (2003). The effect of the cation composition on the synthesis and properties of ultramarine blue. Dyes and Pigments, 58(1), 73-82. https://doi.org/10.1016/S0143-7208(03)00037-8