New purple-blue ceramic pigments based on CoZr4(PO4)6

Due to the outstanding stability and resistivity to dissolution agents of the compounds related to NaZr2(PO4)3 (NZP family), our attention has been focussed on CoZr4(PO4)6 and its performance as an inorganic ceramic pigment for coloration of ceramic glazes. Mixed cobalt zirconium phosphate has been prepared by a solid state reaction and a sol–gel method and was characterised (through thermal analysis, XRD, heating microscopy, SEM, VIS-spectrophotometry and lightfastness measurement) for the first time as a ceramic pigment. In order to reduce the cobalt content in the samples the series of Co1−xMgxZr4(PO4)6 (x = 0.25; 0.5) have also been prepared using a solid state reaction and were investigated with the same techniques.

It was shown that a solid state reaction provides the formation of CoZr4(PO4)6 through a three component system stage (ZrP2O7, ZrO2 and CoP2O6/Co2P2O7), when employment of the sol–gel method leads to the direct formation of a CoZr4(PO4)6 phase at lower temperatures. During further thermal treatment, with an increase of the calcination temperature up to 1200–1300 °C, an additional phase of Zr2O(PO4)2 appears in the composition. A solid state reaction can be suggested as a preferable method for achieving enhanced thermal stability of this phosphate and the substitution of Co by Mg not only helps to reduce the content of Co in the sample compositions, but also to improve their thermal characteristics.

Thus, the obtained results indicate that employment of the more complicated sol–gel method does not provide any advantages at high calcination temperatures with respect to the phase composition, thermal stability, homogeneity and particle size distribution of the obtained samples and the conventional ceramic route does not deteriorate on the basis of these parameters. An irregular change of the colour parameters was observed for the samples during the calcination and the temperature of 1300 °C and 6–12 h of soaking time were chosen for ceramic pigment synthesis. Colouring ability of the obtained samples has been analysed with two types of ceramic glazes. The mixed phosphates exhibit saturated purple-blue colour, which becomes lighter only with an increase of Mg content to x = 0.5. Enamelled samples showed excellent lightfastness and the investigated compounds can be considered as high performance inorganic ceramic pigments for coloration of ceramic glazes.

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Using ceramic pigment

Depending on the use, ceramic pigments may be used straight and just mixed with water, but they are more commonly added as colorants in clay bodies and glazes. Some ceramic pigments are specifically formulated for clay bodies while some are not suitable at all. When used in clay, ceramic pigments are usually used in engobes and slips as a coating for clay rather than pigmenting the entire body. The exception to this would be using stains to tint porcelain for neriage work.

Use in concentrations of 10–15% in clay, using more or less depending on the intensity needed. Add the ceramic pigment to the slip and sieve through a 120x mesh screen to ensure adequate dispersion.

Ceramic pigments can be used in underglazes for brushing onto greenware or bisque. If used only with water as a medium, some glazes may crawl, so for best results, mix the stains with a frit (for example, Ferro frit 3124). Begin with a mix of 85 frit/15 ceramic pigment and test. Transparent gloss glazes applied over the top will heighten the intensity of the colors.

When using ceramic pigments in glazes, usually in concentrations of 1–10%, a little more care must be taken because some ceramic pigment systems react with materials in a glaze. Some ceramic pigments are affected by the presence, or lack of, boron, zinc, calcium, and magnesia. Manufacturers provide information on specific reactions. While most ceramic pigments can be used in both oxidation and reduction atmospheres, some are limited to certain maximum temperatures. Again, this information is available from manufacturer websites.

To achieve a wider palette, most ceramic pigments can be mixed to achieve even more colors. The exception is that black ceramic pigments cannot be used to obtain shades of gray because blacks are made from a combination of several metallic oxides. If low percentages are used, the final color is affected by the predominant oxide in the black ceramic pigment.

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