Structure and colour of cobalt ceramic pigments from phosphates

Co3−xFexP2O8+x/2 (0 ≤ x ≤ 2.0) and Fe1−xCoxPO4−x/2 (0 ≤ x ≤ 0.5) compositions were synthetized by the chemical coprecipitation method and characterized by differential thermal analysis and thermogravimetric analysis, X-ray diffraction, UV–vis–NIR spectroscopy, CIE L* a* b* parameters measurements and electronic microscopy. At 1000 °C, Co3(PO4)2 solid solutions (Co3−xFexP2O8+x/2; 0 ≤ x ≤ 0.3) and FePO4 solid solutions (Fe1−xCoxPO4−x/2; 0 ≤ x ≤ 0.02) were obtained. From enamelled samples, the blue colouration is detected when an appreciable amount of Co3(PO4)2 (Co3−xFexP2O8+x/2 samples) or Co2P2O7 (Fe1−xCoxPO4−x/2 samples) is present in powdered samples. These materials develop blue colourations in enamelled samples when 0 ≤ x ≤ 1.0 in Co3−xFexP2O8+x/2 (between 48 and 32 wt.% Co) compositions and when x ≥ 0.30 in Fe1−xCoxPO4−x/2 (≥12 wt.% Co) compositions. Fe1−xCoxPO4−x/2 (0.3 ≤ x ≤ 0.5) compositions can be considered to minimize the Co content in blue ceramic pigments.