Preparation of novel blue phosphate pigments in imitation of turquoise

As a novel blue pigment, various hydrated copper aluminum phosphate imitating turquoise, CuAl₆ (PO₄ )₄ (OH)₈ .₄H₂ O, were prepared by mixing copper nitrate solution, aluminum nitrate solution, phosphoric acid, and sodium hydroxide solution. Samples with similar chemical composition with turquoise were prepared. The obtained powders were evaluated with X-ray diffraction, infrared spectra, ultraviolet-visible reflectance spectra, and L*a*b* color space. Further, other related compositions were also investigated. Samples heated at low temperature were light blue powder. By heating at 700 °C, the color of materials changed to deep green. Samples with high copper ratio indicated lower L* value and small a* and b* values. The low copper ratio was suitable to obtain light blue and deep green phosphate pigments. (author)