Preparation of novel blue phosphate pigments in imitation of turquoise

Recently, the use of harmful metals has been restricted in the world. However, because suitable substitutes have not been obtained, some materials containing harmful metals have been used in many fields [1-4]. For example, the inorganic color pigments containing metals such as mercury, cadmium, and lead have some merits, including high light stability, heat-resistant coloring visibility, cost, etc. [5-7]. In addition, because the oxide pigments have low coloring and covering, they are difficult to use for paints and plastics [8]. The sulfate and nitrate pigments have a lower heat resistance than oxide pigments and require harmful and/or combustible gas to synthesize. Furthermore, it is difficult to prepare the sulfide and nitrate pigments with repeatability [9, 10]. Therefore, the novel inorganic pigments are required with suitable properties and without difficult production methods [11, 12]. There are some kinds of inorganic blue pigments that are available for use, for example, Prussian blue, mixed metal oxide, cobalt blue, cerulean, cobalt-chromium blue, and so on [13-16]. Some of these pigments also include harmful metals. Therefore, novel blue pigment without harmful and precious metals is required. We focus on the natural ore, turquoise, CuAl6(PO4)4(OH)8.4H2O, because this ore includes no harmful and precious metals [17, 18]. Natural ores have high light stability and heat resistance. Because of their solidity, they are expected to have applications for plastics, paint, ceramics, and so on. In a previous study, we prepared novel inorganic pigments imitating turquoise, CuAl6(PO4)4(OH)8.4H2O, from copper oxide, aluminum hydroxide, and phosphoric acid by heating [19]. The obtained materials had the X-ray diffraction (XRD) peaks of CuAl6(PO4)4(OH)8.4H2O and CuO. However, the color of samples was dark, therefore, a different process is required to prepare the novel blue inorganic pigments. In this study, novel inorganic pigments imitating turquoise were synthesized by mixing copper nitrate solution, aluminum hydroxide or nitrate solution, phosphoric acid, and sodium hydroxide solution, and then heated to repress their solubility. The obtained materials were estimated from the viewpoint of pigment. Preparation of novel blue phosphate pigments in imitation of turquoise