Influence of aluminum source on the color tone of cobalt blue pigment

Cobalt blue is an inorganic pigment with an intense blue color, and consists mainly of cobalt aluminate (CoAl₂O₄) spinel. Many synthesis methods for cobalt blue have been reported, but there are no reports on the color tone of cobalt blue synthesized from different starting materials. Thus, the relationship between starting material and the color tone of the resulting product has not yet been clarified. We experimentally investigate the influence of different starting materials on the color tone of cobalt blue, synthesized by solid-state reaction. Cobalt oxide (Co₃O₄) was used as the cobalt source, and three types of aluminum source, Al(OH)₃, γ-Al₂O₃, and α-Al₂O₃, were used. The color tone of cobalt blue was evaluated by the lightness L* and Chroma C* values, based on CIE L ^* a ^* b ^* color coordinates. The properties of the products were characterized by spectrophotometry, X-ray diffraction, scanning electron microscopy (SEM), particle size distribution analysis, and thermal gravimetric-differential thermal analyses. SEM images and particle size distribution analysis revealed that the lightness L* value depends on the mass median diameter of the cobalt blue. The Chroma C* value is affected by the crystallinity of the aluminum source. These results suggest that the lightness L* and Chroma C* values of cobalt blue can be controlled by the particle size and crystallinity of the starting materials.