In 1970, Japanese scholars studied the phase diagram of iron oxide microcrystalline formation, which laid a theoretical foundation for the preparation method of iron oxide yellow crystal seed. According to the research results, iron yellow crystal seeds can be formed under acidic or alkaline conditions. Because iron yellow is a crystal structure, in order to crystallize into pigment particles, it must first form crystal nucleus and become crystal seed, and then the crystal nucleus grows into iron yellow. Otherwise, only thin and dim color paste can be obtained, which does not have pigment properties. Acid process can be divided into iron sheet process and drop addition process.

Group 2B carcinogen

The National Cancer Institute tested TiO₂ for possible carcinogenicity by the oral route of exposure by feeding rats and mice with TiO₂ (size not specified) at doses 25,000 or 50,000 ppm TiO₂ for 103 weeks. They concluded that TiO₂ was not carcinogenic.Also, the study with rats fed diets containing up to 5 % TiO₂ coated mica for 130 weeks showed no treatment-related carcinogenicity. Since the size and other TiO₂ properties were not specified or determined, we cannot generalize this conclusion and we have to take into account other possible outcomes of this scenario in different exposure conditions (other size/crystalline structure of TiO₂ etc.).