Abstract: Yellow iron oxide crystals having lengths in the range of 100 to 1000 nm and a major axis/motor axis length ratio in the range of 1.5 to 4.5 are produced by adding an aqueous ferric salt solution to an aqueous alkali solution, causing the two solutions to react with each other at a temperature in the range of -5.degree. to 30.degree. C. thereby inducing precipitation of iron hydroxide, allowing the precipitate to age, then subjecting the aged precipitate to a hydrothermal treatment thereby producing crystals and, when necessary, further allowing the crystals to grow.

Abstract: In the manufacture of iron oxide flakes by the steps of mixing an aqueous ferric salt solution with an aqueous alkali solution for thereby inducing an amorphous precipitate and subjecting the precipitate to a hydrothermal treatment, iron oxide flakes of an increased flake size are obtained by adding boric acid or a salt thereof to the mixture prior to the step of hydrothermal treatment.

Abstract: Ferric sulfate obtained by the oxidation of ferrous sulfate or ferric sulfate available in its original state is added to an alkali hydroxide at a ratio such that the amount of said alkali hydroxide will be in excess of its stoichiometric proportion, so as to form a pasty substance. Then, this pasty substance is subjected to a hydrothermal treatment. From the resultant product of the treatment, there are obtained micaceous .alpha.-iron oxide crystals and aqueous solutions of sodium sulfate and alkali hydroxide.

Abstract: Synthetic yellow iron oxide (.alpha.-FeOOH) is dispersed in an aqueous 0 - 10N alkali solution and the resultant dispersion is subjected to a hydrothermal treatment at 100.degree.- 250.degree.C. This treatment improves the synthetic yellow iron oxide in that it converts imperfect crystals present in said iron oxide into perfect crystals, changes particles thereof from the shape of needles to that of rods, makes the particle size substantially uniform and heightens the resistivity thereof to heat by about 50.degree.C. The modified synthetic yellow iron oxide thus obtained is innoxious and can be used as a pigment in paints.