Abstract: Yellow iron oxide pigments having improved heat stability are prepared by subjecting an alkaline slurry of conventional yellow iron oxide, a soluble ferric salt, sodium aluminate, and a third metal component, exemplified by antimony ion or a zinc ion/titanium ion combination, to a hydrothermal (autoclave) treatment at about 100.degree.-240.degree. C. The separated solid product has a heat stability temperature (based on the color change from yellow to red when heated) appreciably higher than that of the starting iron oxide.

Abstract: Yellow iron oxide pigments having improved heat stability are prepared by subjecting an alkaline slurry of conventional yellow iron oxide, a soluble ferric salt, and sodium aluminate to a hydrothermal (autoclave) treatment at 100.degree.-240.degree. C., separating and slurrying the resultant solid product, and subjecting that product to a second hydrothermal treatment of 250.degree.-350.degree. C. The original iron oxide has a heat stability (based on color change from yellow to red) temperature of about 203.degree. C., while the final product shows a change at 270.degree. C.

Abstract: Yellow iron oxide pigments having improved heat stability are prepared by mixing a conventional yellow iron oxide pigment with an aqueous alkaline solution of a soluble antimony compound. This yields a yellow pigment whose particle surface is coated with "antimony-bloom", and this product has a higher heat stability temperature than does the starting yellow iron oxide. When the product coated with "antimony-bloom" is subjected to hydrothermal treatment (autoclaving), the resultant material has a yet-higher heat stability temperature. Other, related treatments furnish products with improved heat stability temperatures.

Abstract: Synthetic yellow iron oxide (.alpha.-FeOOH) is dispersed in an aqueous ferric solution, and an adequate alkali solution and an aqueous aluminum salt solution are added. The resultant dispersion is subjected to a hydrothermal treatment at 150.degree.-250.degree. C. By this treatment, the surface of the synthetic yellow iron oxide is covered with a (FeAl)OOH (solid solution). Yellow iron oxides treated thus have an elevated resistivity to heat of about 50.degree. C. The modified synthetic yellow iron oxide thus obtained is non-toxic and can be used as a pigment in plastics and melt-type traffic paint.