Red oxide is a generic name of a ferric oxide pigment of the red color. Multiple shades based on both anhydrous Fe2O3 and its hydrates were known to painters since prehistory. The pigments were originally sourced from natural sources, since the 20th century they are mostly synthetic. These substances form one of the most commercially important groups of pigments, their names sometimes reflect the location of a natural source, later transferred to the synthetic analog. Well-known examples include the Persian Gulf Oxide with 75% Fe2O3 and 25% silica, Spanish red with 85% of oxide, Tuscan red.
The anhydrous pigment has a dark purple-red or maroon color, hydrates’ colors vary from dull yellow (yellow ochre) to warm red.
The iron oxide red is extremely stable: it is not affected by light and most chemicals (soluble in hot concentrated acids); heat only affects the hydrated variants (the water is removed, and the color darkens).
