Yellow iron oxide
Yellow iron oxide is the durable, economical yellow of the coatings world: lightfast, weather-resistant and chemically inert, with one important limit, heat.
Yellow iron oxide is iron(III) oxide-hydroxide, FeO(OH), with the goethite crystal structure. In the Colour Index it is Pigment Yellow 42 (C.I. 77492). Synthetic grades are made by precipitation or the Penniman process and have needle-shaped (acicular) particles that give an ochre to bright yellow shade.
It is used for yellows, tans, beiges and earth tones in architectural, industrial, floor and concrete coatings, primers and colored concrete. It is often combined with organic yellows when a cleaner, brighter shade is needed, since iron oxide yellow is relatively dull.
Synthetic iron oxide pigments have low toxicity and are generally not classified as hazardous, but fine powders are an inhalable dust. Use dust control and respiratory protection when handling dry pigment, and follow the SDS and occupational dust limits.
Heat drives water out of FeO(OH), converting it toward hematite (Fe₂O₃), which is red. That starts at roughly 180–200 °C.
Iron oxide is cheaper, more opaque and more durable but duller. Organic yellows are brighter and cleaner but usually less opaque and more expensive; blends are common.
Its needle-shaped particles have higher oil absorption, so the same loading raises viscosity more and reduces the critical pigment volume concentration.
